Re: [PATCH 3/6] tap: Cast TAP_BUF_BYTES - ETH_MAX_MTU to ssize_t, not TAP_BUF_BYTES

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

[-- Attachment #1: Type: text/plain, Size: 1739 bytes --]

On Thu, Nov 07, 2024 at 07:43:28PM +0100, Stefano Brivio wrote:
> Given that we're comparing against 'n', which is signed, we cast
> TAP_BUF_BYTES to ssize_t so that the maximum buffer usage, calculated
> as the difference between TAP_BUF_BYTES and ETH_MAX_MTU, will also be
> signed.
> 
> This doesn't necessarily happen on 32-bit architectures, though. On
> armhf and i686, clang-tidy 18.1.8 and 19.1.2 report:
> 
> /home/pi/passt/tap.c:1087:16: error: comparison of integers of different signs: 'ssize_t' (aka 'int') and 'unsigned int' [clang-diagnostic-sign-compare,-warnings-as-errors]
>  1087 |         for (n = 0; n <= (ssize_t)TAP_BUF_BYTES - ETH_MAX_MTU; n += len) {
>       |                     ~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> cast the whole difference to ssize_t, as we know it's going to be
> positive anyway, instead of relying on that side effect.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tap.c b/tap.c
> index f638f2c..a3ba958 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1084,7 +1084,7 @@ static void tap_pasta_input(struct ctx *c, const struct timespec *now)
>  
>  	tap_flush_pools();
>  
> -	for (n = 0; n <= (ssize_t)TAP_BUF_BYTES - ETH_MAX_MTU; n += len) {
> +	for (n = 0; n <= (ssize_t)(TAP_BUF_BYTES - ETH_MAX_MTU); n += len) {
>  		len = read(c->fd_tap, pkt_buf + n, ETH_MAX_MTU);
>  
>  		if (len == 0) {

-- 
David Gibson (he or they)	| 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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]