Re: [PATCH v2][next] jbd2: Avoid dozens of -Wflex-array-member-not-at-end warnings

From: Jan Kara
Date: Mon Nov 04 2024 - 06:17:47 EST


On Fri 01-11-24 14:45:23, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we
> are getting ready to enable it, globally.
>
> Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
> a flexible structure (`struct shash_desc`) where the size of the
> flexible-array member (`__ctx`) is known at compile-time, and
> refactor the rest of the code, accordingly.
>
> So, with this, fix 77 of the following warnings:
>
> include/linux/jbd2.h:1800:35: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

Honza

> ---
> Changes in v2:
> - Use DIV_ROUND_UP() to compute the number of elements for the flex
> array. (Jan Kara)
>
> v1:
> - Link: https://lore.kernel.org/linux-hardening/ZxvyavDjXDaV9cNg@kspp/
>
> include/linux/jbd2.h | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 8aef9bb6ad57..50f7ea8714bf 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -1796,22 +1796,21 @@ static inline unsigned long jbd2_log_space_left(journal_t *journal)
> static inline u32 jbd2_chksum(journal_t *journal, u32 crc,
> const void *address, unsigned int length)
> {
> - struct {
> - struct shash_desc shash;
> - char ctx[JBD_MAX_CHECKSUM_SIZE];
> - } desc;
> + DEFINE_RAW_FLEX(struct shash_desc, desc, __ctx,
> + DIV_ROUND_UP(JBD_MAX_CHECKSUM_SIZE,
> + sizeof(*((struct shash_desc *)0)->__ctx)));
> int err;
>
> BUG_ON(crypto_shash_descsize(journal->j_chksum_driver) >
> JBD_MAX_CHECKSUM_SIZE);
>
> - desc.shash.tfm = journal->j_chksum_driver;
> - *(u32 *)desc.ctx = crc;
> + desc->tfm = journal->j_chksum_driver;
> + *(u32 *)desc->__ctx = crc;
>
> - err = crypto_shash_update(&desc.shash, address, length);
> + err = crypto_shash_update(desc, address, length);
> BUG_ON(err);
>
> - return *(u32 *)desc.ctx;
> + return *(u32 *)desc->__ctx;
> }
>
> /* Return most recent uncommitted transaction */
> --
> 2.43.0
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR