stdhack

fair warning: late night coding, comes with no guarantee, etc...

stdhack runs a child process with standard I/O descriptors redirected through UNIX pipes.

Motivation

For some reason, OpenVZ won't let chrooted processes access standard file descriptors (stdin, stdout and stderr) from the /proc interface.

In particular, the following are unavailable:

/dev/stdin  -> /proc/self/fd/0
/dev/stdout -> /proc/self/fd/1
/dev/stderr -> /proc/self/fd/2

Typical behaviour (from within a chroot environment on OpenVZ):

% echo "test" > /dev/stdout
bash: /dev/stdout: Permission denied

% echo "test" | sudo tee /dev/stdout
tee: /dev/stdout: Permission denied
test

This causes all kinds of integration problems with various applications (for instance, /dev/stdout is currently the only way to have nginx log to its standard output, and not use log files).

Examples

% stdhack sh -c 'ls -l /proc/$$/fd'
total 0
lr-x------ 1 oprs oprs 64 May 28 02:21 0 -> pipe:[15847754]
l-wx------ 1 oprs oprs 64 May 28 02:21 1 -> pipe:[15847755]
l-wx------ 1 oprs oprs 64 May 28 02:21 2 -> pipe:[15847756]

% stdhack ps
  PID TTY          TIME CMD
11503 pts/13   00:00:01 bash
19849 pts/13   00:00:00 stdhack
19850 pts/13   00:00:00 ps