linux-kernel - Re: [PATCH v3 5/5] selftests/ftrace: Update fprobe test to check enabled

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Message-ID: <20250226105028.16208A53-hca@linux.ibm.com>
Date: Wed, 26 Feb 2025 11:50:28 +0100
From: Heiko Carstens <hca@...ux.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sven Schnelle <svens@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Subject: Re: [PATCH v3 5/5] selftests/ftrace: Update fprobe test to check
 enabled_functions file

On Thu, Feb 20, 2025 at 03:20:14PM -0500, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
> 
> A few bugs were found in the fprobe accounting logic along with it using
> the function graph infrastructure. Update the fprobe selftest to catch
> those bugs in case they or something similar shows up in the future.
> 
> The test now checks the enabled_functions file which shows all the
> functions attached to ftrace or fgraph. When enabling a fprobe, make sure
> that its corresponding function is also added to that file. Also add two
> more fprobes to enable to make sure that the fprobe logic works properly
> with multiple probes.
> 
> Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Tested-by: Heiko Carstens <hca@...ux.ibm.com>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  .../test.d/dynevent/add_remove_fprobe.tc      | 54 +++++++++++++++++++
>  1 file changed, 54 insertions(+)
> 
> diff --git a/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc b/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc
> index dc25bcf4f9e2..449f9d8be746 100644
> --- a/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc
> +++ b/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc
> @@ -7,12 +7,38 @@ echo 0 > events/enable
>  echo > dynamic_events
>  
>  PLACE=$FUNCTION_FORK
> +PLACE2="kmem_cache_free"
> +PLACE3="schedule_timeout"
>  
>  echo "f:myevent1 $PLACE" >> dynamic_events
> +
> +# Make sure the event is attached and is the only one
> +grep -q $PLACE enabled_functions
> +cnt=`cat enabled_functions | wc -l`
> +if [ $cnt -ne 1 ]; then
> +	exit_fail
> +fi

Bah.. :) this doesn't work always, since at least with Fedora 41 the
assumption that there are zero enabled functions before this test is
executed is not necessarily true:

# cat tracing/enabled_functions 
free_user_ns (1) R         
bpf_lsm_path_mkdir (1) R   D   M        tramp: ftrace_regs_caller+0x0/0x68 (call_direct_funcs+0x0/0x20)
        direct-->bpf_trampoline_6442505669+0x0/0x148
bpf_lsm_path_mknod (1) R   D   M        tramp: ftrace_regs_caller+0x0/0x68 (call_direct_funcs+0x0/0x20)
        direct-->bpf_trampoline_6442505671+0x0/0x14e
...

I didn't stumble across this before, since I tried a monolithic kernel
without modules when verifying your series; and then there aren't any
enabled functions. But with modules there are.

This could be worked around for example with something like the patch
below (against linux-next). But no idea what your preferred way to
handle this would be.

diff --git a/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc b/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc
index 449f9d8be746..b0f24c57b8e1 100644
--- a/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc
+++ b/tools/testing/selftests/ftrace/test.d/dynevent/add_remove_fprobe.tc
@@ -10,12 +10,14 @@ PLACE=$FUNCTION_FORK
 PLACE2="kmem_cache_free"
 PLACE3="schedule_timeout"
 
+ocnt=`cat enabled_functions | wc -l`
+
 echo "f:myevent1 $PLACE" >> dynamic_events
 
 # Make sure the event is attached and is the only one
 grep -q $PLACE enabled_functions
 cnt=`cat enabled_functions | wc -l`
-if [ $cnt -ne 1 ]; then
+if [ $cnt -ne $((ocnt + 1)) ]; then
 	exit_fail
 fi
 
@@ -23,7 +25,7 @@ echo "f:myevent2 $PLACE%return" >> dynamic_events
 
 # It should till be the only attached function
 cnt=`cat enabled_functions | wc -l`
-if [ $cnt -ne 1 ]; then
+if [ $cnt -ne $((ocnt + 1)) ]; then
 	exit_fail
 fi
 
@@ -32,7 +34,7 @@ echo "f:myevent3 $PLACE2" >> dynamic_events
 
 grep -q $PLACE2 enabled_functions
 cnt=`cat enabled_functions | wc -l`
-if [ $cnt -ne 2 ]; then
+if [ $cnt -ne $((ocnt + 2)) ]; then
 	exit_fail
 fi
 
@@ -49,7 +51,7 @@ grep -q myevent1 dynamic_events
 
 # should still have 2 left
 cnt=`cat enabled_functions | wc -l`
-if [ $cnt -ne 2 ]; then
+if [ $cnt -ne $((ocnt + 2)) ]; then
 	exit_fail
 fi
 
@@ -57,7 +59,7 @@ echo > dynamic_events
 
 # Should have none left
 cnt=`cat enabled_functions | wc -l`
-if [ $cnt -ne 0 ]; then
+if [ $cnt -ne $ocnt ]; then
 	exit_fail
 fi
 
@@ -65,7 +67,7 @@ echo "f:myevent4 $PLACE" >> dynamic_events
 
 # Should only have one enabled
 cnt=`cat enabled_functions | wc -l`
-if [ $cnt -ne 1 ]; then
+if [ $cnt -ne $((ocnt + 1)) ]; then
 	exit_fail
 fi
 
@@ -73,7 +75,7 @@ echo > dynamic_events
 
 # Should have none left
 cnt=`cat enabled_functions | wc -l`
-if [ $cnt -ne 0 ]; then
+if [ $cnt -ne $ocnt ]; then
 	exit_fail
 fi