Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1

[Linus Torvalds]

On Wed, Apr 29, 2020 at 3:38 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> If you do it properly, with a helper function instead of repeating
> that fragile nasty thing, maybe it will look better to me.

Side note: if it has a special helper function for the "get lock,
repeat if it was invalid", you can do a better job than return
-EAGAIN.

In particular, you can do this

        set_thread_flag(TIF_SIGPENDING);
        return -RESTARTNOINTR;

which will actually restart the system call. So a ptrace() user (or
somebody doing a "write()" to /proc/<pid>/attr/xyz, wouldn't even see
the impossible EAGAIN error.

But that all requires that you have some locking helper routines like

    int lock_exec_creds(struct task_struct *);
    void unlock_exec_guard(struct task_struct *);

because there's no way we put that kind of super-fragile code in
several places. It would be very much one single routine with a *HUGE*
comment on it.

             Linus