svn commit: r301572 - in head/lib/libcasper: libcasper services/cap_dns services/cap_grp services/cap_pwd services/cap_random services/cap_sysctl

[Jilles Tjoelker]

On Wed, Jun 08, 2016 at 02:03:53AM +0000, Mariusz Zaborski wrote:
> Author: oshogbo
> Date: Wed Jun  8 02:03:53 2016
> New Revision: 301572
> URL: https://svnweb.freebsd.org/changeset/base/301572

> Log:
>   Add flags to the Casper services.

>   CASPER_SERVICE_STDIO - Casper will not close the first three descriptors (stdin,
>   		       stdout and stderr) this can be helpful for debugging.
>   CASPER_SERVICE_FD - Capser will not close all other descriptors, this can
>   		    be useful for a filesystem service.

This reminds me that there are some common cases where it is wrong to
close descriptors you don't know about. The non-POSIX command
  diff <(cmd1) <(cmd2)
that compares the outputs of the two commands, when executed with bash
that was compiled with the full /dev/fd visible (as in poudriere), will
actually run something like
  diff /dev/fd/63 /dev/fd/62
passing two file descriptors to pipes.

When created by a shell, these pathnames will start with /dev/fd/, but
people could create symlinks to these special files.

> [snip]
> +static void
> +stdnull(void)
> +{
> +	int fd;
> +
> +	fd = open(_PATH_DEVNULL, O_RDWR);
> +	if (fd == -1)
> +		errx(1, "Unable to open %s", _PATH_DEVNULL);
> +
> +	if (setsid() == -1)
> +		errx(1, "Unable to detach from session");

There is an implicit assumption here that stdnull() is only called from
a process that was forked off from here, since setsid() will not and
cannot work when called from a process that is already a session leader.

If the application is running from a shell, this setsid() will exclude
the process from most signals, including terminal ^C/^\/^Z, kill % and
hangups. More generally, this might make it more likely for the process
to hang around indefinitely after the parent is gone.

> +
> +	if (dup2(fd, STDIN_FILENO) == -1)
> +		errx(1, "Unable to cover stdin");
> +	if (dup2(fd, STDOUT_FILENO) == -1)
> +		errx(1, "Unable to cover stdout");
> +	if (dup2(fd, STDERR_FILENO) == -1)
> +		errx(1, "Unable to cover stderr");
> +
> +	close(fd);

This was not broken by this commit, but fd should not be closed if it is
equal to STDIN_FILENO, STDOUT_FILENO or STDERR_FILENO.

> [snip]

-- 
Jilles Tjoelker