linux-hardening - Re: [PATCH][next] readdir: Replace one-element arrays with flexible-array members

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Date:   Tue, 20 Jun 2023 11:42:50 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Christian Brauner <brauner@...nel.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] readdir: Replace one-element arrays with
 flexible-array members

On Tue, Jun 20, 2023 at 11:30:36AM -0600, Gustavo A. R. Silva wrote:
> One-element arrays are deprecated, and we are replacing them with flexible
> array members instead. So, replace one-element arrays with flexible-array
> members in multiple structures.
> 
> Address the following -Wstringop-overflow warnings seen when built
> m68k architecture with m5307c3_defconfig configuration:
> In function '__put_user_fn',
>     inlined from 'fillonedir' at fs/readdir.c:170:2:
> include/asm-generic/uaccess.h:49:35: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
>    49 |                 *(u8 __force *)to = *(u8 *)from;
>       |                 ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
> fs/readdir.c: In function 'fillonedir':
> fs/readdir.c:134:25: note: at offset 1 into destination object 'd_name' of size 1
>   134 |         char            d_name[1];
>       |                         ^~~~~~
> In function '__put_user_fn',
>     inlined from 'filldir' at fs/readdir.c:257:2:
> include/asm-generic/uaccess.h:49:35: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
>    49 |                 *(u8 __force *)to = *(u8 *)from;
>       |                 ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
> fs/readdir.c: In function 'filldir':
> fs/readdir.c:211:25: note: at offset 1 into destination object 'd_name' of size 1
>   211 |         char            d_name[1];
>       |                         ^~~~~~
> 
> This helps with the ongoing efforts to globally enable
> -Wstringop-overflow.
> 
> This results in no differences in binary output.
> 
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/312
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks! We're getting closer every day to killing this code pattern
for good. :)

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook