runtime: ignore SPWRITE in syscall functions

netbsd/amd64's Syscall9 changes SP using ADD and SUB,
which are treated as SPWRITEs (they are not accounted for
in the sp-adjust tracking, and there are too many functions that
would report mismatched stack adjustments at RET if they were).
A traceback starting in Syscall9 as saved by entersyscall complains
about the SPWRITE-ness unnecessarily, since the PC/SP are saved
at the start of the function. Ignore SPWRITE in that case.

netbsd/arm's Syscall6 also changes SP (R13), using a direct write.
So even if we could handle the ADD/SUB in the amd64 case or
rewrote that assembly, we'd still be stuck with a more difficult
problem in this case. Ignoring the SPWRITE fixes it.

Example crashes:
https://build.golang.org/log/160fc7b051a2cf90782b75a99984fff129329e66
https://build.golang.org/log/7879e2fecdb400eee616294285e1f952e5b17301

Change-Id: I0c8e9696066e90dafed6d4a93d11697da23f0080
Reviewed-on: https://go-review.googlesource.com/c/go/+/294072
Reviewed-by: Cherry Zhang <[email protected]>
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>

src/runtime/traceback.go

@@ -174,6 +174,12 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 			// So we don't need to exclude it with the other SP-writing functions.
 			flag &^= funcFlag_SPWRITE
 		}
+		if frame.pc == pc0 && frame.sp == sp0 && pc0 == gp.syscallpc && sp0 == gp.syscallsp {
+			// Some Syscall functions write to SP, but they do so only after
+			// saving the entry PC/SP using entersyscall.
+			// Since we are using the entry PC/SP, the later SP write doesn't matter.
+			flag &^= funcFlag_SPWRITE
+		}
 
 		// Found an actual function.
 		// Derive frame pointer and link register.