liblink, runtime: diagnose and fix C code running on Go stack

This CL contains compiler+runtime changes that detect C code
running on Go (not g0, not gsignal) stacks, and it contains
corrections for what it detected.

The detection works by changing the C prologue to use a different
stack guard word in the G than Go prologue does. On the g0 and
gsignal stacks, that stack guard word is set to the usual
stack guard value. But on ordinary Go stacks, that stack
guard word is set to ^0, which will make any stack split
check fail. The C prologue then calls morestackc instead
of morestack, and morestackc aborts the program with
a message about running C code on a Go stack.

This check catches all C code running on the Go stack
except NOSPLIT code. The NOSPLIT code is allowed,
so the check is complete. Since it is a dynamic check,
the code must execute to be caught. But unlike the static
checks we've been using in cmd/ld, the dynamic check
works with function pointers and other indirect calls.
For example it caught sigpanic being pushed onto Go
stacks in the signal handlers.

Fixes issue 8667.

LGTM=khr, iant
R=golang-codereviews, khr, iant
CC=golang-codereviews, r
https://codereview.appspot.com/133700043

1 files changed, 0 insertions, 1 deletions

diff --git a/src/runtime/stubs.go b/src/runtime/stubs.go
index bf6c33a41..f8ea1092e 100644
--- a/src/runtime/stubs.go
+++ b/src/runtime/stubs.go
@@ -223,7 +223,6 @@ func lessstack()
 func morestack()
 func mstart()
 func rt0_go()
-func sigpanic()
 
 // return0 is a stub used to return 0 from deferproc.
 // It is called at the very end of deferproc to signal