pmalmgren / launch-buddies

Launch a pair of programs in order

Launch Buddies

A program that launches two other programs in order. I use it to run BCC tools on short-lived processes, most of which accept a -p argument but do not support launching commands like perf does.

How it works

The first program gets launched with fork(). After fork() returns, the child process performs a blocking read on a pipe created by the parent.

The second program also gets launched with fork() and immediately calls exec(). The parent process sleeps for 5 seconds and then sends data on a pipe to the first process. Once the first process reads data from the pipe, it calls exec().

The parent waits for its children. After the first child exits, the parent will send a signal (currently SIGINT) to the second process to kill that one.

Compiling

Make sure you have gcc and make installed. Then run:

$ make

Alternatives

If you use bpftrace, you can just filter by comm (process name) and launch your program in another window.

$ sudo bpftrace -e 'kprobe:vfs_read /comm=="ls"/ { printf("vs_read called from %d\n", pid); }'

Sometimes programs will offer a -n where you can filter by process name.

Using

The general use is:

$ lb [cmd1] [cmd2] [pid-arg-flag]

Here's how you could use it to launch ls and profile-bpfcc.

$ sudo lb "ls -lah" "profile-bpfcc" "-p "
Child launched with pid 483635
Profiler launched with pid 483636
Parent sleeping while profiler launches...
launching profile-bpfcc -F 999 -p 483635
Sampling at 999 Hertz of PID 483635 by user + kernel stack... Hit Ctrl-C to end.
Received ?, launching child...
total 88K
drwxrwxr-x  3 petermalmgren petermalmgren 4.0K Nov 20 18:07 .
drwxr-xr-x 24 petermalmgren petermalmgren 4.0K Nov 20 17:32 ..
drwxrwxr-x  8 petermalmgren petermalmgren 4.0K Nov 20 17:35 .git
-rw-rw-r--  1 petermalmgren petermalmgren  430 Nov 20 17:35 .gitignore
-rwxrwxr-x  1 petermalmgren petermalmgren  18K Nov 20 17:48 lb
-rw-rw-r--  1 petermalmgren petermalmgren 5.2K Nov 20 17:30 lb.c
-rw-rw-r--  1 petermalmgren petermalmgren  35K Nov 20 17:35 LICENSE
-rw-rw-r--  1 petermalmgren petermalmgren   77 Nov 20 17:48 Makefile
-rw-rw-r--  1 petermalmgren petermalmgren 1.2K Nov 20 18:01 README.md
Child finished, killing profiler...

    b'apparmor_file_free_security'
    b'apparmor_file_free_security'
    b'security_file_free'
    b'__fput'
    b'____fput'
    b'task_work_run'
    b'exit_to_usermode_loop'
    b'__prepare_exit_to_usermode'
    b'__syscall_return_slowpath'
    b'do_syscall_64'
    b'entry_SYSCALL_64_after_hwframe'
    [unknown]
    -                ls (483635)
        1

Profiler finished, exiting...