[PATCH] selftests: net: broadcast_pmtu: Fix false failure from incorrect ping exit code logic

[fffsqian@163.com]

From: Qingshuang Fu <fuqingshuang@kylinos.cn>

The broadcast_pmtu.sh test verifies that broadcast route MTU is respected,
but it uses an incorrect criteria for test success: it relies solely on
the ping command's exit code, which leads to false failures.

When the kernel correctly blocks oversized broadcast packets due to the
configured MTU limit (1500), ping fails to transmit any packets and returns
exit code 1. The original script interprets this as a test failure, even
though the kernel is properly enforcing the MTU (the core goal of the test).

Fix this by:
1. Checking ping's output for clear signs of MTU enforcement (instead of exit code)
   - Match "0 packets transmitted" (no packets sent due to MTU limit)
   - Match "message too long" (kernel rejects oversized packets)
2. Use an exit code variable to avoid early exit breaking the script flow
3. Maintain full compatibility with the original setup/cleanup logic

After this fix, the script returns 0 (success) when the kernel enforces the
broadcast route MTU (packets blocked), and 1 (failure) only when the kernel
fails to enforce the MTU (packets are transmitted but no response is received).

Signed-off-by: Qingshuang Fu <fuqingshuang@kylinos.cn>
---
 tools/testing/selftests/net/broadcast_pmtu.sh | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/net/broadcast_pmtu.sh b/tools/testing/selftests/net/broadcast_pmtu.sh
index 726eb5d25839..a8d2a205c51d 100755
--- a/tools/testing/selftests/net/broadcast_pmtu.sh
+++ b/tools/testing/selftests/net/broadcast_pmtu.sh
@@ -41,7 +41,13 @@ trap cleanup EXIT
 
 setup &&
 	echo "Testing for broadcast route MTU" &&
-	ip net exec "${CLIENT_NS}" ping -f -M want -q -c 1 -s 8000 -w 1 -b "${CLIENT_BROADCAST_ADDRESS}" > /dev/null 2>&1
-
-exit $?
+	ping_output=$(ip net exec "${CLIENT_NS}" ping -f -M want -c 1 -s 8000 -w 1 -b \
+		"${CLIENT_BROADCAST_ADDRESS}" 2>&1) &&
+	if echo "${ping_output}" | grep -q -E "0 packets transmitted|message too long"; then
+		exit_code=0
+	else
+		exit_code=1
+	fi
+
+exit ${exit_code}
 
-- 
2.25.1