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When we create a thread with signals blocked. But glibc's pthread_sigmask doesn't really allow us to block SIGSETXID. So we may get a signal early on before the signal stack is set. If we get a signal on the current stack, it will clobber anything below the SP. This CL makes it to save LR and decrement SP in a single MOVD.W instruction for small frames, so we don't write below the SP. We used to use a single MOVD.W instruction before CL 379075. CL 379075 changed to use an STP instruction to save the LR and FP, then decrementing the SP. This CL changes it back, just this part (epilogues and large frame prologues are unchanged). For small frames, it is the same number of instructions either way. This decreases the size of a "small" frame from 0x1f0 to 0xf0. For frame sizes in between, it could benefit from using an STP instruction instead of using the prologue for the "large" frame case. We don't bother it for now as this is a stop-gap solution anyway. This only addresses the issue with small frames. Luckily, all functions from thread entry to setting up the signal stack have samll frames. Other possible ideas: - Expand the unwind info metadata, separate SP delta and the location of the return address, so we can express "SP is decremented but the return address is in the LR register". Then we can always create the frame first then write the LR, without writing anything below the SP (except the frame pointer at SP-8, which is minor because it doesn't really affect program execution). - Set up the signal stack immediately in mstart in assembly. For Go 1.19 we do this simple fix. We plan to do the metadata fix in Go 1.20 ( #53609 ). Other LR architectures are addressed in CL 413428. Fix #53374. Change-Id: I9d6582ab14ccb06ac61ad43852943d9555e22ae5 Reviewed-on: https://go-review.googlesource.com/c/go/+/412474 Run-TryBot: Cherry Mui <cherryyz@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: Eric Fang <eric.fang@arm.com> |
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