Please cc patches to the xfs mailing list.
On Mon, Mar 09, 2020 at 08:28:42PM +0000, 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]
Are they subject to the same padding rules & quirks as zero-length
arrays? I ask this mostly because we've had trouble with unaligned
flexible array members in struct xfs_agfl for years. Though I guess
xfs_ondisk.h checks that now, so my question becomes: have you tracked
down the compilers that have gotten it wrong and made sure it's no more
broken than before? :)
--D
> 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@xxxxxxxxxxxxxx>
> ---
> fs/xfs/libxfs/xfs_format.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> index 77e9fa385980..4b2748e669a7 100644
> --- a/fs/xfs/libxfs/xfs_format.h
> +++ b/fs/xfs/libxfs/xfs_format.h
> @@ -1673,7 +1673,7 @@ struct xfs_acl_entry {
>
> struct xfs_acl {
> __be32 acl_cnt;
> - struct xfs_acl_entry acl_entry[0];
> + struct xfs_acl_entry acl_entry[];
> };
>
> /*
> --
> 2.25.0
>
