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

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Date:   Thu, 25 Mar 2021 16:12:28 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     David Laight <David.Laight@...LAB.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



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].

--
Gustavo

[1] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays