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

YiFei Zhu
2020-10-01T16:19:14Z

On Thu, Oct 1, 2020 at 11:05 AM Jann Horn <jannh@google.com> wrote:
> Yeah.
>
> The ONE() entry you're adding to tgid_base_stuff is used to help
> instantiate a "struct inode" when someone looks up the path
> "/proc/$tgid/seccomp_cache"; then when that path is opened, a "struct
> file" is created that holds a reference to the inode; and while that
> file exists, your proc_pid_seccomp_cache() can be invoked.
>
> proc_pid_seccomp_cache() is invoked from proc_single_show()
> ("PROC_I(inode)->op.proc_show" is proc_pid_seccomp_cache), and
> proc_single_show() obtains a temporary reference to the task_struct
> using get_pid_task() on a "struct pid" and drops that reference
> afterwards with put_task_struct(). The "struct pid" is obtained from
> the "struct proc_inode", which is essentially a subclass of "struct
> inode". The "struct pid" is kept refererenced until the inode goes
> away, via proc_pid_evict_inode(), called by proc_evict_inode().
>
> By looking at put_task_struct() and its callees, you can figure out
> which parts of the "struct task" are kept alive by the reference to
> it.

Ah I see. Thanks for the explanation.

> By the way, maybe it'd make sense to add this to tid_base_stuff as
> well? That should just be one extra line of code. Seccomp filters are
> technically per-thread, so it would make sense to have them visible in
> the per-thread subdirectories /proc/$pid/task/$tid/.

Right. Will do.

YiFei Zhu