Re: [PATCH v2 1/4] pidfd: support PIDFD_NONBLOCK in pidfd_open()

Christian Brauner
2020-09-03T15:25:38Z

On Thu, Sep 03, 2020 at 04:58:09PM +0200, Oleg Nesterov wrote:
> Christian, off-topic question...
>
> On 09/02, Christian Brauner wrote:
> >
> > -static int pidfd_create(struct pid *pid)
> > +static int pidfd_create(struct pid *pid, unsigned int flags)
> > {
> > int fd;
> >
> > fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
> > - O_RDWR | O_CLOEXEC);
> > + flags | O_RDWR | O_CLOEXEC);
>
> I just noticed this comment above pidfd_create:
>
> * Note, that this function can only be called after the fd table has
> * been unshared to avoid leaking the pidfd to the new process.
>
> what does it mean?
>
> Of course, if fd table is shared then pidfd can "leak" to another process,
> but this is true for any file and sys_pidfd_open() doesn't do any check?

It's the same comment we added in kernel/fork.c to make callers aware
that they can leak a pidfd to another process unintentionally. Sure,
this is true of any fd but since pidfds were a new type of handle and on
another process at that we felt that this was important to spell out. The
"can only" should've arguably been "should probably".

>
>
>
> In fact I think this helper buys nothing but adds the unnecessary get/put_pid,
> we can kill it and change pidfd_open() to do
>
> SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
> {
> int fd;
> struct pid *p;
>
> if (flags & ~PIDFD_NONBLOCK)
> return -EINVAL;
>
> if (pid <= 0)
> return -EINVAL;
>
> p = find_get_pid(pid);
> if (!p)
> return -ESRCH;
>
> fd = -EINVAL;
> if (pid_has_task(p, PIDTYPE_TGID)) {
> fd = anon_inode_getfd("[pidfd]", &pidfd_fops, pid,
> flags | O_RDWR | O_CLOEXEC);
> }
> if (fd < 0)
> put_pid(p);
> return fd;
> }

Sure, I'd totally take a patch like that!

>
> but this is cosmetic and off-topic too.

No, much appreciated. Good-looking code is important. :)

Christian