runtime: clear frame pointer in mcall

On amd64, mcall leaves BP untouched, so the callback will push BP,
connecting the g0 stack to the calling g stack. This seems OK (frame
pointer unwinders like Linux perf can see what user code called into the
scheduler), but the "scheduler" part is problematic.

mcall is used when calling into the scheduler to deschedule the current
goroutine (e.g., in goyield). Once the goroutine is descheduled, it may
be picked up by another M and continue execution. The other thread is
mutating the goroutine stack, but our M still has a frame pointer
pointing to the goroutine stack.

A frame pointer unwinder like Linux perf could get bogus values off of
the mutating stack. Note that though the execution tracer uses
framepointer unwinding, it never unwinds a g0, so it isn't affected.

Clear the frame pointer in mcall so that unwinding always stops at
mcall.

On arm64, mcall stores the frame pointer from g0.sched.bp. This doesn't
really make any sense. mcall wasn't called by whatever used g0 last, so
at best unwinding will get misleading results (e.g., it might look like
cgocallback calls mcall?).

Also clear the frame pointer on arm64.

Other architectures don't use frame pointers.

For #63630.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-linux-arm64-longtest
Change-Id: I6a6a636cb6404f3c95ecabdb969c9b8184615cee
Reviewed-on: https://go-review.googlesource.com/c/go/+/669615
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Nick Ripley <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: prattmic

src/runtime/asm_amd64.s

@@ -452,6 +452,7 @@ goodm:
 	get_tls(CX)		// Set G in TLS
 	MOVQ	R14, g(CX)
 	MOVQ	(g_sched+gobuf_sp)(R14), SP	// sp = g0.sched.sp
+	MOVQ	$0, BP	// clear frame pointer, as caller may execute on another M
 	PUSHQ	AX	// open up space for fn's arg spill slot
 	MOVQ	0(DX), R12
 	CALL	R12		// fn(g)

src/runtime/asm_arm64.s

@@ -233,7 +233,7 @@ TEXT runtime·mcall<ABIInternal>(SB), NOSPLIT|NOFRAME, $0-8
 
 	MOVD	(g_sched+gobuf_sp)(g), R0
 	MOVD	R0, RSP	// sp = m->g0->sched.sp
-	MOVD	(g_sched+gobuf_bp)(g), R29
+	MOVD	$0, R29				// clear frame pointer, as caller may execute on another M
 	MOVD	R3, R0				// arg = g
 	MOVD	$0, -16(RSP)			// dummy LR
 	SUB	$16, RSP