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

[Paolo Abeni]

On 3/26/26 5:06 AM, fffsqian@xxxxxxx wrote:
> From: Qingshuang Fu <fuqingshuang@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> ---
>  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}

Apparently now the CI test is consistently failing:

# selftests: net: broadcast_pmtu.sh
# 0.03 [+0.03] Testing for broadcast route MTU
not ok 1 selftests: net: broadcast_pmtu.sh # exit=1

/P