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x/sys/windows: clarify safety of Proc.Call, LazyProc.Call #42680

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bradfitz opened this issue Nov 17, 2020 · 5 comments
Open

x/sys/windows: clarify safety of Proc.Call, LazyProc.Call #42680

bradfitz opened this issue Nov 17, 2020 · 5 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. Documentation NeedsFix The path to resolution is known, but the work has not been done.
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Unreleased

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@bradfitz
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https://godoc.org/golang.org/x/sys/windows#LazyProc.Call and https://godoc.org/golang.org/x/sys/windows#Proc.Call are syscall.SysCall wrappers but take a bunch of uintptrs.

I assume that those wrappers are not covered by the uintptr(unsafe.Pointer) conversion exception number (4) from https://golang.org/pkg/unsafe/#Pointer, correct?

Could we document that either way in x/sys/windows?

I assume that many callers will require runtime.KeepAlive?

/cc @ianlancetaylor @mdempsky @alexbrainman @josharian @crawshaw
@gopherbot gopherbot added this to the Unreleased milestone Nov 17, 2020
@mdempsky
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I think this is Q1 of #34684.

@mdempsky
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Also, runtime.KeepAlive would only guarantee that the actual pointer is still live. It doesn't guarantee that uintptr conversion stays valid. Calling Proc.Call might require growing the stack, which could invalidate a pointer to stack memory.

@bradfitz bradfitz mentioned this issue Nov 18, 2020
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@bradfitz
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bradfitz commented Nov 18, 2020 •
edited

Yeah, this could be closed as a dup of #34684 I suppose.

But I'm not exactly sure the status of that bug. It sure sounds like {Lazy,}Proc.Call just aren't safe to use today, at least for arguments involving pointers.

Should we clarify that in the docs for now while we wait for something to happen with #34684?

@mdempsky
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Direct calls to {Lazy,}Proc.Call should be fine, at least since b3bd7ab (first included in Go 1.14).

Either way, I'm happy to have the docs clarified.

@bradfitz
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Ah! I had just missed the //go:uintptrescapes lines in that file. Okay, that makes me feel much better.

I'll send some docs, thanks!

@bradfitz bradfitz self-assigned this Nov 20, 2020
@petemoore petemoore mentioned this issue Jan 28, 2022
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petemoore added a commit to taskcluster/taskcluster that referenced this issue Feb 3, 2022
@petemoore
generic-worker no longer relies on the undocumented go pragma //go:ui…
ee51f2d 
…ntptrescapes

The unofficial documentation for this pragma can be found in the source code:
  * https://github.com/golang/go/blob/go1.17.6/src/cmd/compile/internal/noder/lex.go#L71-L81

Instead of relying on this pragma, generic-worker now calls
syscall.Syscall<x> directly. This change has been made since I suspect
that nested go:uintptrescapes functions may not work as expected, but
furthmore, since this pragma is not an official feature, it presumably
may change over time, so we probably should not assume it will always be
supported.

There have also been several issues raised against functions and methods
that rely on its behaviour, so it is not clear whether it is entirely
safe to use:

  * golang/go#16035
  * golang/go#23045
  * golang/go#34474
  * golang/go#34642
  * golang/go#34684
  * golang/go#34810
  * golang/go#42680

Note, in future an option might be to generate the function bodies using mkwinsyscall:
  * https://github.com/golang/go/blob/go1.17.6/src/internal/syscall/windows/mksyscall.go#L9
  * https://github.com/golang/go/blob/go1.17.6/src/syscall/mksyscall_windows.go#L47
  * https://pkg.go.dev/golang.org/x/sys/windows/mkwinsyscall
  * https://github.com/golang/sys/blob/master/windows/mkwinsyscall/mkwinsyscall.go

One advantage of this approach is that we can probably log the syscalls
again using:

  * https://github.com/golang/sys/blob/50617c2ba19781ae46f34bb505064996b8fa32e8/windows/mkwinsyscall/mkwinsyscall.go#L43-L44

Alternatively if tracing doesn't do what we expect, we could write our own generator.
@cagedmantis cagedmantis added the NeedsFix The path to resolution is known, but the work has not been done. label Jun 23, 2022
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jul 7, 2022
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compiler/runtime Issues related to the Go compiler and/or runtime. Documentation NeedsFix The path to resolution is known, but the work has not been done.
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