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3288bc1ea1
Our calling convention requires BP to be preserved (it is callee-save). But that only happened with the introduction of framepointers in 1.5. Furthermore, nothing checks it, so there is assembly from before and since which violates the calling convention. The frame pointer is not used by Go, but only by kernel traceback code (for sampled profiling), so no one noticed. Also, the frame pointer tends to "fix itself", since after it is clobbered by an assembly function f, the calling function fixes the frame pointer up when that function returns. I've fixed the stdlib, CLs 248260, 248261, 248262. This CL is a simple check, intended to be used in vet, to catch assembly that violates the calling convention by overwriting the frame pointer without saving/restoring it. It is not intended to catch all violations, just ones that are easy to find. We look for a write to the frame pointer before any use of the frame pointer or any control flow instruction. In such a situation, there's no way the code could restore the value of BP before returning. This analysis is very conservative; it gives up in lots of cases. False positive should be very rare, though. Possible false positives: // looks like a write to BP, but isn't. CMPQ BP, BP // BP actually isn't clobbered, just swapped with AX. XORQ AX, BP XORQ BP, AX XORQ AX, BP The first is unlikely, as it is using the contents of an incoming register, which is junk. The second is a general problem of op= assembly operations that are indistiguishable in syntax from = operations. But anything other than the swap above also depends on the incoming junk value in BP. The swap itself is pointless (XCHQ works better). Change-Id: Ie9d91ab3396409486f7022380ad46ac76c3fbed4 Reviewed-on: https://go-review.googlesource.com/c/tools/+/248686 Trust: Keith Randall <khr@golang.org> Trust: Emmanuel Odeke <emmanuel@orijtech.com> Run-TryBot: Keith Randall <khr@golang.org> gopls-CI: kokoro <noreply+kokoro@google.com> TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Michael Matloob <matloob@golang.org> |
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