runtime: keep g->syscallsp consistent after cgo->Go callbacks

Normally, the caller to runtime.entersyscall() must not return before
calling runtime.exitsyscall(), lest g->syscallsp become a dangling
pointer. runtime.cgocallbackg() violates this constraint. To work around
this, save g->syscallsp and g->syscallpc around cgo->Go callbacks, then
restore them after calling runtime.entersyscall(), which restores the
syscall stack frame pointer saved by cgocall. This allows the GC to
correctly trace a goroutine that is currently returning from a
Go->cgo->Go chain.

This also adds a check to proc.c that panics if g->syscallsp is clearly
invalid. It is not 100% foolproof, as it will not catch a case where the
stack was popped then pushed back beyond g->syscallsp, but it does catch
the present cgo issue and makes existing tests fail without the bugfix.

Fixes #7978.

LGTM=dvyukov, rsc
R=golang-codereviews, dvyukov, minux, bradfitz, iant, gobot, rsc
CC=golang-codereviews, rsc
https://golang.org/cl/131910043

1 files changed, 2 insertions, 0 deletions

diff --git a/src/run.bash b/src/run.bash
index b5f061d885..d6e53304d8 100755
--- a/src/run.bash
+++ b/src/run.bash
@@ -119,6 +119,8 @@ go run $GOROOT/test/run.go - . || exit 1
 
 [ "$CGO_ENABLED" != 1 ] ||
 (xcd ../misc/cgo/test
+# cgo tests inspect the traceback for runtime functions
+export GOTRACEBACK=2
 go test -ldflags '-linkmode=auto' || exit 1
 # linkmode=internal fails on dragonfly since errno is a TLS relocation.
 [ "$GOHOSTOS" == dragonfly ] || go test -ldflags '-linkmode=internal' || exit 1