Question about LTTng Kernel Probe Tracking

[Amir Najafi Zadeh]

Hello all,

I hope you are doing well.

I have a quick question about LTTng tracking when using kernel probes. I
mainly use LTTng to trace file system functions in the Linux kernel, and
for that I use the LTTng kernel probe feature, which works very well.
However, when I tried filtering logs for a specific group ID, I noticed
that filters are not supported for kernel probes.

Then I attempted to use the tracking feature to trace only a specific GID,
but this also did not work. From what I see in my logs, it appears that for
kernel probes all events are captured and recorded, regardless of the
tracking options.

I wanted to know if this is a known limitation, and whether there is any
in-progress work or reason why this is not currently supported.

For reference, here is my code. I am using LTTng 2.13.11 with kernel
6.8.0-101-generic:

```
#!/bin/sh
SESSION_NAME="lttng-gid-test"
OUTPUT_DIR="/tmp/lttng/${SESSION_NAME}"
GROUP_ID=1002

# create the lttng session
lttng create "$SESSION_NAME" -o "$OUTPUT_DIR"

# create the lttng ring buffer channel
lttng enable-channel --session="$SESSION_NAME" --kernel channel0 \
  --subbuf-size=16M \
  --num-subbuf=2

# add pid, tid, proc, and gid for tracing collection
lttng add-context --session="$SESSION_NAME" --channel=channel0
--kernel --type pid
lttng add-context --session="$SESSION_NAME" --channel=channel0
--kernel --type gid

# enable the target probe
lttng enable-event --session="$SESSION_NAME" --channel=channel0
--kernel --probe=ext4_get_inode_loc ext4_get_inode_loc

# disable all tracks
# NOTE: untracking doesn't make a difference
# lttng untrack --session="$SESSION_NAME" --kernel --all --gid
# lttng untrack --session="$SESSION_NAME" --kernel --all --pid
# lttng untrack --session="$SESSION_NAME" --kernel --all --vpid
# lttng untrack --session="$SESSION_NAME" --kernel --all --uid
# lttng untrack --session="$SESSION_NAME" --kernel --all --vuid
# lttng untrack --session="$SESSION_NAME" --kernel --all --vgid

# only enable gid 1002 track
lttng track --kernel --gid=1002

# start lttng
lttng start $SESSION_NAME

```

Thank you for your time and guidance.

Best regards,
Amir
-- 
*Amirhossein Najafizadeh*



*PhD Student, Computer Science Department, Stony Brook University, N.Y.File
systems and Storage Lab (FSL)*
najafizadeh21 at gmail.com
https://amirhnajafiz.github.io/
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