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

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Message-ID: <e516146427db45439c02afe57ce06e97@AcuMS.aculab.com>
Date:   Fri, 26 Mar 2021 08:17:13 +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 21:12
> 
> On 3/25/21 10:29, David Laight wrote:
> 
> >>>
> >>> 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.
> 
> We don't want to introduce zero-length arrays [1].

I probably meant to write [] not [0] - doesn't affect the idea.

The real problem is that the compiler is likely to start rejecting
references to a flex array that go beyond the end of the outer
structure.

Thinking back, isn't fb_auth[] at least one entry long?
So it could be:

struct nfs_fhbase_new {
         __u8       fb_version;
         __u8       fb_auth_type;
         __u8       fb_fsid_type;
         __u8       fb_fileid_type;
         __u32      fb_auth[1];
         __u32      fb_auth_extra[];
};

(I've missed the 0 out this time...)

	David

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