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SubjectRe: [PATCH][next] ext2: xattr.h: Replace zero-length array with flexible-array member
On Mon 09-03-20 13:04:41, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Thanks. I've added the patch to my tree.

Honza

> ---
> fs/ext2/xattr.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext2/xattr.h b/fs/ext2/xattr.h
> index cee888cdc235..16272e6ddcf4 100644
> --- a/fs/ext2/xattr.h
> +++ b/fs/ext2/xattr.h
> @@ -39,7 +39,7 @@ struct ext2_xattr_entry {
> __le32 e_value_block; /* disk block attribute is stored on (n/i) */
> __le32 e_value_size; /* size of attribute value */
> __le32 e_hash; /* hash value of name and value */
> - char e_name[0]; /* attribute name */
> + char e_name[]; /* attribute name */
> };
>
> #define EXT2_XATTR_PAD_BITS 2
> --
> 2.25.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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