Source file src/runtime/panic.go

Documentation: runtime

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package runtime
     6  
     7  import (
     8  	"runtime/internal/atomic"
     9  	"runtime/internal/sys"
    10  	"unsafe"
    11  )
    12  
    13  // We have two different ways of doing defers. The older way involves creating a
    14  // defer record at the time that a defer statement is executing and adding it to a
    15  // defer chain. This chain is inspected by the deferreturn call at all function
    16  // exits in order to run the appropriate defer calls. A cheaper way (which we call
    17  // open-coded defers) is used for functions in which no defer statements occur in
    18  // loops. In that case, we simply store the defer function/arg information into
    19  // specific stack slots at the point of each defer statement, as well as setting a
    20  // bit in a bitmask. At each function exit, we add inline code to directly make
    21  // the appropriate defer calls based on the bitmask and fn/arg information stored
    22  // on the stack. During panic/Goexit processing, the appropriate defer calls are
    23  // made using extra funcdata info that indicates the exact stack slots that
    24  // contain the bitmask and defer fn/args.
    25  
    26  // Check to make sure we can really generate a panic. If the panic
    27  // was generated from the runtime, or from inside malloc, then convert
    28  // to a throw of msg.
    29  // pc should be the program counter of the compiler-generated code that
    30  // triggered this panic.
    31  func panicCheck1(pc uintptr, msg string) {
    32  	if sys.GoarchWasm == 0 && hasPrefix(funcname(findfunc(pc)), "runtime.") {
    33  		// Note: wasm can't tail call, so we can't get the original caller's pc.
    34  		throw(msg)
    35  	}
    36  	// TODO: is this redundant? How could we be in malloc
    37  	// but not in the runtime? runtime/internal/*, maybe?
    38  	gp := getg()
    39  	if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
    40  		throw(msg)
    41  	}
    42  }
    43  
    44  // Same as above, but calling from the runtime is allowed.
    45  //
    46  // Using this function is necessary for any panic that may be
    47  // generated by runtime.sigpanic, since those are always called by the
    48  // runtime.
    49  func panicCheck2(err string) {
    50  	// panic allocates, so to avoid recursive malloc, turn panics
    51  	// during malloc into throws.
    52  	gp := getg()
    53  	if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
    54  		throw(err)
    55  	}
    56  }
    57  
    58  // Many of the following panic entry-points turn into throws when they
    59  // happen in various runtime contexts. These should never happen in
    60  // the runtime, and if they do, they indicate a serious issue and
    61  // should not be caught by user code.
    62  //
    63  // The panic{Index,Slice,divide,shift} functions are called by
    64  // code generated by the compiler for out of bounds index expressions,
    65  // out of bounds slice expressions, division by zero, and shift by negative.
    66  // The panicdivide (again), panicoverflow, panicfloat, and panicmem
    67  // functions are called by the signal handler when a signal occurs
    68  // indicating the respective problem.
    69  //
    70  // Since panic{Index,Slice,shift} are never called directly, and
    71  // since the runtime package should never have an out of bounds slice
    72  // or array reference or negative shift, if we see those functions called from the
    73  // runtime package we turn the panic into a throw. That will dump the
    74  // entire runtime stack for easier debugging.
    75  //
    76  // The entry points called by the signal handler will be called from
    77  // runtime.sigpanic, so we can't disallow calls from the runtime to
    78  // these (they always look like they're called from the runtime).
    79  // Hence, for these, we just check for clearly bad runtime conditions.
    80  //
    81  // The panic{Index,Slice} functions are implemented in assembly and tail call
    82  // to the goPanic{Index,Slice} functions below. This is done so we can use
    83  // a space-minimal register calling convention.
    84  
    85  // failures in the comparisons for s[x], 0 <= x < y (y == len(s))
    86  func goPanicIndex(x int, y int) {
    87  	panicCheck1(getcallerpc(), "index out of range")
    88  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsIndex})
    89  }
    90  func goPanicIndexU(x uint, y int) {
    91  	panicCheck1(getcallerpc(), "index out of range")
    92  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsIndex})
    93  }
    94  
    95  // failures in the comparisons for s[:x], 0 <= x <= y (y == len(s) or cap(s))
    96  func goPanicSliceAlen(x int, y int) {
    97  	panicCheck1(getcallerpc(), "slice bounds out of range")
    98  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAlen})
    99  }
   100  func goPanicSliceAlenU(x uint, y int) {
   101  	panicCheck1(getcallerpc(), "slice bounds out of range")
   102  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAlen})
   103  }
   104  func goPanicSliceAcap(x int, y int) {
   105  	panicCheck1(getcallerpc(), "slice bounds out of range")
   106  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAcap})
   107  }
   108  func goPanicSliceAcapU(x uint, y int) {
   109  	panicCheck1(getcallerpc(), "slice bounds out of range")
   110  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAcap})
   111  }
   112  
   113  // failures in the comparisons for s[x:y], 0 <= x <= y
   114  func goPanicSliceB(x int, y int) {
   115  	panicCheck1(getcallerpc(), "slice bounds out of range")
   116  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceB})
   117  }
   118  func goPanicSliceBU(x uint, y int) {
   119  	panicCheck1(getcallerpc(), "slice bounds out of range")
   120  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceB})
   121  }
   122  
   123  // failures in the comparisons for s[::x], 0 <= x <= y (y == len(s) or cap(s))
   124  func goPanicSlice3Alen(x int, y int) {
   125  	panicCheck1(getcallerpc(), "slice bounds out of range")
   126  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Alen})
   127  }
   128  func goPanicSlice3AlenU(x uint, y int) {
   129  	panicCheck1(getcallerpc(), "slice bounds out of range")
   130  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Alen})
   131  }
   132  func goPanicSlice3Acap(x int, y int) {
   133  	panicCheck1(getcallerpc(), "slice bounds out of range")
   134  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Acap})
   135  }
   136  func goPanicSlice3AcapU(x uint, y int) {
   137  	panicCheck1(getcallerpc(), "slice bounds out of range")
   138  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Acap})
   139  }
   140  
   141  // failures in the comparisons for s[:x:y], 0 <= x <= y
   142  func goPanicSlice3B(x int, y int) {
   143  	panicCheck1(getcallerpc(), "slice bounds out of range")
   144  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3B})
   145  }
   146  func goPanicSlice3BU(x uint, y int) {
   147  	panicCheck1(getcallerpc(), "slice bounds out of range")
   148  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3B})
   149  }
   150  
   151  // failures in the comparisons for s[x:y:], 0 <= x <= y
   152  func goPanicSlice3C(x int, y int) {
   153  	panicCheck1(getcallerpc(), "slice bounds out of range")
   154  	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3C})
   155  }
   156  func goPanicSlice3CU(x uint, y int) {
   157  	panicCheck1(getcallerpc(), "slice bounds out of range")
   158  	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3C})
   159  }
   160  
   161  // Implemented in assembly, as they take arguments in registers.
   162  // Declared here to mark them as ABIInternal.
   163  func panicIndex(x int, y int)
   164  func panicIndexU(x uint, y int)
   165  func panicSliceAlen(x int, y int)
   166  func panicSliceAlenU(x uint, y int)
   167  func panicSliceAcap(x int, y int)
   168  func panicSliceAcapU(x uint, y int)
   169  func panicSliceB(x int, y int)
   170  func panicSliceBU(x uint, y int)
   171  func panicSlice3Alen(x int, y int)
   172  func panicSlice3AlenU(x uint, y int)
   173  func panicSlice3Acap(x int, y int)
   174  func panicSlice3AcapU(x uint, y int)
   175  func panicSlice3B(x int, y int)
   176  func panicSlice3BU(x uint, y int)
   177  func panicSlice3C(x int, y int)
   178  func panicSlice3CU(x uint, y int)
   179  
   180  var shiftError = error(errorString("negative shift amount"))
   181  
   182  func panicshift() {
   183  	panicCheck1(getcallerpc(), "negative shift amount")
   184  	panic(shiftError)
   185  }
   186  
   187  var divideError = error(errorString("integer divide by zero"))
   188  
   189  func panicdivide() {
   190  	panicCheck2("integer divide by zero")
   191  	panic(divideError)
   192  }
   193  
   194  var overflowError = error(errorString("integer overflow"))
   195  
   196  func panicoverflow() {
   197  	panicCheck2("integer overflow")
   198  	panic(overflowError)
   199  }
   200  
   201  var floatError = error(errorString("floating point error"))
   202  
   203  func panicfloat() {
   204  	panicCheck2("floating point error")
   205  	panic(floatError)
   206  }
   207  
   208  var memoryError = error(errorString("invalid memory address or nil pointer dereference"))
   209  
   210  func panicmem() {
   211  	panicCheck2("invalid memory address or nil pointer dereference")
   212  	panic(memoryError)
   213  }
   214  
   215  func panicmemAddr(addr uintptr) {
   216  	panicCheck2("invalid memory address or nil pointer dereference")
   217  	panic(errorAddressString{msg: "invalid memory address or nil pointer dereference", addr: addr})
   218  }
   219  
   220  // Create a new deferred function fn with siz bytes of arguments.
   221  // The compiler turns a defer statement into a call to this.
   222  //go:nosplit
   223  func deferproc(siz int32, fn *funcval) { // arguments of fn follow fn
   224  	gp := getg()
   225  	if gp.m.curg != gp {
   226  		// go code on the system stack can't defer
   227  		throw("defer on system stack")
   228  	}
   229  
   230  	// the arguments of fn are in a perilous state. The stack map
   231  	// for deferproc does not describe them. So we can't let garbage
   232  	// collection or stack copying trigger until we've copied them out
   233  	// to somewhere safe. The memmove below does that.
   234  	// Until the copy completes, we can only call nosplit routines.
   235  	sp := getcallersp()
   236  	argp := uintptr(unsafe.Pointer(&fn)) + unsafe.Sizeof(fn)
   237  	callerpc := getcallerpc()
   238  
   239  	d := newdefer(siz)
   240  	if d._panic != nil {
   241  		throw("deferproc: d.panic != nil after newdefer")
   242  	}
   243  	d.link = gp._defer
   244  	gp._defer = d
   245  	d.fn = fn
   246  	d.pc = callerpc
   247  	d.sp = sp
   248  	switch siz {
   249  	case 0:
   250  		// Do nothing.
   251  	case sys.PtrSize:
   252  		*(*uintptr)(deferArgs(d)) = *(*uintptr)(unsafe.Pointer(argp))
   253  	default:
   254  		memmove(deferArgs(d), unsafe.Pointer(argp), uintptr(siz))
   255  	}
   256  
   257  	// deferproc returns 0 normally.
   258  	// a deferred func that stops a panic
   259  	// makes the deferproc return 1.
   260  	// the code the compiler generates always
   261  	// checks the return value and jumps to the
   262  	// end of the function if deferproc returns != 0.
   263  	return0()
   264  	// No code can go here - the C return register has
   265  	// been set and must not be clobbered.
   266  }
   267  
   268  // deferprocStack queues a new deferred function with a defer record on the stack.
   269  // The defer record must have its siz and fn fields initialized.
   270  // All other fields can contain junk.
   271  // The defer record must be immediately followed in memory by
   272  // the arguments of the defer.
   273  // Nosplit because the arguments on the stack won't be scanned
   274  // until the defer record is spliced into the gp._defer list.
   275  //go:nosplit
   276  func deferprocStack(d *_defer) {
   277  	gp := getg()
   278  	if gp.m.curg != gp {
   279  		// go code on the system stack can't defer
   280  		throw("defer on system stack")
   281  	}
   282  	// siz and fn are already set.
   283  	// The other fields are junk on entry to deferprocStack and
   284  	// are initialized here.
   285  	d.started = false
   286  	d.heap = false
   287  	d.openDefer = false
   288  	d.sp = getcallersp()
   289  	d.pc = getcallerpc()
   290  	d.framepc = 0
   291  	d.varp = 0
   292  	// The lines below implement:
   293  	//   d.panic = nil
   294  	//   d.fd = nil
   295  	//   d.link = gp._defer
   296  	//   gp._defer = d
   297  	// But without write barriers. The first three are writes to
   298  	// the stack so they don't need a write barrier, and furthermore
   299  	// are to uninitialized memory, so they must not use a write barrier.
   300  	// The fourth write does not require a write barrier because we
   301  	// explicitly mark all the defer structures, so we don't need to
   302  	// keep track of pointers to them with a write barrier.
   303  	*(*uintptr)(unsafe.Pointer(&d._panic)) = 0
   304  	*(*uintptr)(unsafe.Pointer(&d.fd)) = 0
   305  	*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
   306  	*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))
   307  
   308  	return0()
   309  	// No code can go here - the C return register has
   310  	// been set and must not be clobbered.
   311  }
   312  
   313  // Small malloc size classes >= 16 are the multiples of 16: 16, 32, 48, 64, 80, 96, 112, 128, 144, ...
   314  // Each P holds a pool for defers with small arg sizes.
   315  // Assign defer allocations to pools by rounding to 16, to match malloc size classes.
   316  
   317  const (
   318  	deferHeaderSize = unsafe.Sizeof(_defer{})
   319  	minDeferAlloc   = (deferHeaderSize + 15) &^ 15
   320  	minDeferArgs    = minDeferAlloc - deferHeaderSize
   321  )
   322  
   323  // defer size class for arg size sz
   324  //go:nosplit
   325  func deferclass(siz uintptr) uintptr {
   326  	if siz <= minDeferArgs {
   327  		return 0
   328  	}
   329  	return (siz - minDeferArgs + 15) / 16
   330  }
   331  
   332  // total size of memory block for defer with arg size sz
   333  func totaldefersize(siz uintptr) uintptr {
   334  	if siz <= minDeferArgs {
   335  		return minDeferAlloc
   336  	}
   337  	return deferHeaderSize + siz
   338  }
   339  
   340  // Ensure that defer arg sizes that map to the same defer size class
   341  // also map to the same malloc size class.
   342  func testdefersizes() {
   343  	var m [len(p{}.deferpool)]int32
   344  
   345  	for i := range m {
   346  		m[i] = -1
   347  	}
   348  	for i := uintptr(0); ; i++ {
   349  		defersc := deferclass(i)
   350  		if defersc >= uintptr(len(m)) {
   351  			break
   352  		}
   353  		siz := roundupsize(totaldefersize(i))
   354  		if m[defersc] < 0 {
   355  			m[defersc] = int32(siz)
   356  			continue
   357  		}
   358  		if m[defersc] != int32(siz) {
   359  			print("bad defer size class: i=", i, " siz=", siz, " defersc=", defersc, "\n")
   360  			throw("bad defer size class")
   361  		}
   362  	}
   363  }
   364  
   365  // The arguments associated with a deferred call are stored
   366  // immediately after the _defer header in memory.
   367  //go:nosplit
   368  func deferArgs(d *_defer) unsafe.Pointer {
   369  	if d.siz == 0 {
   370  		// Avoid pointer past the defer allocation.
   371  		return nil
   372  	}
   373  	return add(unsafe.Pointer(d), unsafe.Sizeof(*d))
   374  }
   375  
   376  var deferType *_type // type of _defer struct
   377  
   378  func init() {
   379  	var x interface{}
   380  	x = (*_defer)(nil)
   381  	deferType = (*(**ptrtype)(unsafe.Pointer(&x))).elem
   382  }
   383  
   384  // Allocate a Defer, usually using per-P pool.
   385  // Each defer must be released with freedefer.  The defer is not
   386  // added to any defer chain yet.
   387  //
   388  // This must not grow the stack because there may be a frame without
   389  // stack map information when this is called.
   390  //
   391  //go:nosplit
   392  func newdefer(siz int32) *_defer {
   393  	var d *_defer
   394  	sc := deferclass(uintptr(siz))
   395  	gp := getg()
   396  	if sc < uintptr(len(p{}.deferpool)) {
   397  		pp := gp.m.p.ptr()
   398  		if len(pp.deferpool[sc]) == 0 && sched.deferpool[sc] != nil {
   399  			// Take the slow path on the system stack so
   400  			// we don't grow newdefer's stack.
   401  			systemstack(func() {
   402  				lock(&sched.deferlock)
   403  				for len(pp.deferpool[sc]) < cap(pp.deferpool[sc])/2 && sched.deferpool[sc] != nil {
   404  					d := sched.deferpool[sc]
   405  					sched.deferpool[sc] = d.link
   406  					d.link = nil
   407  					pp.deferpool[sc] = append(pp.deferpool[sc], d)
   408  				}
   409  				unlock(&sched.deferlock)
   410  			})
   411  		}
   412  		if n := len(pp.deferpool[sc]); n > 0 {
   413  			d = pp.deferpool[sc][n-1]
   414  			pp.deferpool[sc][n-1] = nil
   415  			pp.deferpool[sc] = pp.deferpool[sc][:n-1]
   416  		}
   417  	}
   418  	if d == nil {
   419  		// Allocate new defer+args.
   420  		systemstack(func() {
   421  			total := roundupsize(totaldefersize(uintptr(siz)))
   422  			d = (*_defer)(mallocgc(total, deferType, true))
   423  		})
   424  	}
   425  	d.siz = siz
   426  	d.heap = true
   427  	return d
   428  }
   429  
   430  // Free the given defer.
   431  // The defer cannot be used after this call.
   432  //
   433  // This must not grow the stack because there may be a frame without a
   434  // stack map when this is called.
   435  //
   436  //go:nosplit
   437  func freedefer(d *_defer) {
   438  	if d._panic != nil {
   439  		freedeferpanic()
   440  	}
   441  	if d.fn != nil {
   442  		freedeferfn()
   443  	}
   444  	if !d.heap {
   445  		return
   446  	}
   447  	sc := deferclass(uintptr(d.siz))
   448  	if sc >= uintptr(len(p{}.deferpool)) {
   449  		return
   450  	}
   451  	pp := getg().m.p.ptr()
   452  	if len(pp.deferpool[sc]) == cap(pp.deferpool[sc]) {
   453  		// Transfer half of local cache to the central cache.
   454  		//
   455  		// Take this slow path on the system stack so
   456  		// we don't grow freedefer's stack.
   457  		systemstack(func() {
   458  			var first, last *_defer
   459  			for len(pp.deferpool[sc]) > cap(pp.deferpool[sc])/2 {
   460  				n := len(pp.deferpool[sc])
   461  				d := pp.deferpool[sc][n-1]
   462  				pp.deferpool[sc][n-1] = nil
   463  				pp.deferpool[sc] = pp.deferpool[sc][:n-1]
   464  				if first == nil {
   465  					first = d
   466  				} else {
   467  					last.link = d
   468  				}
   469  				last = d
   470  			}
   471  			lock(&sched.deferlock)
   472  			last.link = sched.deferpool[sc]
   473  			sched.deferpool[sc] = first
   474  			unlock(&sched.deferlock)
   475  		})
   476  	}
   477  
   478  	// These lines used to be simply `*d = _defer{}` but that
   479  	// started causing a nosplit stack overflow via typedmemmove.
   480  	d.siz = 0
   481  	d.started = false
   482  	d.openDefer = false
   483  	d.sp = 0
   484  	d.pc = 0
   485  	d.framepc = 0
   486  	d.varp = 0
   487  	d.fd = nil
   488  	// d._panic and d.fn must be nil already.
   489  	// If not, we would have called freedeferpanic or freedeferfn above,
   490  	// both of which throw.
   491  	d.link = nil
   492  
   493  	pp.deferpool[sc] = append(pp.deferpool[sc], d)
   494  }
   495  
   496  // Separate function so that it can split stack.
   497  // Windows otherwise runs out of stack space.
   498  func freedeferpanic() {
   499  	// _panic must be cleared before d is unlinked from gp.
   500  	throw("freedefer with d._panic != nil")
   501  }
   502  
   503  func freedeferfn() {
   504  	// fn must be cleared before d is unlinked from gp.
   505  	throw("freedefer with d.fn != nil")
   506  }
   507  
   508  // Run a deferred function if there is one.
   509  // The compiler inserts a call to this at the end of any
   510  // function which calls defer.
   511  // If there is a deferred function, this will call runtime·jmpdefer,
   512  // which will jump to the deferred function such that it appears
   513  // to have been called by the caller of deferreturn at the point
   514  // just before deferreturn was called. The effect is that deferreturn
   515  // is called again and again until there are no more deferred functions.
   516  //
   517  // Declared as nosplit, because the function should not be preempted once we start
   518  // modifying the caller's frame in order to reuse the frame to call the deferred
   519  // function.
   520  //
   521  // The single argument isn't actually used - it just has its address
   522  // taken so it can be matched against pending defers.
   523  //go:nosplit
   524  func deferreturn(arg0 uintptr) {
   525  	gp := getg()
   526  	d := gp._defer
   527  	if d == nil {
   528  		return
   529  	}
   530  	sp := getcallersp()
   531  	if d.sp != sp {
   532  		return
   533  	}
   534  	if d.openDefer {
   535  		done := runOpenDeferFrame(gp, d)
   536  		if !done {
   537  			throw("unfinished open-coded defers in deferreturn")
   538  		}
   539  		gp._defer = d.link
   540  		freedefer(d)
   541  		return
   542  	}
   543  
   544  	// Moving arguments around.
   545  	//
   546  	// Everything called after this point must be recursively
   547  	// nosplit because the garbage collector won't know the form
   548  	// of the arguments until the jmpdefer can flip the PC over to
   549  	// fn.
   550  	switch d.siz {
   551  	case 0:
   552  		// Do nothing.
   553  	case sys.PtrSize:
   554  		*(*uintptr)(unsafe.Pointer(&arg0)) = *(*uintptr)(deferArgs(d))
   555  	default:
   556  		memmove(unsafe.Pointer(&arg0), deferArgs(d), uintptr(d.siz))
   557  	}
   558  	fn := d.fn
   559  	d.fn = nil
   560  	gp._defer = d.link
   561  	freedefer(d)
   562  	// If the defer function pointer is nil, force the seg fault to happen
   563  	// here rather than in jmpdefer. gentraceback() throws an error if it is
   564  	// called with a callback on an LR architecture and jmpdefer is on the
   565  	// stack, because the stack trace can be incorrect in that case - see
   566  	// issue #8153).
   567  	_ = fn.fn
   568  	jmpdefer(fn, uintptr(unsafe.Pointer(&arg0)))
   569  }
   570  
   571  // Goexit terminates the goroutine that calls it. No other goroutine is affected.
   572  // Goexit runs all deferred calls before terminating the goroutine. Because Goexit
   573  // is not a panic, any recover calls in those deferred functions will return nil.
   574  //
   575  // Calling Goexit from the main goroutine terminates that goroutine
   576  // without func main returning. Since func main has not returned,
   577  // the program continues execution of other goroutines.
   578  // If all other goroutines exit, the program crashes.
   579  func Goexit() {
   580  	// Run all deferred functions for the current goroutine.
   581  	// This code is similar to gopanic, see that implementation
   582  	// for detailed comments.
   583  	gp := getg()
   584  
   585  	// Create a panic object for Goexit, so we can recognize when it might be
   586  	// bypassed by a recover().
   587  	var p _panic
   588  	p.goexit = true
   589  	p.link = gp._panic
   590  	gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
   591  
   592  	addOneOpenDeferFrame(gp, getcallerpc(), unsafe.Pointer(getcallersp()))
   593  	for {
   594  		d := gp._defer
   595  		if d == nil {
   596  			break
   597  		}
   598  		if d.started {
   599  			if d._panic != nil {
   600  				d._panic.aborted = true
   601  				d._panic = nil
   602  			}
   603  			if !d.openDefer {
   604  				d.fn = nil
   605  				gp._defer = d.link
   606  				freedefer(d)
   607  				continue
   608  			}
   609  		}
   610  		d.started = true
   611  		d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
   612  		if d.openDefer {
   613  			done := runOpenDeferFrame(gp, d)
   614  			if !done {
   615  				// We should always run all defers in the frame,
   616  				// since there is no panic associated with this
   617  				// defer that can be recovered.
   618  				throw("unfinished open-coded defers in Goexit")
   619  			}
   620  			if p.aborted {
   621  				// Since our current defer caused a panic and may
   622  				// have been already freed, just restart scanning
   623  				// for open-coded defers from this frame again.
   624  				addOneOpenDeferFrame(gp, getcallerpc(), unsafe.Pointer(getcallersp()))
   625  			} else {
   626  				addOneOpenDeferFrame(gp, 0, nil)
   627  			}
   628  		} else {
   629  
   630  			// Save the pc/sp in reflectcallSave(), so we can "recover" back to this
   631  			// loop if necessary.
   632  			reflectcallSave(&p, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz))
   633  		}
   634  		if p.aborted {
   635  			// We had a recursive panic in the defer d we started, and
   636  			// then did a recover in a defer that was further down the
   637  			// defer chain than d. In the case of an outstanding Goexit,
   638  			// we force the recover to return back to this loop. d will
   639  			// have already been freed if completed, so just continue
   640  			// immediately to the next defer on the chain.
   641  			p.aborted = false
   642  			continue
   643  		}
   644  		if gp._defer != d {
   645  			throw("bad defer entry in Goexit")
   646  		}
   647  		d._panic = nil
   648  		d.fn = nil
   649  		gp._defer = d.link
   650  		freedefer(d)
   651  		// Note: we ignore recovers here because Goexit isn't a panic
   652  	}
   653  	goexit1()
   654  }
   655  
   656  // Call all Error and String methods before freezing the world.
   657  // Used when crashing with panicking.
   658  func preprintpanics(p *_panic) {
   659  	defer func() {
   660  		if recover() != nil {
   661  			throw("panic while printing panic value")
   662  		}
   663  	}()
   664  	for p != nil {
   665  		switch v := p.arg.(type) {
   666  		case error:
   667  			p.arg = v.Error()
   668  		case stringer:
   669  			p.arg = v.String()
   670  		}
   671  		p = p.link
   672  	}
   673  }
   674  
   675  // Print all currently active panics. Used when crashing.
   676  // Should only be called after preprintpanics.
   677  func printpanics(p *_panic) {
   678  	if p.link != nil {
   679  		printpanics(p.link)
   680  		if !p.link.goexit {
   681  			print("\t")
   682  		}
   683  	}
   684  	if p.goexit {
   685  		return
   686  	}
   687  	print("panic: ")
   688  	printany(p.arg)
   689  	if p.recovered {
   690  		print(" [recovered]")
   691  	}
   692  	print("\n")
   693  }
   694  
   695  // addOneOpenDeferFrame scans the stack for the first frame (if any) with
   696  // open-coded defers and if it finds one, adds a single record to the defer chain
   697  // for that frame. If sp is non-nil, it starts the stack scan from the frame
   698  // specified by sp. If sp is nil, it uses the sp from the current defer record
   699  // (which has just been finished). Hence, it continues the stack scan from the
   700  // frame of the defer that just finished. It skips any frame that already has an
   701  // open-coded _defer record, which would have been been created from a previous
   702  // (unrecovered) panic.
   703  //
   704  // Note: All entries of the defer chain (including this new open-coded entry) have
   705  // their pointers (including sp) adjusted properly if the stack moves while
   706  // running deferred functions. Also, it is safe to pass in the sp arg (which is
   707  // the direct result of calling getcallersp()), because all pointer variables
   708  // (including arguments) are adjusted as needed during stack copies.
   709  func addOneOpenDeferFrame(gp *g, pc uintptr, sp unsafe.Pointer) {
   710  	var prevDefer *_defer
   711  	if sp == nil {
   712  		prevDefer = gp._defer
   713  		pc = prevDefer.framepc
   714  		sp = unsafe.Pointer(prevDefer.sp)
   715  	}
   716  	systemstack(func() {
   717  		gentraceback(pc, uintptr(sp), 0, gp, 0, nil, 0x7fffffff,
   718  			func(frame *stkframe, unused unsafe.Pointer) bool {
   719  				if prevDefer != nil && prevDefer.sp == frame.sp {
   720  					// Skip the frame for the previous defer that
   721  					// we just finished (and was used to set
   722  					// where we restarted the stack scan)
   723  					return true
   724  				}
   725  				f := frame.fn
   726  				fd := funcdata(f, _FUNCDATA_OpenCodedDeferInfo)
   727  				if fd == nil {
   728  					return true
   729  				}
   730  				// Insert the open defer record in the
   731  				// chain, in order sorted by sp.
   732  				d := gp._defer
   733  				var prev *_defer
   734  				for d != nil {
   735  					dsp := d.sp
   736  					if frame.sp < dsp {
   737  						break
   738  					}
   739  					if frame.sp == dsp {
   740  						if !d.openDefer {
   741  							throw("duplicated defer entry")
   742  						}
   743  						return true
   744  					}
   745  					prev = d
   746  					d = d.link
   747  				}
   748  				if frame.fn.deferreturn == 0 {
   749  					throw("missing deferreturn")
   750  				}
   751  
   752  				maxargsize, _ := readvarintUnsafe(fd)
   753  				d1 := newdefer(int32(maxargsize))
   754  				d1.openDefer = true
   755  				d1._panic = nil
   756  				// These are the pc/sp to set after we've
   757  				// run a defer in this frame that did a
   758  				// recover. We return to a special
   759  				// deferreturn that runs any remaining
   760  				// defers and then returns from the
   761  				// function.
   762  				d1.pc = frame.fn.entry + uintptr(frame.fn.deferreturn)
   763  				d1.varp = frame.varp
   764  				d1.fd = fd
   765  				// Save the SP/PC associated with current frame,
   766  				// so we can continue stack trace later if needed.
   767  				d1.framepc = frame.pc
   768  				d1.sp = frame.sp
   769  				d1.link = d
   770  				if prev == nil {
   771  					gp._defer = d1
   772  				} else {
   773  					prev.link = d1
   774  				}
   775  				// Stop stack scanning after adding one open defer record
   776  				return false
   777  			},
   778  			nil, 0)
   779  	})
   780  }
   781  
   782  // readvarintUnsafe reads the uint32 in varint format starting at fd, and returns the
   783  // uint32 and a pointer to the byte following the varint.
   784  //
   785  // There is a similar function runtime.readvarint, which takes a slice of bytes,
   786  // rather than an unsafe pointer. These functions are duplicated, because one of
   787  // the two use cases for the functions would get slower if the functions were
   788  // combined.
   789  func readvarintUnsafe(fd unsafe.Pointer) (uint32, unsafe.Pointer) {
   790  	var r uint32
   791  	var shift int
   792  	for {
   793  		b := *(*uint8)((unsafe.Pointer(fd)))
   794  		fd = add(fd, unsafe.Sizeof(b))
   795  		if b < 128 {
   796  			return r + uint32(b)<<shift, fd
   797  		}
   798  		r += ((uint32(b) &^ 128) << shift)
   799  		shift += 7
   800  		if shift > 28 {
   801  			panic("Bad varint")
   802  		}
   803  	}
   804  }
   805  
   806  // runOpenDeferFrame runs the active open-coded defers in the frame specified by
   807  // d. It normally processes all active defers in the frame, but stops immediately
   808  // if a defer does a successful recover. It returns true if there are no
   809  // remaining defers to run in the frame.
   810  func runOpenDeferFrame(gp *g, d *_defer) bool {
   811  	done := true
   812  	fd := d.fd
   813  
   814  	// Skip the maxargsize
   815  	_, fd = readvarintUnsafe(fd)
   816  	deferBitsOffset, fd := readvarintUnsafe(fd)
   817  	nDefers, fd := readvarintUnsafe(fd)
   818  	deferBits := *(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset)))
   819  
   820  	for i := int(nDefers) - 1; i >= 0; i-- {
   821  		// read the funcdata info for this defer
   822  		var argWidth, closureOffset, nArgs uint32
   823  		argWidth, fd = readvarintUnsafe(fd)
   824  		closureOffset, fd = readvarintUnsafe(fd)
   825  		nArgs, fd = readvarintUnsafe(fd)
   826  		if deferBits&(1<<i) == 0 {
   827  			for j := uint32(0); j < nArgs; j++ {
   828  				_, fd = readvarintUnsafe(fd)
   829  				_, fd = readvarintUnsafe(fd)
   830  				_, fd = readvarintUnsafe(fd)
   831  			}
   832  			continue
   833  		}
   834  		closure := *(**funcval)(unsafe.Pointer(d.varp - uintptr(closureOffset)))
   835  		d.fn = closure
   836  		deferArgs := deferArgs(d)
   837  		// If there is an interface receiver or method receiver, it is
   838  		// described/included as the first arg.
   839  		for j := uint32(0); j < nArgs; j++ {
   840  			var argOffset, argLen, argCallOffset uint32
   841  			argOffset, fd = readvarintUnsafe(fd)
   842  			argLen, fd = readvarintUnsafe(fd)
   843  			argCallOffset, fd = readvarintUnsafe(fd)
   844  			memmove(unsafe.Pointer(uintptr(deferArgs)+uintptr(argCallOffset)),
   845  				unsafe.Pointer(d.varp-uintptr(argOffset)),
   846  				uintptr(argLen))
   847  		}
   848  		deferBits = deferBits &^ (1 << i)
   849  		*(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset))) = deferBits
   850  		p := d._panic
   851  		reflectcallSave(p, unsafe.Pointer(closure), deferArgs, argWidth)
   852  		if p != nil && p.aborted {
   853  			break
   854  		}
   855  		d.fn = nil
   856  		// These args are just a copy, so can be cleared immediately
   857  		memclrNoHeapPointers(deferArgs, uintptr(argWidth))
   858  		if d._panic != nil && d._panic.recovered {
   859  			done = deferBits == 0
   860  			break
   861  		}
   862  	}
   863  
   864  	return done
   865  }
   866  
   867  // reflectcallSave calls reflectcall after saving the caller's pc and sp in the
   868  // panic record. This allows the runtime to return to the Goexit defer processing
   869  // loop, in the unusual case where the Goexit may be bypassed by a successful
   870  // recover.
   871  func reflectcallSave(p *_panic, fn, arg unsafe.Pointer, argsize uint32) {
   872  	if p != nil {
   873  		p.argp = unsafe.Pointer(getargp(0))
   874  		p.pc = getcallerpc()
   875  		p.sp = unsafe.Pointer(getcallersp())
   876  	}
   877  	reflectcall(nil, fn, arg, argsize, argsize)
   878  	if p != nil {
   879  		p.pc = 0
   880  		p.sp = unsafe.Pointer(nil)
   881  	}
   882  }
   883  
   884  // The implementation of the predeclared function panic.
   885  func gopanic(e interface{}) {
   886  	gp := getg()
   887  	if gp.m.curg != gp {
   888  		print("panic: ")
   889  		printany(e)
   890  		print("\n")
   891  		throw("panic on system stack")
   892  	}
   893  
   894  	if gp.m.mallocing != 0 {
   895  		print("panic: ")
   896  		printany(e)
   897  		print("\n")
   898  		throw("panic during malloc")
   899  	}
   900  	if gp.m.preemptoff != "" {
   901  		print("panic: ")
   902  		printany(e)
   903  		print("\n")
   904  		print("preempt off reason: ")
   905  		print(gp.m.preemptoff)
   906  		print("\n")
   907  		throw("panic during preemptoff")
   908  	}
   909  	if gp.m.locks != 0 {
   910  		print("panic: ")
   911  		printany(e)
   912  		print("\n")
   913  		throw("panic holding locks")
   914  	}
   915  
   916  	var p _panic
   917  	p.arg = e
   918  	p.link = gp._panic
   919  	gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
   920  
   921  	atomic.Xadd(&runningPanicDefers, 1)
   922  
   923  	// By calculating getcallerpc/getcallersp here, we avoid scanning the
   924  	// gopanic frame (stack scanning is slow...)
   925  	addOneOpenDeferFrame(gp, getcallerpc(), unsafe.Pointer(getcallersp()))
   926  
   927  	for {
   928  		d := gp._defer
   929  		if d == nil {
   930  			break
   931  		}
   932  
   933  		// If defer was started by earlier panic or Goexit (and, since we're back here, that triggered a new panic),
   934  		// take defer off list. An earlier panic will not continue running, but we will make sure below that an
   935  		// earlier Goexit does continue running.
   936  		if d.started {
   937  			if d._panic != nil {
   938  				d._panic.aborted = true
   939  			}
   940  			d._panic = nil
   941  			if !d.openDefer {
   942  				// For open-coded defers, we need to process the
   943  				// defer again, in case there are any other defers
   944  				// to call in the frame (not including the defer
   945  				// call that caused the panic).
   946  				d.fn = nil
   947  				gp._defer = d.link
   948  				freedefer(d)
   949  				continue
   950  			}
   951  		}
   952  
   953  		// Mark defer as started, but keep on list, so that traceback
   954  		// can find and update the defer's argument frame if stack growth
   955  		// or a garbage collection happens before reflectcall starts executing d.fn.
   956  		d.started = true
   957  
   958  		// Record the panic that is running the defer.
   959  		// If there is a new panic during the deferred call, that panic
   960  		// will find d in the list and will mark d._panic (this panic) aborted.
   961  		d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
   962  
   963  		done := true
   964  		if d.openDefer {
   965  			done = runOpenDeferFrame(gp, d)
   966  			if done && !d._panic.recovered {
   967  				addOneOpenDeferFrame(gp, 0, nil)
   968  			}
   969  		} else {
   970  			p.argp = unsafe.Pointer(getargp(0))
   971  			reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
   972  		}
   973  		p.argp = nil
   974  
   975  		// reflectcall did not panic. Remove d.
   976  		if gp._defer != d {
   977  			throw("bad defer entry in panic")
   978  		}
   979  		d._panic = nil
   980  
   981  		// trigger shrinkage to test stack copy. See stack_test.go:TestStackPanic
   982  		//GC()
   983  
   984  		pc := d.pc
   985  		sp := unsafe.Pointer(d.sp) // must be pointer so it gets adjusted during stack copy
   986  		if done {
   987  			d.fn = nil
   988  			gp._defer = d.link
   989  			freedefer(d)
   990  		}
   991  		if p.recovered {
   992  			gp._panic = p.link
   993  			if gp._panic != nil && gp._panic.goexit && gp._panic.aborted {
   994  				// A normal recover would bypass/abort the Goexit.  Instead,
   995  				// we return to the processing loop of the Goexit.
   996  				gp.sigcode0 = uintptr(gp._panic.sp)
   997  				gp.sigcode1 = uintptr(gp._panic.pc)
   998  				mcall(recovery)
   999  				throw("bypassed recovery failed") // mcall should not return
  1000  			}
  1001  			atomic.Xadd(&runningPanicDefers, -1)
  1002  
  1003  			// Remove any remaining non-started, open-coded
  1004  			// defer entries after a recover, since the
  1005  			// corresponding defers will be executed normally
  1006  			// (inline). Any such entry will become stale once
  1007  			// we run the corresponding defers inline and exit
  1008  			// the associated stack frame.
  1009  			d := gp._defer
  1010  			var prev *_defer
  1011  			if !done {
  1012  				// Skip our current frame, if not done. It is
  1013  				// needed to complete any remaining defers in
  1014  				// deferreturn()
  1015  				prev = d
  1016  				d = d.link
  1017  			}
  1018  			for d != nil {
  1019  				if d.started {
  1020  					// This defer is started but we
  1021  					// are in the middle of a
  1022  					// defer-panic-recover inside of
  1023  					// it, so don't remove it or any
  1024  					// further defer entries
  1025  					break
  1026  				}
  1027  				if d.openDefer {
  1028  					if prev == nil {
  1029  						gp._defer = d.link
  1030  					} else {
  1031  						prev.link = d.link
  1032  					}
  1033  					newd := d.link
  1034  					freedefer(d)
  1035  					d = newd
  1036  				} else {
  1037  					prev = d
  1038  					d = d.link
  1039  				}
  1040  			}
  1041  
  1042  			gp._panic = p.link
  1043  			// Aborted panics are marked but remain on the g.panic list.
  1044  			// Remove them from the list.
  1045  			for gp._panic != nil && gp._panic.aborted {
  1046  				gp._panic = gp._panic.link
  1047  			}
  1048  			if gp._panic == nil { // must be done with signal
  1049  				gp.sig = 0
  1050  			}
  1051  			// Pass information about recovering frame to recovery.
  1052  			gp.sigcode0 = uintptr(sp)
  1053  			gp.sigcode1 = pc
  1054  			mcall(recovery)
  1055  			throw("recovery failed") // mcall should not return
  1056  		}
  1057  	}
  1058  
  1059  	// ran out of deferred calls - old-school panic now
  1060  	// Because it is unsafe to call arbitrary user code after freezing
  1061  	// the world, we call preprintpanics to invoke all necessary Error
  1062  	// and String methods to prepare the panic strings before startpanic.
  1063  	preprintpanics(gp._panic)
  1064  
  1065  	fatalpanic(gp._panic) // should not return
  1066  	*(*int)(nil) = 0      // not reached
  1067  }
  1068  
  1069  // getargp returns the location where the caller
  1070  // writes outgoing function call arguments.
  1071  //go:nosplit
  1072  //go:noinline
  1073  func getargp(x int) uintptr {
  1074  	// x is an argument mainly so that we can return its address.
  1075  	return uintptr(noescape(unsafe.Pointer(&x)))
  1076  }
  1077  
  1078  // The implementation of the predeclared function recover.
  1079  // Cannot split the stack because it needs to reliably
  1080  // find the stack segment of its caller.
  1081  //
  1082  // TODO(rsc): Once we commit to CopyStackAlways,
  1083  // this doesn't need to be nosplit.
  1084  //go:nosplit
  1085  func gorecover(argp uintptr) interface{} {
  1086  	// Must be in a function running as part of a deferred call during the panic.
  1087  	// Must be called from the topmost function of the call
  1088  	// (the function used in the defer statement).
  1089  	// p.argp is the argument pointer of that topmost deferred function call.
  1090  	// Compare against argp reported by caller.
  1091  	// If they match, the caller is the one who can recover.
  1092  	gp := getg()
  1093  	p := gp._panic
  1094  	if p != nil && !p.goexit && !p.recovered && argp == uintptr(p.argp) {
  1095  		p.recovered = true
  1096  		return p.arg
  1097  	}
  1098  	return nil
  1099  }
  1100  
  1101  //go:linkname sync_throw sync.throw
  1102  func sync_throw(s string) {
  1103  	throw(s)
  1104  }
  1105  
  1106  //go:nosplit
  1107  func throw(s string) {
  1108  	// Everything throw does should be recursively nosplit so it
  1109  	// can be called even when it's unsafe to grow the stack.
  1110  	systemstack(func() {
  1111  		print("fatal error: ", s, "\n")
  1112  	})
  1113  	gp := getg()
  1114  	if gp.m.throwing == 0 {
  1115  		gp.m.throwing = 1
  1116  	}
  1117  	fatalthrow()
  1118  	*(*int)(nil) = 0 // not reached
  1119  }
  1120  
  1121  // runningPanicDefers is non-zero while running deferred functions for panic.
  1122  // runningPanicDefers is incremented and decremented atomically.
  1123  // This is used to try hard to get a panic stack trace out when exiting.
  1124  var runningPanicDefers uint32
  1125  
  1126  // panicking is non-zero when crashing the program for an unrecovered panic.
  1127  // panicking is incremented and decremented atomically.
  1128  var panicking uint32
  1129  
  1130  // paniclk is held while printing the panic information and stack trace,
  1131  // so that two concurrent panics don't overlap their output.
  1132  var paniclk mutex
  1133  
  1134  // Unwind the stack after a deferred function calls recover
  1135  // after a panic. Then arrange to continue running as though
  1136  // the caller of the deferred function returned normally.
  1137  func recovery(gp *g) {
  1138  	// Info about defer passed in G struct.
  1139  	sp := gp.sigcode0
  1140  	pc := gp.sigcode1
  1141  
  1142  	// d's arguments need to be in the stack.
  1143  	if sp != 0 && (sp < gp.stack.lo || gp.stack.hi < sp) {
  1144  		print("recover: ", hex(sp), " not in [", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n")
  1145  		throw("bad recovery")
  1146  	}
  1147  
  1148  	// Make the deferproc for this d return again,
  1149  	// this time returning 1. The calling function will
  1150  	// jump to the standard return epilogue.
  1151  	gp.sched.sp = sp
  1152  	gp.sched.pc = pc
  1153  	gp.sched.lr = 0
  1154  	gp.sched.ret = 1
  1155  	gogo(&gp.sched)
  1156  }
  1157  
  1158  // fatalthrow implements an unrecoverable runtime throw. It freezes the
  1159  // system, prints stack traces starting from its caller, and terminates the
  1160  // process.
  1161  //
  1162  //go:nosplit
  1163  func fatalthrow() {
  1164  	pc := getcallerpc()
  1165  	sp := getcallersp()
  1166  	gp := getg()
  1167  	// Switch to the system stack to avoid any stack growth, which
  1168  	// may make things worse if the runtime is in a bad state.
  1169  	systemstack(func() {
  1170  		startpanic_m()
  1171  
  1172  		if dopanic_m(gp, pc, sp) {
  1173  			// crash uses a decent amount of nosplit stack and we're already
  1174  			// low on stack in throw, so crash on the system stack (unlike
  1175  			// fatalpanic).
  1176  			crash()
  1177  		}
  1178  
  1179  		exit(2)
  1180  	})
  1181  
  1182  	*(*int)(nil) = 0 // not reached
  1183  }
  1184  
  1185  // fatalpanic implements an unrecoverable panic. It is like fatalthrow, except
  1186  // that if msgs != nil, fatalpanic also prints panic messages and decrements
  1187  // runningPanicDefers once main is blocked from exiting.
  1188  //
  1189  //go:nosplit
  1190  func fatalpanic(msgs *_panic) {
  1191  	pc := getcallerpc()
  1192  	sp := getcallersp()
  1193  	gp := getg()
  1194  	var docrash bool
  1195  	// Switch to the system stack to avoid any stack growth, which
  1196  	// may make things worse if the runtime is in a bad state.
  1197  	systemstack(func() {
  1198  		if startpanic_m() && msgs != nil {
  1199  			// There were panic messages and startpanic_m
  1200  			// says it's okay to try to print them.
  1201  
  1202  			// startpanic_m set panicking, which will
  1203  			// block main from exiting, so now OK to
  1204  			// decrement runningPanicDefers.
  1205  			atomic.Xadd(&runningPanicDefers, -1)
  1206  
  1207  			printpanics(msgs)
  1208  		}
  1209  
  1210  		docrash = dopanic_m(gp, pc, sp)
  1211  	})
  1212  
  1213  	if docrash {
  1214  		// By crashing outside the above systemstack call, debuggers
  1215  		// will not be confused when generating a backtrace.
  1216  		// Function crash is marked nosplit to avoid stack growth.
  1217  		crash()
  1218  	}
  1219  
  1220  	systemstack(func() {
  1221  		exit(2)
  1222  	})
  1223  
  1224  	*(*int)(nil) = 0 // not reached
  1225  }
  1226  
  1227  // startpanic_m prepares for an unrecoverable panic.
  1228  //
  1229  // It returns true if panic messages should be printed, or false if
  1230  // the runtime is in bad shape and should just print stacks.
  1231  //
  1232  // It must not have write barriers even though the write barrier
  1233  // explicitly ignores writes once dying > 0. Write barriers still
  1234  // assume that g.m.p != nil, and this function may not have P
  1235  // in some contexts (e.g. a panic in a signal handler for a signal
  1236  // sent to an M with no P).
  1237  //
  1238  //go:nowritebarrierrec
  1239  func startpanic_m() bool {
  1240  	_g_ := getg()
  1241  	if mheap_.cachealloc.size == 0 { // very early
  1242  		print("runtime: panic before malloc heap initialized\n")
  1243  	}
  1244  	// Disallow malloc during an unrecoverable panic. A panic
  1245  	// could happen in a signal handler, or in a throw, or inside
  1246  	// malloc itself. We want to catch if an allocation ever does
  1247  	// happen (even if we're not in one of these situations).
  1248  	_g_.m.mallocing++
  1249  
  1250  	// If we're dying because of a bad lock count, set it to a
  1251  	// good lock count so we don't recursively panic below.
  1252  	if _g_.m.locks < 0 {
  1253  		_g_.m.locks = 1
  1254  	}
  1255  
  1256  	switch _g_.m.dying {
  1257  	case 0:
  1258  		// Setting dying >0 has the side-effect of disabling this G's writebuf.
  1259  		_g_.m.dying = 1
  1260  		atomic.Xadd(&panicking, 1)
  1261  		lock(&paniclk)
  1262  		if debug.schedtrace > 0 || debug.scheddetail > 0 {
  1263  			schedtrace(true)
  1264  		}
  1265  		freezetheworld()
  1266  		return true
  1267  	case 1:
  1268  		// Something failed while panicking.
  1269  		// Just print a stack trace and exit.
  1270  		_g_.m.dying = 2
  1271  		print("panic during panic\n")
  1272  		return false
  1273  	case 2:
  1274  		// This is a genuine bug in the runtime, we couldn't even
  1275  		// print the stack trace successfully.
  1276  		_g_.m.dying = 3
  1277  		print("stack trace unavailable\n")
  1278  		exit(4)
  1279  		fallthrough
  1280  	default:
  1281  		// Can't even print! Just exit.
  1282  		exit(5)
  1283  		return false // Need to return something.
  1284  	}
  1285  }
  1286  
  1287  var didothers bool
  1288  var deadlock mutex
  1289  
  1290  func dopanic_m(gp *g, pc, sp uintptr) bool {
  1291  	if gp.sig != 0 {
  1292  		signame := signame(gp.sig)
  1293  		if signame != "" {
  1294  			print("[signal ", signame)
  1295  		} else {
  1296  			print("[signal ", hex(gp.sig))
  1297  		}
  1298  		print(" code=", hex(gp.sigcode0), " addr=", hex(gp.sigcode1), " pc=", hex(gp.sigpc), "]\n")
  1299  	}
  1300  
  1301  	level, all, docrash := gotraceback()
  1302  	_g_ := getg()
  1303  	if level > 0 {
  1304  		if gp != gp.m.curg {
  1305  			all = true
  1306  		}
  1307  		if gp != gp.m.g0 {
  1308  			print("\n")
  1309  			goroutineheader(gp)
  1310  			traceback(pc, sp, 0, gp)
  1311  		} else if level >= 2 || _g_.m.throwing > 0 {
  1312  			print("\nruntime stack:\n")
  1313  			traceback(pc, sp, 0, gp)
  1314  		}
  1315  		if !didothers && all {
  1316  			didothers = true
  1317  			tracebackothers(gp)
  1318  		}
  1319  	}
  1320  	unlock(&paniclk)
  1321  
  1322  	if atomic.Xadd(&panicking, -1) != 0 {
  1323  		// Some other m is panicking too.
  1324  		// Let it print what it needs to print.
  1325  		// Wait forever without chewing up cpu.
  1326  		// It will exit when it's done.
  1327  		lock(&deadlock)
  1328  		lock(&deadlock)
  1329  	}
  1330  
  1331  	printDebugLog()
  1332  
  1333  	return docrash
  1334  }
  1335  
  1336  // canpanic returns false if a signal should throw instead of
  1337  // panicking.
  1338  //
  1339  //go:nosplit
  1340  func canpanic(gp *g) bool {
  1341  	// Note that g is m->gsignal, different from gp.
  1342  	// Note also that g->m can change at preemption, so m can go stale
  1343  	// if this function ever makes a function call.
  1344  	_g_ := getg()
  1345  	_m_ := _g_.m
  1346  
  1347  	// Is it okay for gp to panic instead of crashing the program?
  1348  	// Yes, as long as it is running Go code, not runtime code,
  1349  	// and not stuck in a system call.
  1350  	if gp == nil || gp != _m_.curg {
  1351  		return false
  1352  	}
  1353  	if _m_.locks != 0 || _m_.mallocing != 0 || _m_.throwing != 0 || _m_.preemptoff != "" || _m_.dying != 0 {
  1354  		return false
  1355  	}
  1356  	status := readgstatus(gp)
  1357  	if status&^_Gscan != _Grunning || gp.syscallsp != 0 {
  1358  		return false
  1359  	}
  1360  	if GOOS == "windows" && _m_.libcallsp != 0 {
  1361  		return false
  1362  	}
  1363  	return true
  1364  }
  1365  
  1366  // shouldPushSigpanic reports whether pc should be used as sigpanic's
  1367  // return PC (pushing a frame for the call). Otherwise, it should be
  1368  // left alone so that LR is used as sigpanic's return PC, effectively
  1369  // replacing the top-most frame with sigpanic. This is used by
  1370  // preparePanic.
  1371  func shouldPushSigpanic(gp *g, pc, lr uintptr) bool {
  1372  	if pc == 0 {
  1373  		// Probably a call to a nil func. The old LR is more
  1374  		// useful in the stack trace. Not pushing the frame
  1375  		// will make the trace look like a call to sigpanic
  1376  		// instead. (Otherwise the trace will end at sigpanic
  1377  		// and we won't get to see who faulted.)
  1378  		return false
  1379  	}
  1380  	// If we don't recognize the PC as code, but we do recognize
  1381  	// the link register as code, then this assumes the panic was
  1382  	// caused by a call to non-code. In this case, we want to
  1383  	// ignore this call to make unwinding show the context.
  1384  	//
  1385  	// If we running C code, we're not going to recognize pc as a
  1386  	// Go function, so just assume it's good. Otherwise, traceback
  1387  	// may try to read a stale LR that looks like a Go code
  1388  	// pointer and wander into the woods.
  1389  	if gp.m.incgo || findfunc(pc).valid() {
  1390  		// This wasn't a bad call, so use PC as sigpanic's
  1391  		// return PC.
  1392  		return true
  1393  	}
  1394  	if findfunc(lr).valid() {
  1395  		// This was a bad call, but the LR is good, so use the
  1396  		// LR as sigpanic's return PC.
  1397  		return false
  1398  	}
  1399  	// Neither the PC or LR is good. Hopefully pushing a frame
  1400  	// will work.
  1401  	return true
  1402  }
  1403  
  1404  // isAbortPC reports whether pc is the program counter at which
  1405  // runtime.abort raises a signal.
  1406  //
  1407  // It is nosplit because it's part of the isgoexception
  1408  // implementation.
  1409  //
  1410  //go:nosplit
  1411  func isAbortPC(pc uintptr) bool {
  1412  	return pc == funcPC(abort) || ((GOARCH == "arm" || GOARCH == "arm64") && pc == funcPC(abort)+sys.PCQuantum)
  1413  }
  1414  

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