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

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Message-ID: <20241104110509.4ioef4oqzvlg7u6e@quack3>
Date: Mon, 4 Nov 2024 12:05:09 +0100
From: Jan Kara <jack@...e.cz>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] jbd2: Avoid dozens of
 -Wflex-array-member-not-at-end warnings

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@...nel.org>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								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@...e.com>
SUSE Labs, CR