For a given PID, resolve file descriptors in /proc/$PID/fd to their underlying /dev/pts/$X dynamically allocated pty
Usage: ptmx_resolve $PID [ target file descriptor ID]
Elevated privileges are required.
Pseudoterminals in use by $PID are difficult to resolve from a perspective outside of the running program. Within the actual process, these ptys are referenced only by file descriptors. Such file descriptors correspond to a dynamically created path of the actual pseudoterminal, e.g., /dev/pts/12. These descriptors are obtained at runtime by opening /dev/ptmx, implicitly via getpt(3) / posix_openpt(3), or manually via open("/dev/ptmx", O_RDWR | O_NOCTTY). Within a running program, psuedoterminal file descriptors can be passed to ptsname(3) to obtain the corresponding device path, e.g. /dev/pts/12.
Occasionally, one desires to connect to a dynamic pseudoterminal bound by a program. For example, it is often useful for advanced management of virtual machines to connect to the qemu monitor via a serial emulator such as screen or minicom. Unfortunately, from a context external to a running process, there is no mechanism to acquire the allocated pseudoterminal unless the program logs this information or otherwise makes it available. Indeed, the path to a pty is not accessible even via the data shown in /proc/$PID/fd for a running process. Furthermore, inspecting /proc/$PID/fd shows that these file descriptors are pseudo-symlinks to /dev/ptmx, rather than revealing the device path in /dev/pts.
This rather godawful little utility exists to solve this problem.
This utility : 1) finds file descriptors of a process that are pseudotermianl candidates (i.e., they reference /dev/ptmx when the /proc/$PID/fd/$FD symlink is resolved), then 2) attaches to a running process using ptrace() 3) creates a child process that can be used sacrificially to obtain access to the parent's file descriptors 3) injects a system call to obtain the path in /dev/pts, in essence performing the ioctl used internally by ptsname() 4) restores process state and resumes the program
Please forgive me for this abomination. It should not be used in any production context, ever...most especially because of process-thread equivalence and the fairly braindead nature of ptrace().
It does work, however.
That said, if someone finds a better way, please for the love of god let me know what it is.
Copyright 2013 Author Steve Maresca steve@zentific.com
Thanks go to Sam Hocevar et al and neercs for some of the helpful tracing abstraction around ptrace().