runtime: fix TestSigStackSwapping on aix/ppc64

This commit fixes TestSigStackSwapping by increasing the signal stack
size. This is needed because SIGSTKSZ is too small when VMX is used on
AIX.

Change-Id: Ic2b5faa65745228d0768383b3d6ebd4b6f9f532c
Reviewed-on: https://go-review.googlesource.com/c/go/+/164012
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/testdata/testprogcgo/sigstack.go

@@ -17,11 +17,18 @@ package main
 #include <stdlib.h>
 #include <sys/mman.h>
 
+#ifdef _AIX
+// On AIX, SIGSTKSZ is too small to handle Go sighandler.
+#define CSIGSTKSZ 0x4000
+#else
+#define CSIGSTKSZ SIGSTKSZ
+#endif
+
 extern void SigStackCallback();
 
 static void* WithSigStack(void* arg __attribute__((unused))) {
 	// Set up an alternate system stack.
-	void* base = mmap(0, SIGSTKSZ, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
+	void* base = mmap(0, CSIGSTKSZ, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
 	if (base == MAP_FAILED) {
 		perror("mmap failed");
 		abort();
@@ -29,7 +36,7 @@ static void* WithSigStack(void* arg __attribute__((unused))) {
 	stack_t st = {}, ost = {};
 	st.ss_sp = (char*)base;
 	st.ss_flags = 0;
-	st.ss_size = SIGSTKSZ;
+	st.ss_size = CSIGSTKSZ;
 	if (sigaltstack(&st, &ost) < 0) {
 		perror("sigaltstack failed");
 		abort();
@@ -42,13 +49,13 @@ static void* WithSigStack(void* arg __attribute__((unused))) {
 	if (ost.ss_flags & SS_DISABLE) {
 		// Darwin libsystem has a bug where it checks ss_size
 		// even if SS_DISABLE is set. (The kernel gets it right.)
-		ost.ss_size = SIGSTKSZ;
+		ost.ss_size = CSIGSTKSZ;
 	}
 	if (sigaltstack(&ost, NULL) < 0) {
 		perror("sigaltstack restore failed");
 		abort();
 	}
-	mprotect(base, SIGSTKSZ, PROT_NONE);
+	mprotect(base, CSIGSTKSZ, PROT_NONE);
 	return NULL;
 }