How to write to terminal device stdin of running process from a different terminal without any prior preparation? [duplicate]

Question

There seem to be plenty of similar questions, for example:

https://stackoverflow.com/questions/5374255/how-to-write-data-to-existing-processs-stdin-from-external-process

However the answer given is to prepare the program beforehand by making its stdin a pipe.

There's also:

https://serverfault.com/questions/178457/can-i-send-some-text-to-the-stdin-of-an-active-process-running-in-a-screen-sessi

Which is answered by commanding a terminal multiplexer under which the program in question is running. That means it also needs prior preparation.

This question is about how to do this without having done any preparation beforehand.

At first I thought it might be as simple as:

echo foo > /proc/$p/fd/0

but that just writes to the terminal. So then I tried:

echo foo > /proc/$terminal_emulator/fd/$ptmx_fd

but that also fails because it just opens up a new terminal device slave for echo.

I already have an answer using gdb (sigh) which I'll be posting below, but I wonder if anyone knows of a simpler and better alternative.

Answer 1

#!/usr/bin/zsh

pid="$1"
stdin_data="$2"

for p in $(pstree -sp "$pid" | grep -Po '(?<=\()\d+(?=\))' | tac); do
  [[ -r "/proc/$p/fd" ]] || continue
  for f in /proc/$p/fd/*; do
    if [[ "$(readlink "$f")" = /dev/ptmx ]]; then
      fd="$(basename "$f")"
      break 2
    fi
  done
done

sudo gdb -p "$p" --batch -x =(printf '%s' '            
  p (int)write($fd, "$stdin_data", (int)strlen("$stdin_data"))
')

I don't like that the script argument can be a C injection.

Answer 2

Apparently you can't, writing to a process' stdin file doesn't make the process read what you appended. See this answer for a full explanation: https://serverfault.com/a/962956