Re: [PATCH] tcp: Work around gcc 12 bogus warning in tcp_rtt_dst_check()

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, May 20, 2022 at 11:01:11AM +0200, Stefano Brivio wrote:
> gcc 12.1.x (e.g. current OpenSUSE Tumbleweed, x86_64 only,
> gcc-12-1.4.x86_64) reports:
> 
> tcp.c: In function ‘tcp_send_flag’:
> tcp.c:1014:9: warning: writing 16 bytes into a region of size 0 [-Wstringop-overflow=]
>  1014 |         memcpy(low_rtt_dst + hole++, &conn->a.a6, sizeof(conn->a.a6));
>       |         ^
> tcp.c:559:24: note: at offset -16 into destination object ‘low_rtt_dst’ of size 128
>   559 | static struct in6_addr low_rtt_dst[LOW_RTT_TABLE_SIZE];
>       |
> 
> but 'hole' can't be -1, because the low_rtt_dst table is guaranteed
> to have a hole: if we happened to write to the last entry, we'll go
> back to index 0 and clear that one.
> 
> Signed-off-by: Stefano Brivio <sbrivio(a)redhat.com>

Reviewed-by: David Gibson <david(a)gibson.dropbear.id.au>

> ---
>  tcp.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/tcp.c b/tcp.c
> index e68409a..53af3db 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1011,6 +1011,12 @@ static void tcp_rtt_dst_check(const struct tcp_conn *conn,
>  			hole = i;
>  	}
>  
> +	/* Keep gcc 12 happy: this won't actually happen because the table is
> +	 * guaranteed to have a hole, see the second memcpy() below.
> +	 */
> +	if (hole == -1)
> +		return;
> +
>  	memcpy(low_rtt_dst + hole++, &conn->a.a6, sizeof(conn->a.a6));
>  	if (hole == LOW_RTT_TABLE_SIZE)
>  		hole = 0;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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