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

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Message-ID: <57b70bbc-c8d0-4181-98a9-5174517270a0@oracle.com>
Date: Tue, 18 Feb 2025 13:10:32 -0500
From: Chuck Lever <chuck.lever@...cle.com>
To: Trond Myklebust <trondmy@...nel.org>, Anna Schumaker <anna@...nel.org>
Cc: linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva"
 <gustavoars@...nel.org>,
        Dai Ngo <Dai.Ngo@...cle.com>, Jeff Layton <jlayton@...nel.org>,
        Olga Kornievskaia <okorniev@...hat.com>, Tom Talpey <tom@...pey.com>,
        Neil Brown <neilb@...e.de>
Subject: Re: [RESEND PATCH][next] fs: nfs: acl: Avoid
 -Wflex-array-member-not-at-end warning

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@...nel.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@...cle.com>

Gentle ping: Still waiting for a response.


-- 
Chuck Lever