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 <[e-mail]>
Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Pavel Emelyanov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Stas Sergeev <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

tools/testing/selftests/sigaltstack/sas.c

···
 #include <alloca.h>
 #include <string.h>
 #include <assert.h>
+#include <errno.h>
 
 #ifndef SS_AUTODISARM
 #define SS_AUTODISARM  (1 << 4)
···
 	stk.ss_flags = SS_ONSTACK | SS_AUTODISARM;
 	err = sigaltstack(&stk, NULL);
 	if (err) {
-		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;
+		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)");
+			return EXIT_FAILURE;
+		}
 	}
 
 	ustack = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,