linux-hardening - Re: [PATCH] vt: Replace 0-length array with flexible array

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Date:   Mon, 9 Jan 2023 13:03:44 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Helge Deller <deller@....de>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Yangxi Xiang <xyangxi5@...il.com>,
        Xuezhi Zhang <zhangxuezhi1@...lpad.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] vt: Replace 0-length array with flexible array

On 05. 01. 23, 23:05, Kees Cook wrote:
> Zero-length arrays are deprecated[1]. Replace struct uni_screen's
> "lines" 0-length array with a flexible array. Detected with GCC 13,
> using -fstrict-flex-arrays=3:
> 
> ../drivers/tty/vt/vt.c: In function 'vc_uniscr_copy_area':
> ../drivers/tty/vt/vt.c:488:48: warning: array subscript dst_row is outside array bounds of 'char32_t *[0]' {aka 'unsigned int *[]'} [-Warray-bounds=]
>    488 |                 char32_t *dst_line = dst->lines[dst_row];
>        |                                      ~~~~~~~~~~^~~~~~~~~
> ../drivers/tty/vt/vt.c:335:19: note: while referencing 'lines'
>    335 |         char32_t *lines[0];
>        |                   ^~~~~
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jiri Slaby <jirislaby@...nel.org>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: "Ilpo Järvinen" <ilpo.jarvinen@...ux.intel.com>
> Cc: Helge Deller <deller@....de>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>   drivers/tty/vt/vt.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 981d2bfcf9a5..b1445f00f616 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -332,7 +332,7 @@ typedef uint32_t char32_t;
>    * scrolling only implies some pointer shuffling.
>    */
>   struct uni_screen {
> -	char32_t *lines[0];
> +	DECLARE_FLEX_ARRAY(char32_t *, lines);
>   };

Perhaps I am missing something, but why don't we simply have:
   char32_t **uni_screen_lines;
? And even if we don't, we should likely convert to array_size() on the 
alloc site.

And a side note: what's actually the purpose of all that 
NO_VC_UNI_SCREEN and closely connected get_vc_uniscr()?

thanks,
-- 
js
suse labs