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, 7 insertions, 0 deletions

diff --git a/src/run.bat b/src/run.bat
index 62692acaf2..309e06d507 100644
--- a/src/run.bat
+++ b/src/run.bat
@@ -90,11 +90,18 @@ go run "%GOROOT%\test\run.go" - ..\misc\cgo\stdio
 if errorlevel 1 goto fail
 echo.
 
+# cgo tests inspect the traceback for runtime functions
+set OLDGOTRACEBACK=%GOTRACEBACK%
+set GOTRACEBACK=2
+
 echo # ..\misc\cgo\test
 go test ..\misc\cgo\test
 if errorlevel 1 goto fail
 echo.
 
+set GOTRACEBACK=%OLDGOTRACEBACK%
+set OLDGOTRACEBACK=
+
 echo # ..\misc\cgo\testso
 cd ..\misc\cgo\testso
 set FAIL=0