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Date:   Thu, 25 Mar 2021 15:29:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Gustavo A. R. Silva'" <gustavo@...eddedor.com>,
        "'Gustavo A. R. Silva'" <gustavoars@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        "Chuck Lever" <chuck.lever@...cle.com>
CC:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: RE: [PATCH][next] UAPI: nfsfh.h: Replace one-element array with
 flexible-array member

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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