How to trace just system call events with ftrace without showing any other functions in the Linux kernel?

Question

For example, to monitor all mkdir calls made, the best I could come up with was:

#!/bin/sh
set -eux

d=debug/tracing

mkdir -p debug
if ! mountp         


        

Answer 1

Use the nop tracer

As proposed by sruffell, all we have to do is to use the nop tracer instead of function, and that will disable the function traces, but not the events.

Run with sudo:

#!/bin/sh
set -eux

d=debug/tracing

mkdir -p debug
if ! mountpoint -q debug; then
  mount -t debugfs nodev debug
fi

# Stop tracing.
echo 0 > "${d}/tracing_on"

# Clear previous traces.
echo > "${d}/trace"

# Find the tracer name.
cat "${d}/available_tracers"

# Disable tracing functions, show only system call events.
echo nop > "${d}/current_tracer"

# Find the event name with.
grep mkdir "${d}/available_events"

# Enable tracing mkdir.
# Both statements below seem to do the exact same thing,
# just with different interfaces.
# https://www.kernel.org/doc/html/v4.18/trace/events.html
echo sys_enter_mkdir > "${d}/set_event"
# echo 1 > "${d}/events/syscalls/sys_enter_mkdir/enable"

# Start tracing.
echo 1 > "${d}/tracing_on"

# Generate two mkdir calls by two different processes.
rm -rf /tmp/a /tmp/b
mkdir /tmp/a
mkdir /tmp/b

# View the trace.
cat "${d}/trace"

# Stop tracing.
echo 0 > "${d}/tracing_on"

umount debug

This gives the desired output:

   mkdir-26064 [007] .... 2014370.909743: sys_mkdir(pathname: 7fffbd461c77, mode: 1ff)
   mkdir-26065 [014] .... 2014370.911615: sys_mkdir(pathname: 7ffea53bac77, mode: 1ff)

Alternative not best solution

This also works, but definitely less nice, replace:

echo SyS_mkdir > "${d}/set_ftrace_filter"

with:

echo '*' > "${d}/set_ftrace_notrace"

This turns off all functions as mentioned in the docs: https://www.kernel.org/doc/html/v4.18/trace/ftrace.html#the-file-system

set_ftrace_notrace:

This has an effect opposite to that of set_ftrace_filter. Any function that is added here will not be traced. If a function exists in both set_ftrace_filter and set_ftrace_notrace, the function will not be traced.