cmd/link, runtime: mark goexit as the top of the call stack

This CL adds a new attribute, TOPFRAME, which can be used to mark
functions that should be treated as being at the top of the call
stack. The function `runtime.goexit` has been marked this way on
architectures that use a link register.

This will stop programs that use DWARF to unwind the call stack
from unwinding past `runtime.goexit` on architectures that use a
link register. For example, it eliminates "corrupt stack?"
warnings when generating a backtrace that hits `runtime.goexit`
in GDB on s390x.

Similar code should be added for non-link-register architectures
(i.e. amd64, 386). They mark the top of the call stack slightly
differently to link register architectures so I haven't added
that code (they need to mark "rip" as undefined).

Fixes #24385.

Change-Id: I15b4c69ac75b491daa0acf0d981cb80eb06488de
Reviewed-on: https://go-review.googlesource.com/c/go/+/169726
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/cmd/internal/obj/textflag.go b/src/cmd/internal/obj/textflag.go
index d8a52da4af..d2cec734b1 100644
--- a/src/cmd/internal/obj/textflag.go
+++ b/src/cmd/internal/obj/textflag.go
@@ -47,4 +47,8 @@ const (
 
 	// Function can call reflect.Type.Method or reflect.Type.MethodByName.
 	REFLECTMETHOD = 1024
+
+	// Function is the top of the call stack. Call stack unwinders should stop
+	// at this function.
+	TOPFRAME = 2048
 )