Re: [PATCH][next] ext4: Avoid -Wflex-array-member-not-at-end warning

Kees Cook
2025-04-07T18:58:20Z

On Wed, Mar 26, 2025 at 04:55:51PM -0600, 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 where the size of the flexible-array member
> is known at compile-time, and refactor the rest of the code,
> accordingly.
>
> So, with these changes, fix the following warning:
>
> fs/ext4/mballoc.c:3041:40: 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@kernel.org>
> ---
> fs/ext4/mballoc.c | 18 ++++++++----------
> 1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 0d523e9fb3d5..f88424c28194 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3037,10 +3037,8 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
> unsigned char blocksize_bits = min_t(unsigned char,
> sb->s_blocksize_bits,
> EXT4_MAX_BLOCK_LOG_SIZE);
> - struct sg {
> - struct ext4_group_info info;
> - ext4_grpblk_t counters[EXT4_MAX_BLOCK_LOG_SIZE + 2];
> - } sg;
> + DEFINE_RAW_FLEX(struct ext4_group_info, sg, bb_counters,
> + EXT4_MAX_BLOCK_LOG_SIZE + 2);

Yup, struct ext4_group_info::bb_counters is ext4_grpblk_t, so everything
matches up.

>
> group--;
> if (group == 0)
> @@ -3048,7 +3046,7 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
> " 2^0 2^1 2^2 2^3 2^4 2^5 2^6 "
> " 2^7 2^8 2^9 2^10 2^11 2^12 2^13 ]\n");
>
> - i = (blocksize_bits + 2) * sizeof(sg.info.bb_counters[0]) +
> + i = (blocksize_bits + 2) * sizeof(sg->bb_counters[0]) +
> sizeof(struct ext4_group_info);
>
> grinfo = ext4_get_group_info(sb, group);
> @@ -3068,14 +3066,14 @@ static int ext4_mb_seq_groups_show(struct seq_file *seq, void *v)
> * We care only about free space counters in the group info and
> * these are safe to access even after the buddy has been unloaded
> */
> - memcpy(&sg, grinfo, i);
> - seq_printf(seq, "#%-5u: %-5u %-5u %-5u [", group, sg.info.bb_free,
> - sg.info.bb_fragments, sg.info.bb_first_free);
> + memcpy(sg, grinfo, i);
> + seq_printf(seq, "#%-5u: %-5u %-5u %-5u [", group, sg->bb_free,
> + sg->bb_fragments, sg->bb_first_free);
> for (i = 0; i <= 13; i++)
> seq_printf(seq, " %-5u", i <= blocksize_bits + 1 ?
> - sg.info.bb_counters[i] : 0);
> + sg->bb_counters[i] : 0);
> seq_puts(seq, " ]");
> - if (EXT4_MB_GRP_BBITMAP_CORRUPT(&sg.info))
> + if (EXT4_MB_GRP_BBITMAP_CORRUPT(sg))
> seq_puts(seq, " Block bitmap corrupted!");
> seq_putc(seq, '\n');
> return 0;

Replacements looks good.

Reviewed-by: Kees Cook <kees@kernel.org>

--
Kees Cook