Preventing file descriptor leak to execl'd processes

Preventing file descriptor leak to execl'd processes

[Marius Bakke]

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Hello Guilers,

I recently had the insa^W bright idea of making my login shell a Guile
script instead of fiddling so much with bashrc and the likes.

For example, here is how I start my window manager when logging into
TTY2 or TTY3:

$ cat /tmp/test-shell
#!/gnu/store/18hp7flyb3yid3yp49i6qcdq0sbi5l1n-guile-3.0.2/bin/guile --no-auto-compile
!#
(let ((bash "/gnu/store/87kif0bpf0anwbsaw0jvg8fyciw4sz67-bash-5.0.16/bin/bash")
      (sway "/gnu/store/9zffdhn3yrfim36ya1g0dwbj012pjk2p-sway-1.5.1/bin/sway")
      (tty (readlink "/proc/self/fd/0"))
      (args (cdr (program-arguments))))
  (if (and (string-prefix? "/dev/tty" tty)
           (or (string-suffix? "2" tty) (string-suffix? "3" tty))
           (not (getenv "DISPLAY")))
      (execl bash bash "--login" "-c"
                  (string-append "exec sway --config /etc/sway/config"))
      (apply execl bash bash args)))

It works great, except that the script filename (/tmp/test-shell) has
an open file descriptor which leaks into the new process:

$ ls -l /proc/self/fd
lrwx------ 1 marius marius 64 Mar  6 17:41 0 -> /dev/pts/18
lrwx------ 1 marius marius 64 Mar  6 17:41 1 -> /dev/pts/18
lrwx------ 1 marius marius 64 Mar  6 17:41 2 -> /dev/pts/18
lr-x------ 1 marius marius 64 Mar  6 17:41 3 -> /proc/9940/fd

$ /tmp/test-shell -c 'ls -l /proc/self/fd'
lrwx------ 1 marius marius 64 Mar  6 17:41 0 -> /dev/pts/18
lrwx------ 1 marius marius 64 Mar  6 17:41 1 -> /dev/pts/18
lrwx------ 1 marius marius 64 Mar  6 17:41 2 -> /dev/pts/18
lr-x------ 1 marius marius 64 Mar  6 17:41 3 -> /proc/9951/fd
lr-x------ 1 marius marius 64 Mar  6 17:41 7 -> /tmp/test-shell

I've managed to work around it by setting FD_CLOEXEC on it:

  (port-for-each (lambda (port)
                   (let ((name (port-filename port))
                         (self (car (program-arguments))))
                     (when (and name (string=? name self))
                       (fcntl port F_SETFD (logior FD_CLOEXEC
                                                   (fcntl port F_GETFD)))))))

But it seems heavy-handed.  Is there an easier way to access the "script
port"?  Perhaps Guile itself should make it FD_CLOEXEC by default?

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Re: Preventing file descriptor leak to execl'd processes

[Maxime Devos]

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On Sat, 2021-03-06 at 17:55 +0100, Marius Bakke wrote:
> Hello Guilers,
> 
> [...]
> 
> It works great, except that the script filename (/tmp/test-shell) has
> an open file descriptor which leaks into the new process:
> 
> [...]
> 
> I've managed to work around it by setting FD_CLOEXEC on it:
> 
> [code using port-for-each and port-filename]
> 
> But it seems heavy-handed.  Is there an easier way to access the "script
> port"?  Perhaps Guile itself should make it FD_CLOEXEC by default?

Easy way to access the ‘script port’: the Scheme procedure current-load-port.
Take a look at the output of the attached script.

Greetings,
Maxime

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#!/gnu/store/m5iprcg6pb5ch86r9agmqwd8v6kp7999-guile-3.0.5/bin/guile --no-auto-compile
!#
(eval-when (expand)
  (pk 'expand (current-load-port) (fileno (current-load-port))))
(eval-when (load)
  (pk 'load (current-load-port) (fileno (current-load-port))))
(eval-when (eval)
  (pk 'eval (current-load-port) (fileno (current-load-port))))
(eval-when (compile)
  (pk 'compile (current-load-port) (fileno (current-load-port))))

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Re: Preventing file descriptor leak to execl'd processes

[Andy Wingo]

On Sat 06 Mar 2021 17:55, Marius Bakke <marius@gnu.org> writes:

> $ ls -l /proc/self/fd
> lrwx------ 1 marius marius 64 Mar  6 17:41 0 -> /dev/pts/18
> lrwx------ 1 marius marius 64 Mar  6 17:41 1 -> /dev/pts/18
> lrwx------ 1 marius marius 64 Mar  6 17:41 2 -> /dev/pts/18
> lr-x------ 1 marius marius 64 Mar  6 17:41 3 -> /proc/9940/fd
>
> $ /tmp/test-shell -c 'ls -l /proc/self/fd'
> lrwx------ 1 marius marius 64 Mar  6 17:41 0 -> /dev/pts/18
> lrwx------ 1 marius marius 64 Mar  6 17:41 1 -> /dev/pts/18
> lrwx------ 1 marius marius 64 Mar  6 17:41 2 -> /dev/pts/18
> lr-x------ 1 marius marius 64 Mar  6 17:41 3 -> /proc/9951/fd
> lr-x------ 1 marius marius 64 Mar  6 17:41 7 -> /tmp/test-shell
>
> I've managed to work around it by setting FD_CLOEXEC on it:
>
>   (port-for-each (lambda (port)
>                    (let ((name (port-filename port))
>                          (self (car (program-arguments))))
>                      (when (and name (string=? name self))
>                        (fcntl port F_SETFD (logior FD_CLOEXEC
>                                                    (fcntl port F_GETFD)))))))
>
> But it seems heavy-handed.  Is there an easier way to access the "script
> port"?  Perhaps Guile itself should make it FD_CLOEXEC by default?

I think Guile itself should make the load port FD_CLOEXEC / O_CLOEXEC.
More broadly there are a number of file descriptors in Guile that should
be cloexec but aren't yet.  Want to make a patch for the load port ? :)

Andy