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@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++)
>
> Trond, Anna -
>
> Let me know if I need to take this one via the NFSD tree. If not,
>
> Acked-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
Gentle ping: Still waiting for a response.
--
Chuck Lever
