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Subject: Re: [PATCH v2] livepatch: Skip livepatch tests if ftrace cannot be
 configured
To:     David Vernet <void@manifault.com>, live-patching@vger.kernel.org,
        linux-kernel@vger.kernel.org, jpoimboe@redhat.com,
        pmladek@suse.com, jikos@kernel.org, mbenes@suse.cz, corbet@lwn.net
Cc:     kernel-team@fb.com
References: <20220203233205.1554034-1-void@manifault.com>
 <20220204205625.2628328-1-void@manifault.com>
From:   Joe Lawrence <joe.lawrence@redhat.com>
Message-ID: <89a430e9-6da4-761a-68aa-187a3faa9c53@redhat.com>
Date:   Tue, 8 Feb 2022 10:45:02 -0500
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On 2/4/22 3:56 PM, David Vernet wrote:
> livepatch has a set of selftests that are used to validate the behavior of
> the livepatching subsystem.  One of the testcases in the livepatch
> testsuite is test-ftrace.sh, which among other things, validates that
> livepatching gracefully fails when ftrace is disabled.  In the event that
> ftrace cannot be disabled using 'sysctl kernel.ftrace_enabled=0', the test
> will fail later due to it unexpectedly successfully loading the
> test_klp_livepatch module.
> 
> While the livepatch selftests are careful to remove any of the livepatch
> test modules between testcases to avoid this situation, ftrace may still
> fail to be disabled if another trace is active on the system that was
> enabled with FTRACE_OPS_FL_PERMANENT.  For example, any active BPF programs
> that use trampolines will cause this test to fail due to the trampoline
> being implemented with register_ftrace_direct().  The following is an
> example of such a trace:
> 
> tcp_drop (1) R I D      tramp: ftrace_regs_caller+0x0/0x58
> (call_direct_funcs+0x0/0x30)
>         direct-->bpf_trampoline_6442550536_0+0x0/0x1000
> 
> In order to make the test more resilient to system state that is out of its
> control, this patch adds a check to set_ftrace_enabled() to skip the tests
> if the sysctl invocation fails.
> 
> Signed-off-by: David Vernet <void@manifault.com>
> ---
> v2:
>   - Fix typo in newly added comment (s/permament/permanent).
>   - Adjust the location of the added newline to be before the new comment
>     rather than that the end of the function.
>   - Make the failure-path check a bit less brittle by checking for the
>     exact expected string, rather than specifically for "Device or resource
>     busy".
> 
>  tools/testing/selftests/livepatch/functions.sh | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/tools/testing/selftests/livepatch/functions.sh b/tools/testing/selftests/livepatch/functions.sh
> index 846c7ed71556..32970324dd7e 100644
> --- a/tools/testing/selftests/livepatch/functions.sh
> +++ b/tools/testing/selftests/livepatch/functions.sh
> @@ -78,6 +78,12 @@ function set_ftrace_enabled() {
>  	result=$(sysctl -q kernel.ftrace_enabled="$1" 2>&1 && \
>  		 sysctl kernel.ftrace_enabled 2>&1)
>  	echo "livepatch: $result" > /dev/kmsg
> +
> +	# Skip the test if ftrace is busy.  This can happen under normal system
> +	# conditions if a trace is marked as permanent.
> +	if [[ "$result" != "kernel.ftrace_enabled = $1" ]]; then
> +		skip "failed to set kernel.ftrace_enabled=$1"
> +	fi
>  }
>  
>  function cleanup() {
> 

Thanks for updating.

Acked-by: Joe Lawrence <joe.lawrence@redhat.com>

-- 
Joe