cmd/compile: minimally align variables in memory when -d=checkptr is used #35128

mdempsky · 2019-10-24T06:53:31Z

-d=checkptr should report a warning for:

var x [4]byte
p := (*uint32)(unsafe.Pointer(&x[0]))

In general, this is unsafe because x only requires 1-byte alignment, but *uint32 requires 4-byte alignment.

Currently, this isn't reliably reported, because efficiently allocating variables often results in over alignment. But when -d=checkptr is in use, cmd/compile and the runtime could burn a few bytes of memory to intentionally minimally align things in the heap and on the stack (e.g., 1-byte-alignment variables always on an odd address).

Relatedly, when the runtime allocates a chunk of memory for a heap variable allocation, it could align the variable as close to the end of the allocation (rather than at the beginning), so that we can more easily detect pointer arithmetic overflows (at the expense of pointer arithmetic underflows, which seem less common).

These both would have helped catch issues that I've noticed in CLs to address errors reported by -d=checkptr.

(Prior art: I think OpenBSD's malloc used to prefer aligning allocations towards the end of an mmap'ed page so that buffer overflows were more likely to trigger a segmentation fault. Not sure if it still does that. I don't see it mentioned specifically in the man page.)

The text was updated successfully, but these errors were encountered:

mdempsky · 2019-10-24T07:04:25Z

Hrm:

go/src/runtime/malloc.go

Lines 889 to 904 in 68981bf

    
           // TODO(austin): This should be just 
        
           //   align = uintptr(typ.align) 
        
           // but that's only 4 on 32-bit platforms, 
        
           // even if there's a uint64 field in typ (see #599). 
        
           // This causes 64-bit atomic accesses to panic. 
        
           // Hence, we use stricter alignment that matches 
        
           // the normal allocator better. 
        
           if size&7 == 0 { 
        
           	align = 8 
        
           } else if size&3 == 0 { 
        
           	align = 4 
        
           } else if size&1 == 0 { 
        
           	align = 2 
        
           } else { 
        
           	align = 1 
        
           }

mdempsky · 2019-11-12T23:31:47Z

Bumping to Backlog, since I don't think this is going to happen for Go 1.14.

gopherbot · 2020-03-11T13:33:08Z

Change https://golang.org/cl/222855 mentions this issue: sha3: mark xorInUnaligned with go:nocheckptr

It is unclear whether unaligned reads should be allowed, or if they are even actually a good idea here. However, while we figure that out, we should un-break 'go test -race' for users of this package. Updates golang/go#37644 Updates golang/go#37298 Updates golang/go#37715 Updates golang/go#34972 Updates golang/go#35128 Change-Id: I088f5703023e4f05ee274a6753e925973f12ac1b Reviewed-on: https://go-review.googlesource.com/c/crypto/+/222855 Run-TryBot: Bryan C. Mills <bcmills@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>

mdempsky added help wanted NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Oct 24, 2019

mdempsky added this to the Go1.14 milestone Oct 24, 2019

mdempsky modified the milestones: Go1.14, Backlog Nov 12, 2019

cuonglm mentioned this issue Jul 1, 2020

cmd/compile,runtime: checkptr false negatives for tiny-alloced objects #38872

Closed

bcmills mentioned this issue May 11, 2021

cmd/vet: warn about concurrent modification of a map (values added in a for loop) #46097

Open

gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jul 13, 2022

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cmd/compile: minimally align variables in memory when -d=checkptr is used #35128

cmd/compile: minimally align variables in memory when -d=checkptr is used #35128

mdempsky commented Oct 24, 2019

mdempsky commented Oct 24, 2019

mdempsky commented Nov 12, 2019

gopherbot commented Mar 11, 2020

cmd/compile: minimally align variables in memory when -d=checkptr is used #35128

cmd/compile: minimally align variables in memory when -d=checkptr is used #35128

Comments

mdempsky commented Oct 24, 2019

mdempsky commented Oct 24, 2019

mdempsky commented Nov 12, 2019

gopherbot commented Mar 11, 2020