Re: [PATCH] jbd2: enforce power-of-two default revoke hash size at compile time

[Jan Kara]

On Mon 13-04-26 14:27:24, Milos Nikic wrote:
> The jbd2 revoke table relies on bitwise AND operations for fast hash
> indexing, which requires the hash table size to be a strict power of two.
> 
> Currently, this requirement is only enforced at runtime via a J_ASSERT
> in jbd2_journal_init_revoke(). While this successfully catches invalid
> dynamic allocations, it means a developer accidentally modifying the
> hardcoded JOURNAL_REVOKE_DEFAULT_HASH macro will experience a system
> panic upon mounting the filesystem during testing.
> 
> Add a BUILD_BUG_ON() in journal_init_common() to validate the default
> macro at compile time. This acts as an immediate, zero-overhead
> safeguard, preventing compilation entirely if the default hash size is
> mathematically invalid.
> 
> Signed-off-by: Milos Nikic <nikic.milos@xxxxxxxxx>

Eh, if you modify JOURNAL_REVOKE_DEFAULT_HASH you should better know what
you are doing and if you mess up, then the kernel failing with assertion
isn't that difficult to diagnose. So sorry I don't think this "cleanup" is
useful either.

								Honza

> ---
>  fs/jbd2/journal.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 4f397fcdb13c..62b36a2fc4e2 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -1565,6 +1565,7 @@ static journal_t *journal_init_common(struct block_device *bdev,
>  	/* The journal is marked for error until we succeed with recovery! */
>  	journal->j_flags = JBD2_ABORT;
>  
> +	BUILD_BUG_ON(!is_power_of_2(JOURNAL_REVOKE_DEFAULT_HASH));
>  	/* Set up a default-sized revoke table for the new mount. */
>  	err = jbd2_journal_init_revoke(journal, JOURNAL_REVOKE_DEFAULT_HASH);
>  	if (err)
> -- 
> 2.53.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR