[release-branch.go1.17] runtime: use saved LR when unwinding through morestack

On LR machine, consider F calling G calling H, which grows stack.
The stack looks like
...
G's frame:
	... locals ...
	saved LR = return PC in F  <- SP points here at morestack
H's frame (to be created)

At morestack, we save
	gp.sched.pc = H's morestack call
	gp.sched.sp = H's entry SP (the arrow above)
	gp.sched.lr = return PC in G

Currently, when unwinding through morestack (if _TraceJumpStack
is set), we switch PC and SP but not LR. We then have
	frame.pc = H's morestack call
	frame.sp = H's entry SP (the arrow above)
As LR is not set, we load it from stack at *sp, so
	frame.lr = return PC in F
As the SP hasn't decremented at the morestack call,
	frame.fp = frame.sp = H's entry SP

Unwinding a frame, we have
	frame.pc = old frame.lr = return PC in F
	frame.sp = old frame.fp = H's entry SP a.k.a. G's SP
The PC and SP don't match. The unwinding will go off if F and G
have different frame sizes.

Fix this by preserving the LR when switching stack.

Also add code to detect infinite loop in unwinding.

TODO: add some test. I can reproduce the infinite loop (or throw
with added check) but the frequency is low.

Fixes #53111.
Updates #52116.

Change-Id: I6e1294f1c6e55f664c962767a1cf6c466a0c0eff
Reviewed-on: https://go-review.googlesource.com/c/go/+/400575
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Eric Fang <eric.fang@arm.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
(cherry picked from commit 74f00094220f26c80fbaab6eca28c3a664897d24)
Reviewed-on: https://go-review.googlesource.com/c/go/+/408822
Reviewed-by: Austin Clements <austin@google.com>

diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index 814c3236345404e63f7d37c11e0076b67d824d52..de593491a13ca103a2049b09d5f07384601473d5 100644 (file)
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -116,6 +116,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
        }
        waspanic := false
        cgoCtxt := gp.cgoCtxt
+       stack := gp.stack
        printing := pcbuf == nil && callback == nil
 
        // If the PC is zero, it's likely a nil function call.
@@ -134,7 +135,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
        if !f.valid() {
                if callback != nil || printing {
                        print("runtime: unknown pc ", hex(frame.pc), "\n")
-                       tracebackHexdump(gp.stack, &frame, 0)
+                       tracebackHexdump(stack, &frame, 0)
                }
                if callback != nil {
                        throw("unknown pc")
@@ -194,12 +195,15 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
                                        frame.fn = findfunc(frame.pc)
                                        f = frame.fn
                                        flag = f.flag
+                                       frame.lr = gp.m.curg.sched.lr
                                        frame.sp = gp.m.curg.sched.sp
+                                       stack = gp.m.curg.stack
                                        cgoCtxt = gp.m.curg.cgoCtxt
                                case funcID_systemstack:
                                        // systemstack returns normally, so just follow the
                                        // stack transition.
                                        frame.sp = gp.m.curg.sched.sp
+                                       stack = gp.m.curg.stack
                                        cgoCtxt = gp.m.curg.cgoCtxt
                                        flag &^= funcFlag_SPWRITE
                                }
@@ -268,7 +272,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
                                }
                                if callback != nil || doPrint {
                                        print("runtime: unexpected return pc for ", funcname(f), " called from ", hex(frame.lr), "\n")
-                                       tracebackHexdump(gp.stack, &frame, lrPtr)
+                                       tracebackHexdump(stack, &frame, lrPtr)
                                }
                                if callback != nil {
                                        throw("unknown caller pc")
@@ -497,6 +501,13 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
                        break
                }
 
+               if frame.pc == frame.lr && frame.sp == frame.fp {
+                       // If the next frame is identical to the current frame, we cannot make progress.
+                       print("runtime: traceback stuck. pc=", hex(frame.pc), " sp=", hex(frame.sp), "\n")
+                       tracebackHexdump(stack, &frame, frame.sp)
+                       throw("traceback stuck")
+               }
+
                // Unwind to next frame.
                frame.fn = flr
                frame.pc = frame.lr