[PATCH] Btrfs: fix snprintf usage by send's gen_unique_name

[PATCH] Btrfs: fix snprintf usage by send's gen_unique_name

[Filipe David Borba Manana]

The buffer size argument passed to snprintf must account for the
trailing null byte added by snprintf, and it returns a value >= then
sizeof(buffer) when the string can't fit in the buffer.

Since our buffer has a size of 64 characters, and the maximum orphan
name we can generate is 63 characters wide, we must pass 64 as the
buffer size to snprintf, and not 63.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
---
 fs/btrfs/send.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 84aed2f..27edc5e 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -1336,7 +1336,7 @@ static int gen_unique_name(struct send_ctx *sctx,
 		return -ENOMEM;
 
 	while (1) {
-		len = snprintf(tmp, sizeof(tmp) - 1, "o%llu-%llu-%llu",
+		len = snprintf(tmp, sizeof(tmp), "o%llu-%llu-%llu",
 				ino, gen, idx);
 		if (len >= sizeof(tmp)) {
 			/* should really not happen */
-- 
1.7.9.5

Re: [PATCH] Btrfs: fix snprintf usage by send's gen_unique_name

[David Sterba]

On Tue, Jan 21, 2014 at 11:36:38PM +0000, Filipe David Borba Manana wrote:
> The buffer size argument passed to snprintf must account for the
> trailing null byte added by snprintf, and it returns a value >= then
> sizeof(buffer) when the string can't fit in the buffer.
> 
> Since our buffer has a size of 64 characters, and the maximum orphan
> name we can generate is 63 characters wide, we must pass 64 as the
> buffer size to snprintf, and not 63.
> 
> Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>

JFYI, I have a patch to do the same plus cleans the code around, but
it's part of a bigger series that's in testing atm, so I haven't sent it
yet.

Consider it
Reviewed-by: David Sterba <dsterba@suse.cz>

I'll update my version.