Re: [PATCH v2] perf trace: Fix perf trace tracing itself, creating feedback loops

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Tue, Nov 12, 2024 at 02:07:36PM -0300, Arnaldo Carvalho de Melo wrote:
> On Tue, Oct 29, 2024 at 10:24:31PM -0700, Howard Chu wrote:
> > There exists a pids_filtered map in augmented_raw_syscalls.bpf.c that
> > ceases to provide functionality after the BPF skeleton migration:
> > commit 5e6da6be3082 ("perf trace: Migrate BPF augmentation to use a skeleton")
> 
> Thanks, applied to perf-tools-next,

There is a problem with this patch, I'm investigating it now:

root@number:~# perf trace -e syscalls:sys_enter_exit_group true
perf: Segmentation fault
Obtained 12 stack frames.
perf() [0x6229a1]
perf() [0x622a8b]
/lib64/libc.so.6(+0x40d00) [0x7f704364fd00]
perf() [0x4b5f2e]
perf() [0x4ba3e6]
perf() [0x4c03bc]
perf() [0x4c0663]
perf() [0x4c07bc]
perf() [0x4c0b05]
/lib64/libc.so.6(+0x2a088) [0x7f7043639088]
/lib64/libc.so.6(__libc_start_main+0x8b) [0x7f704363914b]
perf() [0x412265]
Segmentation fault (core dumped)
root@number:~#

(gdb) run trace -e syscalls:sys_enter_exit_group true
Starting program: /root/bin/perf trace -e syscalls:sys_enter_exit_group true

This GDB supports auto-downloading debuginfo from the following URLs:
  <https://debuginfod.fedoraproject.org/>
Enable debuginfod for this session? (y or [n]) y
Debuginfod has been enabled.
To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[Detaching after fork from child process 1965197]

Program received signal SIGSEGV, Segmentation fault.
trace__run (trace=0x7fffffffa5e0, argc=1, argv=0x7fffffffde90) at builtin-trace.c:4330
4330		trace->filter_pids.map = trace->skel->maps.pids_filtered;
(gdb) bt
#0  trace__run (trace=0x7fffffffa5e0, argc=1, argv=0x7fffffffde90) at builtin-trace.c:4330
#1  0x00000000004ba3e6 in cmd_trace (argc=1, argv=0x7fffffffde90) at builtin-trace.c:5490
#2  0x00000000004c03bc in run_builtin (p=0xec4068 <commands+648>, argc=4, argv=0x7fffffffde90) at perf.c:351
#3  0x00000000004c0663 in handle_internal_command (argc=4, argv=0x7fffffffde90) at perf.c:404
#4  0x00000000004c07bc in run_argv (argcp=0x7fffffffdc7c, argv=0x7fffffffdc70) at perf.c:448
#5  0x00000000004c0b05 in main (argc=4, argv=0x7fffffffde90) at perf.c:560
(gdb) p trace->skel
$1 = (struct augmented_raw_syscalls_bpf *) 0x0
(gdb)

I.e. when we specify a syscall event tracepoint, we expect to use
augmentation, but are not setting it up.

So this is seems to be exposing a previous bug, I'll check.

- Arnaldo