linux-hardening - Re: [PATCH] ipv6: ioam: Replace 0-length array with flexible array

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Date:   Fri, 6 Jan 2023 10:03:27 -0600
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Justin Iurman <justin.iurman@...ege.be>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] ipv6: ioam: Replace 0-length array with flexible array

On Thu, Jan 05, 2023 at 02:21:16PM -0800, Kees Cook wrote:
> Zero-length arrays are deprecated[1]. Replace struct ioam6_trace_hdr's
> "data" 0-length array with a flexible array. Detected with GCC 13,
> using -fstrict-flex-arrays=3:
> 
> net/ipv6/ioam6_iptunnel.c: In function 'ioam6_build_state':
> net/ipv6/ioam6_iptunnel.c:194:37: warning: array subscript <unknown> is outside array bounds of '__u8[0]' {aka 'unsigned char[]'} [-Warray-bounds=]
>   194 |                 tuninfo->traceh.data[trace->remlen * 4] = IPV6_TLV_PADN;
>       |                 ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
> In file included from include/linux/ioam6.h:11,
>                  from net/ipv6/ioam6_iptunnel.c:13:
> include/uapi/linux/ioam6.h:130:17: note: while referencing 'data'
>   130 |         __u8    data[0];
>       |                 ^~~~
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays
> 
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Justin Iurman <justin.iurman@...ege.be>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: netdev@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks!
--
Gustavo

> ---
>  include/uapi/linux/ioam6.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/ioam6.h b/include/uapi/linux/ioam6.h
> index ac4de376f0ce..8f72b24fefb3 100644
> --- a/include/uapi/linux/ioam6.h
> +++ b/include/uapi/linux/ioam6.h
> @@ -127,7 +127,7 @@ struct ioam6_trace_hdr {
>  #endif
>  
>  #define IOAM6_TRACE_DATA_SIZE_MAX 244
> -	__u8	data[0];
> +	__u8	data[];
>  } __attribute__((packed));
>  
>  #endif /* _UAPI_LINUX_IOAM6_H */
> -- 
> 2.34.1
>