Re: [PATCH][next] UAPI: nfsfh.h: Replace one-element array with flexible-array member

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 3/25/21 08:45, David Laight wrote:
> From: Gustavo A. R. Silva
>> Sent: 23 March 2021 22:49
>>
>> There is a regular need in the kernel to provide a way to declare having
>> a dynamically sized set of trailing elements in a structure. Kernel code
>> should always use “flexible array members”[1] for these cases. The older
>> style of one-element or zero-length arrays should no longer be used[2].
>>
>> Use an anonymous union with a couple of anonymous structs in order to
>> keep userspace unchanged:
>>
>> $ pahole -C nfs_fhbase_new fs/nfsd/nfsfh.o
>> struct nfs_fhbase_new {
>>         union {
>>                 struct {
>>                         __u8       fb_version_aux;       /*     0     1 */
>>                         __u8       fb_auth_type_aux;     /*     1     1 */
>>                         __u8       fb_fsid_type_aux;     /*     2     1 */
>>                         __u8       fb_fileid_type_aux;   /*     3     1 */
>>                         __u32      fb_auth[1];           /*     4     4 */
>>                 };                                       /*     0     8 */
>>                 struct {
>>                         __u8       fb_version;           /*     0     1 */
>>                         __u8       fb_auth_type;         /*     1     1 */
>>                         __u8       fb_fsid_type;         /*     2     1 */
>>                         __u8       fb_fileid_type;       /*     3     1 */
>>                         __u32      fb_auth_flex[0];      /*     4     0 */
>>                 };                                       /*     0     4 */
>>         };                                               /*     0     8 */
>>
>>         /* size: 8, cachelines: 1, members: 1 */
>>         /* last cacheline: 8 bytes */
>> };
> 
> Could you use the simpler:
>> struct nfs_fhbase_new {
>>          __u8       fb_version;
>>          __u8       fb_auth_type;
>>          __u8       fb_fsid_type;
>>          __u8       fb_fileid_type;
>>          union {
>>                 __u32      fb_auth[1];
>>                 __u32      fb_auth_flex[0];
>>          };
>> };
> 
> Although I'm not certain flexible arrays are supported
> as the last element of a union.

Nope; this is not allowed: https://godbolt.org/z/14vd4o8na

--
Gustavo