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.
You might need to use a named anonymous structure for the
four __u8 fields and create two different structures that
include the extra field on the end.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
