On Tue, 2025-02-04 at 13:33 +1030, 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.
>
> So, in order to avoid ending up with a flexible-array member in the
> middle of other structs, we use the `struct_group_tagged()` helper
> to create a new tagged `struct posix_acl_hdr`. This structure
> groups together all the members of the flexible `struct posix_acl`
> except the flexible array.
>
> As a result, the array is effectively separated from the rest of the
> members without modifying the memory layout of the flexible structure.
> We then change the type of the middle struct member currently causing
> trouble from `struct posix_acl` to `struct posix_acl_hdr`.
>
> We also want to ensure that when new members need to be added to the
> flexible structure, they are always included within the newly created
> tagged struct. For this, we use `static_assert()`. This ensures that the
> memory layout for both the flexible structure and the new tagged struct
> is the same after any changes.
>
> This approach avoids having to implement `struct posix_acl_hdr` as a
> completely separate structure, thus preventing having to maintain two
> independent but basically identical structures, closing the door to
> potential bugs in the future.
>
> We also use `container_of()` whenever we need to retrieve a pointer to
> the flexible structure, through which we can access the flexible-array
> member, if necessary.
>
> So, with these changes, fix the following warning:
>
> fs/nfs_common/nfsacl.c:45:26: 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@xxxxxxxxxx>
> ---
> fs/nfs_common/nfsacl.c | 8 +++++---
> include/linux/posix_acl.h | 11 ++++++++---
> 2 files changed, 13 insertions(+), 6 deletions(-)
>
> diff --git a/fs/nfs_common/nfsacl.c b/fs/nfs_common/nfsacl.c
> index ea382b75b26c..e2eaac14fd8e 100644
> --- a/fs/nfs_common/nfsacl.c
> +++ b/fs/nfs_common/nfsacl.c
> @@ -42,7 +42,7 @@ struct nfsacl_encode_desc {
> };
>
> struct nfsacl_simple_acl {
> - struct posix_acl acl;
> + struct posix_acl_hdr acl;
> struct posix_acl_entry ace[4];
> };
>
> @@ -112,7 +112,8 @@ int nfsacl_encode(struct xdr_buf *buf, unsigned int base, struct inode *inode,
> xdr_encode_word(buf, base, entries))
> return -EINVAL;
> if (encode_entries && acl && acl->a_count == 3) {
> - struct posix_acl *acl2 = &aclbuf.acl;
> + struct posix_acl *acl2 =
> + container_of(&aclbuf.acl, struct posix_acl, hdr);
>
> /* Avoid the use of posix_acl_alloc(). nfsacl_encode() is
> * invoked in contexts where a memory allocation failure is
> @@ -177,7 +178,8 @@ bool nfs_stream_encode_acl(struct xdr_stream *xdr, struct inode *inode,
> return false;
>
> if (encode_entries && acl && acl->a_count == 3) {
> - struct posix_acl *acl2 = &aclbuf.acl;
> + struct posix_acl *acl2 =
> + container_of(&aclbuf.acl, struct posix_acl, hdr);
>
> /* Avoid the use of posix_acl_alloc(). nfsacl_encode() is
> * invoked in contexts where a memory allocation failure is
> diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
> index e2d47eb1a7f3..62d497763e25 100644
> --- a/include/linux/posix_acl.h
> +++ b/include/linux/posix_acl.h
> @@ -27,11 +27,16 @@ struct posix_acl_entry {
> };
>
> struct posix_acl {
> - refcount_t a_refcount;
> - unsigned int a_count;
> - struct rcu_head a_rcu;
> + /* New members MUST be added within the struct_group() macro below. */
> + struct_group_tagged(posix_acl_hdr, hdr,
> + refcount_t a_refcount;
> + unsigned int a_count;
> + struct rcu_head a_rcu;
> + );
> struct posix_acl_entry a_entries[] __counted_by(a_count);
> };
> +static_assert(offsetof(struct posix_acl, a_entries) == sizeof(struct posix_acl_hdr),
> + "struct member likely outside of struct_group_tagged()");
>
> #define FOREACH_ACL_ENTRY(pa, acl, pe) \
> for(pa=(acl)->a_entries, pe=pa+(acl)->a_count; pa<pe; pa++)
Acked-by: Jeff Layton <jlayton@xxxxxxxxxx>
