[tip:core/signals] signals/sigaltstack: Report current flag bits in sigaltstack()

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  0318bc8a919ded355eaa5078689924a15c1bf52a
Gitweb:     http://git.kernel.org/tip/0318bc8a919ded355eaa5078689924a15c1bf52a
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 3 May 2016 10:31:51 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 4 May 2016 08:34:14 +0200

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 <luto@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Amanieu d'Antras <amanieu@gmail.com>
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: Michal Hocko <mhocko@suse.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Stas Sergeev <stsp@list.ru>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Cc: linux-api@vger.kernel.org
Link: http://lkml.kernel.org/r/94b291ec9fd47741a9264851e316e158ded0b00d.1462296606.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/signal.c                           |  3 ++-
 tools/testing/selftests/sigaltstack/sas.c | 19 ++++++++++++++++---
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index bf97ea5..ab122a2 100644
--- a/kernel/signal.c
+++ b/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;
diff --git a/tools/testing/selftests/sigaltstack/sas.c b/tools/testing/selftests/sigaltstack/sas.c
index a98c3ef..4280d06 100644
--- a/tools/testing/selftests/sigaltstack/sas.c
+++ b/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;