linux-kernel - Re: [PATCH] livepatch: Skip livepatch tests if ftrace cannot be configured

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Date:   Fri, 4 Feb 2022 15:30:02 -0500
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     David Vernet <void@...ifault.com>, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        pmladek@...e.com, jikos@...nel.org, mbenes@...e.cz, corbet@....net
Cc:     kernel-team@...com
Subject: Re: [PATCH] livepatch: Skip livepatch tests if ftrace cannot be
 configured

On 2/3/22 6:32 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@...ifault.com>

Hi David,

Thanks for this test case, comments below...

> ---
>  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..6857fdcb6b45 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 permament.

sp: s/permament/permanent

> +	if [[ "$result" == *"Device or resource busy"* ]]; then
> +		skip "failed to set kernel.ftrace_enabled=$1"
> +	fi
> +

style nit: move the blank line from here to just before the new # Skip
comment

>  }
>  
>  function cleanup() {
> 

Can we be more paranoid and just look for the exact result that we expect:

if [[ "$result" != "kernel.ftrace_enabled = 1" ]]; then
	skip "failed to set kernel.ftrace_enabled=$1"
fi

and that way we catch any other faults.  What do you think?

Thanks,
-- 
Joe