selftests/sigaltstack: Fix the sigaltstack test on old kernels

The handling for old kernels was wrong, resulting in a segfault.  Fix it.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Stas Sergeev <stsp@list.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-api@vger.kernel.org
Link: http://lkml.kernel.org/r/f3e739bf435beeaecbd5f038f1359d2eac6d1e63.1462296606.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

tools/testing/selftests/sigaltstack/sas.c

@@ -15,6 +15,7 @@
 #include <alloca.h>
 #include <string.h>
 #include <assert.h>
+#include <errno.h>
 
 #ifndef SS_AUTODISARM
 #define SS_AUTODISARM  (1 << 4)
@@ -117,14 +118,20 @@ int main(void)
 	stk.ss_flags = SS_ONSTACK | SS_AUTODISARM;
 	err = sigaltstack(&stk, NULL);
 	if (err) {
+		if (errno == EINVAL) {
+			printf("[NOTE]\tThe running kernel doesn't support SS_AUTODISARM\n");
+			/*
+			 * If test cases for the !SS_AUTODISARM variant were
+			 * added, we could still run them.  We don't have any
+			 * test cases like that yet, so just exit and report
+			 * success.
+			 */
+			return 0;
+		} else {
 			perror("[FAIL]\tsigaltstack(SS_ONSTACK | SS_AUTODISARM)");
-		stk.ss_flags = SS_ONSTACK;
-	}
-	err = sigaltstack(&stk, NULL);
-	if (err) {
-		perror("[FAIL]\tsigaltstack(SS_ONSTACK)");
 			return EXIT_FAILURE;
 		}
+	}
 
 	ustack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
 		      MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);