[PATCH 1/2] sigaltstack: ignore flags if SS_DISABLE is set

[Stas Sergeev]

ss_flags combo of SS_AUTODISARM|SS_DISABLE can be used to check
the support of SS_AUTODISARM. We need to remove the like flags and
only keep SS_DISABLE because many libraries (eg asan runtime)
check if SAS is disabled by just checking "ss_flags == SS_DISABLE".

Also man page mandates that only 1 flag can be returned, so
returning SS_AUTODISARM|SS_DISABLE should be disallowed.

Signed-off-by: Stas Sergeev <stsp2@xxxxxxxxx>
CC: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: Jens Axboe <axboe@xxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Marco Elver <elver@xxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Alexey Gladkov <legion@xxxxxxxxxx>
CC: Andrew Lutomirski <luto@xxxxxxx>
CC: linux-kernel@xxxxxxxxxxxxxxx
---
 kernel/signal.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/signal.c b/kernel/signal.c
index 38602738866e..40634a500317 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -4209,6 +4209,7 @@ do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp,
 		if (ss_mode == SS_DISABLE) {
 			ss_size = 0;
 			ss_sp = NULL;
+			ss_flags = SS_DISABLE;
 		} else {
 			if (unlikely(ss_size < min_ss_size))
 				ret = -ENOMEM;
-- 
2.34.1