runtime: signal handling: sigfwd calls C handlers with improper alignment

[bcmills]

go version go1.7.3 linux/amd64

What did you do?

In misc/cgo/testcarchive/main2.c, add a function call within the signal handler which generates an instruction that requires proper alignment. Calling a varargs function with a floating-point parameter seems to suffice:

bcmills:~/src/go.googlesource.com/go/misc/cgo/testcarchive$ git diff main2.c
diff --git a/misc/cgo/testcarchive/main2.c b/misc/cgo/testcarchive/main2.c
index 3726977..c814047 100644
--- a/misc/cgo/testcarchive/main2.c
+++ b/misc/cgo/testcarchive/main2.c
@@ -7,8 +7,10 @@

 #include <setjmp.h>
 #include <signal.h>
+#include <stdarg.h>
 #include <stddef.h>
 #include <stdio.h>
+#include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
@@ -46,11 +48,19 @@ static void ioHandler(int signo, siginfo_t* info, void* ctxt) {
 static jmp_buf jmp;
 static char* nullPointer;

+static void callWithVarargs(void* dummy, ...) {
+       va_list args;
+       va_start(args, dummy);
+       va_end(args);
+}
+
 // Signal handler for SIGSEGV on a C thread.
 static void segvHandler(int signo, siginfo_t* info, void* ctxt) {
        sigset_t mask;
        int i;

+       callWithVarargs("dummy arg", 3.1415);
+
        if (sigemptyset(&mask) < 0) {
                die("sigemptyset");
        }

Enable the race-detector in the corresponding test:

bcmills:~/src/go.googlesource.com/go/misc/cgo/testcarchive$ git diff carchive_test.go
diff --git a/misc/cgo/testcarchive/carchive_test.go b/misc/cgo/testcarchive/carchive_test.go
index 4999929..30334cf 100644
--- a/misc/cgo/testcarchive/carchive_test.go
+++ b/misc/cgo/testcarchive/carchive_test.go
@@ -198,7 +198,7 @@ func TestEarlySignalHandler(t *testing.T) {
                os.RemoveAll("pkg")
        }()

-       cmd := exec.Command("go", "build", "-buildmode=c-archive", "-o", "libgo2.a", "libgo2")
+       cmd := exec.Command("go", "build", "-race", "-buildmode=c-archive", "-o", "libgo2.a", "libgo2")
        cmd.Env = gopathEnv
        if out, err := cmd.CombinedOutput(); err != nil {
                t.Logf("%s", out)

Then, run the test:

bcmills:~/src/go.googlesource.com/go/misc/cgo/testcarchive$ go test -v -test.run=TestEarlySignalHandler ./carchive_test.go
=== RUN   TestEarlySignalHandler
--- FAIL: TestEarlySignalHandler (6.46s)
        carchive_test.go:215:
        carchive_test.go:216: signal: segmentation fault (core dumped)
FAIL
exit status 1
FAIL    command-line-arguments  7.259s

What did you expect to see?

The test should pass.

What did you see instead?

The test fails due to a second SIGSEGV within the first SIGSEGV handler call. The source of the second SIGSEGV is a movaps instruction which requires the stack to be properly 16-byte aligned.

[bcmills]

As it turns out, at tip it doesn't even require the -race edit: the change to main2.c suffices.

[gopherbot]

CL https://golang.org/cl/32107 mentions this issue.

[bcmills]

There is a related issue that sigtramp and/or cgoSigtramp need to save callee-save registers in case they are themselves forwarded calls from some other signal handler, but I'll file that as a separate issue.