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

[Stefano Brivio <sbrivio@redhat.com>]

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>
---
 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) {
-- 
@@ -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) {
-- 
2.43.0