Re: [RESEND PATCH][next] fs: nfs: acl: Avoid -Wflex-array-member-not-at-end warning

[Chuck Lever <chuck.lever@oracle.com>]

On 2/19/25 12:46 PM, Anna Schumaker wrote:
> Hi Chuck,
> 
> On Tue, Feb 18, 2025 at 1:10 PM Chuck Lever <chuck.lever@oracle.com> wrote:
>>
>> On 2/10/25 2:48 PM, Chuck Lever wrote:
>>> On 2/3/25 10:03 PM, 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@kernel.org>
>>>> ---
>>>>  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++)
>>>
>>> Trond, Anna -
>>>
>>> Let me know if I need to take this one via the NFSD tree. If not,
>>>
>>> Acked-by: Chuck Lever <chuck.lever@oracle.com>
>>
>> Gentle ping: Still waiting for a response.
> 
> If you want it, it's yours! :)
> 
> I am planning a bugfixes pull request soon, so I don't mind taking it
> if you don't have anything else planned for 6.14 at the moment.

This change doesn't look like a "fix" to me, so I would include it in
nfsd-testing / nfsd-next, as long as you (or Trond) can send me an
Acked-by.


-- 
Chuck Lever