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>

src/runtime/cgocall.go

@@ -314,6 +314,7 @@ func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
 	// save syscall* and let reentersyscall restore them.
 	savedsp := unsafe.Pointer(gp.syscallsp)
 	savedpc := gp.syscallpc
+	savedbp := gp.syscallbp
 	exitsyscall() // coming out of cgo call
 	gp.m.incgo = false
 	if gp.m.isextra {
@@ -345,7 +346,7 @@ func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
 	osPreemptExtEnter(gp.m)
 
 	// going back to cgo call
-	reentersyscall(savedpc, uintptr(savedsp))
+	reentersyscall(savedpc, uintptr(savedsp), savedbp)
 
 	gp.m.winsyscall = winsyscall
 }

src/runtime/export_windows_test.go

@@ -33,11 +33,6 @@ func NewContextStub() *ContextStub {
 	var ctx context
 	ctx.set_ip(getcallerpc())
 	ctx.set_sp(getcallersp())
-	fp := getfp()
+	ctx.set_fp(getcallerfp())
-	// getfp is not implemented on windows/386 and windows/arm,
-	// in which case it returns 0.
-	if fp != 0 {
-		ctx.set_fp(*(*uintptr)(unsafe.Pointer(fp)))
-	}
 	return &ContextStub{ctx}
 }

src/runtime/proc.go

@@ -4237,7 +4237,7 @@ func gdestroy(gp *g) {
 //
 //go:nosplit
 //go:nowritebarrierrec
-func save(pc, sp uintptr) {
+func save(pc, sp, bp uintptr) {
 	gp := getg()
 
 	if gp == gp.m.g0 || gp == gp.m.gsignal {
@@ -4253,6 +4253,7 @@ func save(pc, sp uintptr) {
 	gp.sched.sp = sp
 	gp.sched.lr = 0
 	gp.sched.ret = 0
+	gp.sched.bp = bp
 	// We need to ensure ctxt is zero, but can't have a write
 	// barrier here. However, it should always already be zero.
 	// Assert that.
@@ -4285,7 +4286,7 @@ func save(pc, sp uintptr) {
 // entry point for syscalls, which obtains the SP and PC from the caller.
 //
 //go:nosplit
-func reentersyscall(pc, sp uintptr) {
+func reentersyscall(pc, sp, bp uintptr) {
 	trace := traceAcquire()
 	gp := getg()
 
@@ -4301,14 +4302,15 @@ func reentersyscall(pc, sp uintptr) {
 	gp.throwsplit = true
 
 	// Leave SP around for GC and traceback.
-	save(pc, sp)
+	save(pc, sp, bp)
 	gp.syscallsp = sp
 	gp.syscallpc = pc
+	gp.syscallbp = bp
 	casgstatus(gp, _Grunning, _Gsyscall)
 	if staticLockRanking {
 		// When doing static lock ranking casgstatus can call
 		// systemstack which clobbers g.sched.
-		save(pc, sp)
+		save(pc, sp, bp)
 	}
 	if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
 		systemstack(func() {
@@ -4325,18 +4327,18 @@ func reentersyscall(pc, sp uintptr) {
 		// systemstack itself clobbers g.sched.{pc,sp} and we might
 		// need them later when the G is genuinely blocked in a
 		// syscall
-		save(pc, sp)
+		save(pc, sp, bp)
 	}
 
 	if sched.sysmonwait.Load() {
 		systemstack(entersyscall_sysmon)
-		save(pc, sp)
+		save(pc, sp, bp)
 	}
 
 	if gp.m.p.ptr().runSafePointFn != 0 {
 		// runSafePointFn may stack split if run on this stack
 		systemstack(runSafePointFn)
-		save(pc, sp)
+		save(pc, sp, bp)
 	}
 
 	gp.m.syscalltick = gp.m.p.ptr().syscalltick
@@ -4347,7 +4349,7 @@ func reentersyscall(pc, sp uintptr) {
 	atomic.Store(&pp.status, _Psyscall)
 	if sched.gcwaiting.Load() {
 		systemstack(entersyscall_gcwait)
-		save(pc, sp)
+		save(pc, sp, bp)
 	}
 
 	gp.m.locks--
@@ -4360,7 +4362,12 @@ func reentersyscall(pc, sp uintptr) {
 //go:nosplit
 //go:linkname entersyscall
 func entersyscall() {
-	reentersyscall(getcallerpc(), getcallersp())
+	// N.B. getcallerfp cannot be written directly as argument in the call
+	// to reentersyscall because it forces spilling the other arguments to
+	// the stack. This results in exceeding the nosplit stack requirements
+	// on some platforms.
+	fp := getcallerfp()
+	reentersyscall(getcallerpc(), getcallersp(), fp)
 }
 
 func entersyscall_sysmon() {
@@ -4418,7 +4425,8 @@ func entersyscallblock() {
 	// Leave SP around for GC and traceback.
 	pc := getcallerpc()
 	sp := getcallersp()
-	save(pc, sp)
+	bp := getcallerfp()
+	save(pc, sp, bp)
 	gp.syscallsp = gp.sched.sp
 	gp.syscallpc = gp.sched.pc
 	if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
@@ -4441,7 +4449,7 @@ func entersyscallblock() {
 	systemstack(entersyscallblock_handoff)
 
 	// Resave for traceback during blocked call.
-	save(getcallerpc(), getcallersp())
+	save(getcallerpc(), getcallersp(), getcallerfp())
 
 	gp.m.locks--
 }

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
+}

src/runtime/sizeof_test.go

@@ -20,7 +20,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr // size on 32bit platforms
 		_64bit uintptr // size on 64bit platforms
 	}{
-		{runtime.G{}, 268, 432},   // g, but exported for testing
+		{runtime.G{}, 272, 440},   // g, but exported for testing
 		{runtime.Sudog{}, 56, 88}, // sudog, but exported for testing
 	}
 

src/runtime/traceruntime.go

@@ -481,25 +481,10 @@ func emitUnblockStatus(w traceWriter, gp *g, gen uintptr) traceWriter {
 //
 // Must be called with a valid P.
 func (tl traceLocker) GoSysCall() {
-	var skip int
-	switch {
-	case tracefpunwindoff():
-		// Unwind by skipping 1 frame relative to gp.syscallsp which is captured 3
-		// results by hard coding the number of frames in between our caller and the
-		// actual syscall, see cases below.
-		// TODO(felixge): Implement gp.syscallbp to avoid this workaround?
-		skip = 1
-	case GOOS == "solaris" || GOOS == "illumos":
-		// These platforms don't use a libc_read_trampoline.
-		skip = 3
-	default:
-		// Skip the extra trampoline frame used on most systems.
-		skip = 4
-	}
 	// Scribble down the M that the P is currently attached to.
 	pp := tl.mp.p.ptr()
 	pp.trace.mSyscallID = int64(tl.mp.procid)
-	tl.eventWriter(traceGoRunning, traceProcRunning).commit(traceEvGoSyscallBegin, pp.trace.nextSeq(tl.gen), tl.stack(skip))
+	tl.eventWriter(traceGoRunning, traceProcRunning).commit(traceEvGoSyscallBegin, pp.trace.nextSeq(tl.gen), tl.stack(1))
 }
 
 // GoSysExit emits a GoSyscallEnd event, possibly along with a GoSyscallBlocked event

src/runtime/tracestack.go

@@ -92,7 +92,7 @@ func traceStack(skip int, gp *g, gen uintptr) uint64 {
 		if getg() == gp {
 			nstk += fpTracebackPCs(unsafe.Pointer(getfp()), pcBuf[1:])
 		} else if gp != nil {
-			// Two cases:
+			// Three cases:
 			//
 			// (1) We're called on the g0 stack through mcall(fn) or systemstack(fn). To
 			// behave like gcallers above, we start unwinding from sched.bp, which
@@ -100,11 +100,21 @@ func traceStack(skip int, gp *g, gen uintptr) uint64 {
 			// address of the leaf frame is stored in sched.pc, which we manually
 			// capture here.
 			//
-			// (2) We're called against a gp that we're not currently executing on, in
+			// (2) We're called against a gp that we're not currently executing on, but that isn't
-			// which case it's currently not executing. gp.sched contains the most up-to-date
+			// in a syscall, in which case it's currently not executing. gp.sched contains the most
+			// up-to-date information about where it stopped, and like case (1), we match gcallers
+			// here.
+			//
+			// (3) We're called against a gp that we're not currently executing on, but that is in
+			// a syscall, in which case gp.syscallsp != 0. gp.syscall* contains the most up-to-date
 			// information about where it stopped, and like case (1), we match gcallers here.
-			pcBuf[1] = gp.sched.pc
+			if gp.syscallsp != 0 {
-			nstk += 1 + fpTracebackPCs(unsafe.Pointer(gp.sched.bp), pcBuf[2:])
+				pcBuf[1] = gp.syscallpc
+				nstk += 1 + fpTracebackPCs(unsafe.Pointer(gp.syscallbp), pcBuf[2:])
+			} else {
+				pcBuf[1] = gp.sched.pc
+				nstk += 1 + fpTracebackPCs(unsafe.Pointer(gp.sched.bp), pcBuf[2:])
+			}
 		}
 	}
 	if nstk > 0 {