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

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Message-ID: <278bfe19-4a79-4969-950a-28f9fa2fdb82@oracle.com>
Date: Fri, 23 Aug 2024 13:16:19 -0500
From: Dave Kleikamp <dave.kleikamp@...cle.com>
To: Remington Brasga <rbrasga@....edu>,
        Manas Ghandat
 <ghandatmanas@...il.com>,
        Juntong Deng <juntong.deng@...look.com>,
        Andrew Kanner <andrew.kanner@...il.com>,
        Osama Muhammad
 <osmtendev@...il.com>,
        Shuah Khan <skhan@...uxfoundation.org>
Cc: jfs-discussion@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+e38d703eeb410b17b473@...kaller.appspotmail.com
Subject: Re: [PATCH] jfs: UBSAN: shift-out-of-bounds in dbFindBits

On 7/9/24 7:12PM, Remington Brasga wrote:
> Fix issue with UBSAN throwing shift-out-of-bounds warning.
> 
> Reported-by: syzbot+e38d703eeb410b17b473@...kaller.appspotmail.com
> Signed-off-by: Remington Brasga <rbrasga@....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;
>   	}