The expression "inactags << bmp->db_agl2size" in the function
dbFinalizeBmap() is computed using int operands. Although the
values (inactags and db_agl2size) are derived from filesystem
parameters and are usually small, there is a theoretical risk that
the shift could overflow a 32-bit int if extreme values occur.

According to the C standard, shifting a signed 32-bit int can lead
to undefined behavior if the result exceeds its range. In our
case, an overflow could miscalculate free blocks, potentially
leading to erroneous filesystem accounting.

To ensure the arithmetic is performed in 64-bit space, we cast
"inactags" to s64 before shifting. This defensive fix prevents any
risk of overflow and complies with kernel coding best practices.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Rand Deeb <rand.sec96@gmail.com>
---
 fs/jfs/jfs_dmap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
index f9009e4f9ffd..f89f07c9580e 100644
--- a/fs/jfs/jfs_dmap.c
+++ b/fs/jfs/jfs_dmap.c
@@ -3666,8 +3666,8 @@ void dbFinalizeBmap(struct inode *ipbmap)
 	 * system size is not a multiple of the group size).
 	 */
 	inactfree = (inactags && ag_rem) ?
-	    ((inactags - 1) << bmp->db_agl2size) + ag_rem
-	    : inactags << bmp->db_agl2size;
+	    (((s64)inactags - 1) << bmp->db_agl2size) + ag_rem
+	    : ((s64)inactags << bmp->db_agl2size);
 
 	/* determine how many free blocks are in the active
 	 * allocation groups plus the average number of free blocks
-- 
2.34.1

On 2/20/25 3:43AM, Rand Deeb wrote:
> The expression "inactags << bmp->db_agl2size" in the function
> dbFinalizeBmap() is computed using int operands. Although the
> values (inactags and db_agl2size) are derived from filesystem
> parameters and are usually small, there is a theoretical risk that
> the shift could overflow a 32-bit int if extreme values occur.
> 
> According to the C standard, shifting a signed 32-bit int can lead
> to undefined behavior if the result exceeds its range. In our
> case, an overflow could miscalculate free blocks, potentially
> leading to erroneous filesystem accounting.
> 
> To ensure the arithmetic is performed in 64-bit space, we cast
> "inactags" to s64 before shifting. This defensive fix prevents any
> risk of overflow and complies with kernel coding best practices.

Looks good. I'll add this to jfs-next.

Shaggy

> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Signed-off-by: Rand Deeb <rand.sec96@gmail.com>
> ---
>   fs/jfs/jfs_dmap.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/jfs/jfs_dmap.c b/fs/jfs/jfs_dmap.c
> index f9009e4f9ffd..f89f07c9580e 100644
> --- a/fs/jfs/jfs_dmap.c
> +++ b/fs/jfs/jfs_dmap.c
> @@ -3666,8 +3666,8 @@ void dbFinalizeBmap(struct inode *ipbmap)
>   	 * system size is not a multiple of the group size).
>   	 */
>   	inactfree = (inactags && ag_rem) ?
> -	    ((inactags - 1) << bmp->db_agl2size) + ag_rem
> -	    : inactags << bmp->db_agl2size;
> +	    (((s64)inactags - 1) << bmp->db_agl2size) + ag_rem
> +	    : ((s64)inactags << bmp->db_agl2size);
>   
>   	/* determine how many free blocks are in the active
>   	 * allocation groups plus the average number of free blocks