README.md

getenv

When processes update their own environment variables while running, these changes will usually not be reflected in /proc/<pid>/env. This program will attach to a running process, inject code that runs getenv('VAR'), and print the result; this way, variables can be read live.

WARNING: Due to environment variables being handled by the program itself and not the kernel, this will only work for programs that use libc, and only ones using the same version of libc as getenv is linked to.

Usage

You can compile the code with make. You should see that it builds an executable called getenv. Invoke it like this:

getenv -p <pid> -e <envvar>

An easy way to test this is to open two terminals, run echo $$ in the first terminal to get the pid of the shell, and then in the other terminal run getenv with the first shell's pid.

The output of this call will be the content of the corresponding environment variable, or empty if it's not set.

Issues With Yama ptrace_scope

If you get a failure like this:

$ ./getenv -p 1 -e var
PTRACE_ATTACH: Operation not permitted

then you are trying to trace a process that you don't have permissions to trace, i.e. a process with a different user id than you. You can only ptrace a process whose effective user id is the same as yours (or if you are root).

If you instead get a failure like this:

$ ./getenv -p 5603 -e var
PTRACE_ATTACH: Operation not permitted

The likely cause of this failure is that your system has kernel.yama.ptrace_scope = 1
If you would like to disable Yama, you can run: sudo sysctl kernel.yama.ptrace_scope=0

Then the issue is that you have Yama ptrace_scope configured to disallow ptrace. In particular, the default behavior of Ubuntu since Ubuntu 10.10 has been to set kernel.yama.ptrace_scope = 1. If this affects you, you can either run getenv as root, or you can run the command listed in the error message to disable the Yama setting.

Original work

This project is based on https://github.com/eklitzke/ptrace-call-userspace, a POC for calling userspace functions using ptrace. I used most of the code from there to go from a POC to a program solving an actual use case.