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Debugging across pipes and sockets with strace

Niklas Hambüchen, FP Complete


Privileged-server / unprivileged-client

  • Server that runs restricted set of commands (ls, dmesg) sent to it via a socket as root
  • Client that sends argv[0] to the server and prints the output


  • ls case works fine
  • dmesg case just hangs without output

Further complication

  • Assume this is hard to reproduce (only happens every 1000th time), so you really want to debug it on the currently hanging system, and not restart any processes.


Command server

#!/usr/bin/env python2

from __future__ import print_function
import socket
import subprocess
from subprocess import PIPE

serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
serversocket.bind(("localhost", 1234))
serversocket.listen(5) continued:

def run_command_for_client(command, clientsocket):
  if command in ["ls", "dmesg"]:
    p = subprocess.Popen([command], stdout=PIPE)
    out =
    out = b"command not allowed\n"
# Server loop
while True:
  (clientsocket, address) = serversocket.accept()

  command = clientsocket.recv(100).decode('utf-8')
  print("server got command: " + command)

  run_command_for_client(command, clientsocket)


Command client

#!/usr/bin/env python2

from __future__ import print_function
import socket
import sys

name = sys.argv[1]

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(("localhost", 1234))


while True:
  data = sock.recv(100)
  if len(data) == 0:

Command wrappers:

#!/usr/bin/env python2

from __future__ import print_function
import subprocess
from subprocess import PIPE
import sys

p = subprocess.Popen(['./', "ls"],

sys.stdout.write(p.communicate()[0]) (same thing with dmesg)

p = subprocess.Popen(['./', "dmesg"], ...

Program communication

   | |
   | | stdout pipe
   | |
    |  TCP socket
   | |
   | | stdout pipe
   | |
dmesg executable


% ./
server got command: ls
server got command: dmesg
% ./
% ./

95% of computer problems can be solved with strace.


strace part

strace (on the process that hangs)

% sudo strace -fp "$(pgrep -f 'unprivileged-dmesg')"
strace: Process 23572 attached

Add -y (prints paths associated with file descriptor arguments):

% sudo strace -fp "$(pgrep -f 'unprivileged-dmesg')" -y
strace: Process 23572 attached

The pipe number it's trying to read from is 139828372.

Let's chase down that pipe.

Chasing pipes with lsof

% lsof -n -P | grep --color '139828372'
python2 23572 .. 3r FIFO 0,12   .. 139828372 pipe
python2 23573 .. 1w FIFO 0,12   .. 139828372 pipe

3r means process 23572 has a read-end of the pipe open as file descriptior 3. See:

% ls -l /proc/23572/fd/3
...            /proc/23572/fd/3 -> pipe:[139828372]

1w means process 23573 has a write-end of the pipe open as file descriptior 1.

So the only possible producer to unblock our read(3, in strace is process is process 23573.

Let's strace the process that has the wipe write end:

% sudo strace -fp 23573 -y
strace: Process 23573 attached

It's blocked reading from a socket.

Let's chase down that socket.

Chasing sockets with lsof

% lsof -n -P | grep --color '139825870'
python2 23573 .. 3u IPv4 139825870 .. TCP> (ESTABLISHED)


Follow that TCP connection (potentially on a another machine, in our case is the same machine):

% lsof -n -P | grep --color '\b1234\b'
python2 23270 .. TCP (LISTEN)
python2 23270 .. TCP> (ESTABLISHED)
python2 23573 .. TCP> (ESTABLISHED)

So process 23270 has the other end of that socket.

Let's strace it.

General appraoch

Find what a process is blocked reading from / writing to with strace -y.

  • If it's a file strace -y will show it inline.
  • If it's a pipe, look up its number in lsof. Find PID that has the other end, strace that one.
  • If it's a socket, look up its number in lsof. Find PID (or host IP) that has the other end, strace that one.

strace'ing the process that has the other end of the TCP socket (PID 23270 is python2 ./

% sudo strace -fp 23270 -y
strace: Process 23270 attached

Blocked on wait4(), so that's probably this code from the server:

    p = subprocess.Popen([name], stdout=PIPE)

We've found the bug, because this is incorrect code, as the Python Popen() documentation says on wait():

This will deadlock when using stdout=PIPE ... and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use Popen.communicate() when using pipes to avoid that.

dmesg creates more output than fits in the pipe buffer.

This was easy to identify because there's only one wait() invocation in the server.

But how would we find the location of the problem if there were hundreds of wait() invocations in the server software, across many files?

In general, after you've chased down the problematic process using a series of straces, how do you find the userspace location issuing the blocking syscall?

95% of computer problems can be solved with strace. For the remaining 4% there's gdb.


GDB part

Inspecting Python with GDB

From this deleted StackOverflow question and here we learn:

Switch to the frame in the stack which has function:

PyEval_EvalFrameEx (or eval_frame)  # For Python < 3

To get the file name:

x/s ((PyStringObject*)f->f_code->co_filename)->ob_sval

To get the function name:

x/s ((PyStringObject*)f->f_code->co_name)->ob_sval

To get the line number:

print f->f_lineno

GDB preparation

If your GDB shows

Reading symbols from /usr/bin/python2.7...
   (no debugging symbols found)...done.

then install debugging symbols, e.g.

sudo apt-get install python2.7-dbg

GDB and blocking syscalls

When a process is blocked on a syscall, GDB drops you into a shell at that syscall, and you can ask for the backtrace to see how the program got there.

GDB run

% sudo gdb -p $(pgrep -f command-server)
Attaching to process 23270

0x00007f61090aff2a in __waitpid (pid=2946, stat_loc=0x7ffcf7e32a5c, options=0) at ../sysdeps/unix/sysv/linux/waitpid.c:29

(gdb) backtrace
#0  0x00007f61090aff2a in __waitpid (pid=2946, ...) 
#1  0x0000000000576dd6 in posix_waitpid.lto_priv ()
#2  0x00000000004c30ce in ext_do_call (...) 
#3  PyEval_EvalFrameEx ()
#4  0x00000000004b9ab6 in PyEval_EvalCodeEx () 
#5  0x00000000004c1e6f in fast_function (...) 
#6  call_function (...)    
#7  PyEval_EvalFrameEx () 
#8  0x00000000004c136f in fast_function (...)

OK, stuck in __waitpid(). We are in stack frame #0. Let's go up to PyEval_EvalFrameEx ...

(gdb) up
#1  0x0000000000576dd6 in posix_waitpid.lto_priv (...)

(gdb) up
#2  0x00000000004c30ce in ext_do_call (...)

(gdb) up
#3  PyEval_EvalFrameEx ()
(gdb) x/s ((PyStringObject*)f->f_code->co_filename)->ob_sval
0x7f6109388eac:	"/usr/lib/python2.7/"

(gdb) print f->f_lineno
$1 = 473

(gdb) x/s ((PyStringObject*)f->f_code->co_name)->ob_sval
0x7f610932ac94:	"_eintr_retry_call"

We're at the bottom of the Python standard library, in a wrapper that loops around the wait() syscall (code of Let's go futher up until we're in our applications's code.

(gdb) up
#4  0x00000000004b9ab6 in PyEval_EvalCodeEx ()
(gdb) up
#5  0x00000000004c1e6f in fast_function (...)
(gdb) up
#6  call_function (...)
(gdb) up
#7  PyEval_EvalFrameEx ()

(gdb) x/s ((PyStringObject*)f->f_code->co_filename)->ob_sval
0x7f610933fa2c: "/usr/lib/python2.7/"

(gdb) up
#8  0x00000000004c136f in fast_function (...)
(gdb) up
#9  call_function ()
(gdb) up
#10 PyEval_EvalFrameEx ()

(gdb) x/s ((PyStringObject*)f->f_code->co_filename)->ob_sval
0x7f61093924b4: "./"

This is the first/lowest stack frame that's in our code.

(gdb) x/s ((PyStringObject*)f->f_code->co_name)->ob_sval
0x7f879c0d4454:	"run_command_for_client"

(gdb) print f->f_lineno
$1 = 13

So we've tracked down the precise location in our python program's userpace:

It's the wait() call in, in the function run_command_for_client() which starts at line 13.


  1. Investigate issues on the running system via syscalls using strace.
  2. Chase through pipes, sockets and across machines, with strace and lsof.
  3. Find the origin of the final syscall in your userspace program using gdb or a similar debugger.

You can do this to debug hard-to-reproduce problems in production, and knowing very little about the programs you are debugging.

System calls are tye universal inspection point on Linux.