Re: [PATCH v3 seccomp 5/5] seccomp/cache: Report cache data through /proc/pid/seccomp_cache

YiFei Zhu
2020-10-01T12:06:48Z

On Wed, Sep 30, 2020 at 5:01 PM Jann Horn <jannh@google.com> wrote:
> Hmm, this won't work, because the task could be exiting, and seccomp
> filters are detached in release_task() (using
> seccomp_filter_release()). And at the moment, seccomp_filter_release()
> just locklessly NULLs out the tsk->seccomp.filter pointer and drops
> the reference.
>
> The locking here is kind of gross, but basically I think you can
> change this code to use lock_task_sighand() / unlock_task_sighand()
> (see the other examples in fs/proc/base.c), and bail out if
> lock_task_sighand() returns NULL. And in seccomp_filter_release(), add
> something like this:
>
> /* We are effectively holding the siglock by not having any sighand. */
> WARN_ON(tsk->sighand != NULL);

Ah thanks. I was thinking about how tasks exit and get freed and that
sort of stuff, and how this would race against them. The last time I
worked with procfs there was some magic going on that I could not
figure out, so I was thinking if some magic will stop the task_struct
from being released, considering it's an argument here.

I just looked at release_task and related functions; looks like it
will, at the end, decrease the reference count of the task_struct.
Does procfs increase the refcount while calling the procfs functions?
Hence, in procfs functions one can rely on the task_struct still being
a task_struct, but any direct effects of release_task may happen while
the procfs functions are running?

YiFei Zhu