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
