From: Gustavo A. R. Silva
> Sent: 25 March 2021 13:18
>
> 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
Nothing an extra 'struct {__u32 fb_auth_flex[0]; }'; won't solve.
David
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