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tap: Cast TAP_BUF_BYTES - ETH_MAX_MTU to ssize_t, not TAP_BUF_BYTES

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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>
This commit is contained in:
Stefano Brivio 2024-11-07 18:58:49 +01:00
parent 1feb90fe62
commit 87940f9aa7
1 changed files with 1 additions and 1 deletions
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tap.c
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@ -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) {