[PATCH] jfs: UBSAN: shift-out-of-bounds in dbFindBits

[PATCH] jfs: UBSAN: shift-out-of-bounds in dbFindBits

[Remington Brasga]

Fix issue with UBSAN throwing shift-out-of-bounds warning.

Reported-by: syzbot+e38d703eeb410b17b473@syzkaller.appspotmail.com
Signed-off-by: Remington Brasga <rbrasga@uci.edu>
---
When nb = 32, `mask = mask >> nb` or shorthand `mask >>= nb` throws
shift-out-of-bounds warning.
`mask = (mask >> nb)` removes that warning. 

Link to the syzbot bug report: https://lore.kernel.org/all/0000000000006fc563061cbc7f9c@google.com/T/

 fs/jfs/jfs_dmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index cb3cda1390ad..636aae946e84 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -3020,7 +3020,7 @@ static int dbFindBits(u32 word, int l2nb)
 
 	/* scan the word for nb free bits at nb alignments.
 	 */
-	for (bitno = 0; mask != 0; bitno += nb, mask >>= nb) {
+	for (bitno = 0; mask != 0; bitno += nb, mask = (mask >> nb)) {
 		if ((mask & word) == mask)
 			break;
 	}
-- 
2.34.1

Re: [PATCH] jfs: UBSAN: shift-out-of-bounds in dbFindBits

[Dave Kleikamp]

On 7/9/24 7:12PM, Remington Brasga wrote:
> Fix issue with UBSAN throwing shift-out-of-bounds warning.
> 
> Reported-by: syzbot+e38d703eeb410b17b473@syzkaller.appspotmail.com
> Signed-off-by: Remington Brasga <rbrasga@uci.edu>
> ---
> When nb = 32, `mask = mask >> nb` or shorthand `mask >>= nb` throws
> shift-out-of-bounds warning.
> `mask = (mask >> nb)` removes that warning.

Looks good. Applied.

Shaggy

> 
> Link to the syzbot bug report: https://lore.kernel.org/all/0000000000006fc563061cbc7f9c@google.com/T/
> 
>   fs/jfs/jfs_dmap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
> index cb3cda1390ad..636aae946e84 100644
> --- a/fs/jfs/jfs_dmap.c
> +++ b/fs/jfs/jfs_dmap.c
> @@ -3020,7 +3020,7 @@ static int dbFindBits(u32 word, int l2nb)
>   
>   	/* scan the word for nb free bits at nb alignments.
>   	 */
> -	for (bitno = 0; mask != 0; bitno += nb, mask >>= nb) {
> +	for (bitno = 0; mask != 0; bitno += nb, mask = (mask >> nb)) {
>   		if ((mask & word) == mask)
>   			break;
>   	}

[PATCH] jfs: UBSAN: shift-out-of-bounds in dbFindBits

[Matt Jan]

The starting bit number of free bits should not be >= 32 as it is
0-indexed.Assert that the number of bits is < 32, or if it is 32, the
input word must be 0.

Signed-off-by: Matt Jan <zoo868e@gmail.com>
---
 fs/jfs/jfs_dmap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index 974ecf5e0d95..b9eccf8900eb 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -3015,7 +3015,7 @@ static int dbFindBits(u32 word, int l2nb)
 	/* get the number of bits.
 	 */
 	nb = 1 << l2nb;
-	assert(nb <= DBWORD);
+	assert(nb < DBWORD || (nb == DBWORD && !word));
 
 	/* complement the word so we can use a mask (i.e. 0s represent
 	 * free bits) and compute the mask.
-- 
2.25.1