runtime: track frame pointer while in syscall

Currently the runtime only tracks the PC and SP upon entering a syscall,
but not the FP (BP). This is mainly for historical reasons, and because
the tracer (which uses the frame pointer unwinder) does not need it.

Until it did, of course, in CL 567076, where the tracer tries to take a
stack trace of a goroutine that's in a syscall from afar. It tries to
use gp.sched.bp and lots of things go wrong. It *really* should be using
the equivalent of gp.syscallbp, which doesn't exist before this CL.

This change introduces gp.syscallbp and tracks it. It also introduces
getcallerfp which is nice for simplifying some code. Because we now have
gp.syscallbp, we can also delete the frame skip count computation in
traceLocker.GoSysCall, because it's now the same regardless of whether
frame pointer unwinding is used.

Fixes #66889.

Change-Id: Ib6d761c9566055e0a037134138cb0f81be73ecf7
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-nocgo
Reviewed-on: https://go-review.googlesource.com/c/go/+/580255
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 83252abb44..b58255f279 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -437,6 +437,7 @@ type g struct {
 	sched     gobuf
 	syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
 	syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
+	syscallbp uintptr // if status==Gsyscall, syscallbp = sched.bp to use in fpTraceback
 	stktopsp  uintptr // expected sp at top of stack, to check in traceback
 	// param is a generic pointer parameter field used to pass
 	// values in particular contexts where other storage for the
@@ -1263,3 +1264,17 @@ var (
 
 // Must agree with internal/buildcfg.FramePointerEnabled.
 const framepointer_enabled = GOARCH == "amd64" || GOARCH == "arm64"
+
+// getcallerfp returns the frame pointer of the caller of the caller
+// of this function.
+//
+//go:nosplit
+//go:noinline
+func getcallerfp() uintptr {
+	fp := getfp() // This frame's FP.
+	if fp != 0 {
+		fp = *(*uintptr)(unsafe.Pointer(fp)) // The caller's FP.
+		fp = *(*uintptr)(unsafe.Pointer(fp)) // The caller's caller's FP.
+	}
+	return fp
+}