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SubjectRe: [PATCH] ceph: cache: Replace zero-length array with flexible-array member
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On Thu, 2020-02-13 at 10:00 -0600, 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>
> ---
> fs/ceph/cache.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ceph/cache.c b/fs/ceph/cache.c
> index 270b769607a2..2f5cb6bc78e1 100644
> --- a/fs/ceph/cache.c
> +++ b/fs/ceph/cache.c
> @@ -32,7 +32,7 @@ struct ceph_fscache_entry {
> size_t uniq_len;
> /* The following members must be last */
> struct ceph_fsid fsid;
> - char uniquifier[0];
> + char uniquifier[];
> };
>
> static const struct fscache_cookie_def ceph_fscache_fsid_object_def = {

Meh, ok. Merged into the ceph-client/testing branch. Should make v5.7
barring unforseen issues.

Thanks,
--
Jeff Layton <jlayton@kernel.org>

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