Observed issue
==============
A segmentation fault is observed for multiple UST timeout scenarios.
Backtrace:
#0 __memmove_avx_unaligned_erms () at ../sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S:384
#1 0x0000557fe0395df9 in copy_filter_bytecode (orig_f=0x7f9c5802b790) at ust-app.c:1196
#2 0x0000557fe0397702 in shadow_copy_event (ua_event=0x7f9c58025ff0, uevent=0x7f9c58033560) at ust-app.c:1824
#3 0x0000557fe039ac46 in create_ust_app_event (ua_sess=0x7f9c5802ec20, ua_chan=0x7f9c58025cc0, uevent=0x7f9c58033560, app=0x7f9c5c001da0) at ust-app.c:3192
#4 0x0000557fe03a054d in ust_app_channel_synchronize_event (ua_chan=0x7f9c58025cc0, uevent=0x7f9c58033560, ua_sess=0x7f9c5802ec20, app=0x7f9c5c001da0) at ust-app.c:5096
#5 0x0000557fe03a0772 in ust_app_synchronize (usess=0x7f9c580074a0, app=0x7f9c5c001da0) at ust-app.c:5173
#6 0x0000557fe03a0a70 in ust_app_global_update (usess=0x7f9c580074a0, app=0x7f9c5c001da0) at ust-app.c:5255
#7 0x0000557fe03a00e0 in ust_app_start_trace_all (usess=0x7f9c580074a0) at ust-app.c:4987
#8 0x0000557fe0355c6a in cmd_start_trace (session=0x7f9c5800a190) at cmd.c:2668
#9 0x0000557fe0382e70 in process_client_msg (cmd_ctx=0x7f9c58003d70, sock=0x7f9c74bf44e0, sock_error=0x7f9c74bf44e4) at client.c:1527
#10 0x0000557fe03848a2 in thread_manage_clients (data=0x557fe06d9440) at client.c:2200
#11 0x0000557fe037d1cb in launch_thread (data=0x557fe06d94b0) at thread.c:75
#12 0x00007f9c796af609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#13 0x00007f9c795b6293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
The scenario:
# Start an instrumented app
./app
gdb lttng-sessiond
# put a breakpoint on ustctl_set_filter
lttng create my_session
lttng enable-event -u tp:tp_test
lttng start
lttng enable-event -u __dummy --filter 'my_field == "user34"'
# The tracepoint should hit. Do not continue.
kill -s SIGSTOP $(pgrep app)
# Continue lttng-sessiond.
# enable-event will return an error. This a bug in itself, still let's
# continue with the current bug.
lttng stop
# Start a new app that will register.
./app &
sleep 1
lttng start
# lttng-sessiond should segfault.
Cause
=====
During the "lttng enable-event" command, the timeout error bubbles up
all the way to event_ust_enable_tracepoint and is different from
LTTNG_UST_ERR_EXIST. `trace_ust_destroy_event` is called and frees the
`uevent` object. Note that contrary to the comment `uevent` is added to
the channel event hash table at this point.
On the next `lttng start` command, the event node is still present in
the hash table and is iterated on. lttng-sessiond segfault on the first
data access of the previously freed memory.
The problem was introduced by commit
88e3c2f5610b9ac89b0923d448fee34140fc46fb [1]. Which essentially move the
callsite of `add_unique_ust_event` before `ust_app_*_event_glb` calls.
Solution
========
Go to `end` label to prevent freeing of the uevent object.
Note that app synchronization should not force an error at the channel
level, since a single app can fail but the whole channel should not.
The `error` label is now obsolete.
Known drawbacks
=========
None.
References
==========
[1] https://github.com/lttng/lttng-tools/commit/
88e3c2f5610b9ac89b0923d448fee34140fc46fb
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: Ifaf3f4c71bb2da869c7b441aaa4b367f8f7cbdd6
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>