signals/sigaltstack: Report current flag bits in sigaltstack()

sigaltstack()'s reported previous state uses a somewhat odd
convention, but the concept of flag bits is new, and we can do the
flag bits sensibly.  Specifically, let's just report them directly.

This will allow saving and restoring the sigaltstack state using
sigaltstack() to work correctly.

Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Amanieu d'Antras <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Stas Sergeev <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/94b291ec9fd47741a9264851e316e158ded0b00d.1462296606.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>

kernel/signal.c

@@ -3099,7 +3099,8 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 
 	oss.ss_sp = (void __user *) current->sas_ss_sp;
 	oss.ss_size = current->sas_ss_size;
-	oss.ss_flags = sas_ss_flags(sp);
+	oss.ss_flags = sas_ss_flags(sp) |
+		(current->sas_ss_flags & SS_FLAG_BITS);
 
 	if (uss) {
 		void __user *ss_sp;

tools/testing/selftests/sigaltstack/sas.c

@@ -113,6 +113,19 @@ int main(void)
 		perror("mmap()");
 		return EXIT_FAILURE;
 	}
+
+	err = sigaltstack(NULL, &stk);
+	if (err) {
+		perror("[FAIL]\tsigaltstack()");
+		exit(EXIT_FAILURE);
+	}
+	if (stk.ss_flags == SS_DISABLE) {
+		printf("[OK]\tInitial sigaltstack state was SS_DISABLE\n");
+	} else {
+		printf("[FAIL]\tInitial sigaltstack state was %i; should have been SS_DISABLE\n", stk.ss_flags);
+		return EXIT_FAILURE;
+	}
+
 	stk.ss_sp = sstack;
 	stk.ss_size = SIGSTKSZ;
 	stk.ss_flags = SS_ONSTACK | SS_AUTODISARM;
@@ -151,12 +164,12 @@ int main(void)
 		perror("[FAIL]\tsigaltstack()");
 		exit(EXIT_FAILURE);
 	}
-	if (stk.ss_flags != 0) {
-		printf("[FAIL]\tss_flags=%i, should be 0\n",
+	if (stk.ss_flags != SS_AUTODISARM) {
+		printf("[FAIL]\tss_flags=%i, should be SS_AUTODISARM\n",
 				stk.ss_flags);
 		exit(EXIT_FAILURE);
 	}
-	printf("[OK]\tsigaltstack is enabled after signal\n");
+	printf("[OK]\tsigaltstack is still SS_AUTODISARM after signal\n");
 
 	printf("[OK]\tTest passed\n");
 	return 0;