[PATCH] swapon: check validity of swap_flags

[Hugh Dickins]

Most system calls taking flags first check that the flags passed in are
valid, and that helps userspace to detect when new flags are supported.

But swapon never did so: start checking now, to help if we ever want to
support more swap_flags in future.

It's difficult to get stray bits set in an int, and swapon is not widely
used, so this is most unlikely to break any userspace; but we can just
revert if it turns out to do so.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---

 include/linux/swap.h |    3 +++
 mm/swapfile.c        |    3 +++
 2 files changed, 6 insertions(+)

--- linux.git/include/linux/swap.h	2012-03-23 10:19:53.408051631 -0700
+++ linux/include/linux/swap.h	2012-03-23 10:34:02.956071819 -0700
@@ -21,6 +21,9 @@ struct bio;
 #define SWAP_FLAG_PRIO_SHIFT	0
 #define SWAP_FLAG_DISCARD	0x10000 /* discard swap cluster after use */
 
+#define SWAP_FLAGS_VALID	(SWAP_FLAG_PRIO_MASK | SWAP_FLAG_PREFER | \
+				 SWAP_FLAG_DISCARD)
+
 static inline int current_is_kswapd(void)
 {
 	return current->flags & PF_KSWAPD;
--- linux.git/mm/swapfile.c	2012-03-23 10:19:53.588051635 -0700
+++ linux/mm/swapfile.c	2012-03-23 10:35:52.764074181 -0700
@@ -2022,6 +2022,9 @@ SYSCALL_DEFINE2(swapon, const char __use
 	struct page *page = NULL;
 	struct inode *inode = NULL;
 
+	if (swap_flags & ~SWAP_FLAGS_VALID)
+		return -EINVAL;
+
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
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