Re: [PATCH bpf-next v11 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname

[Wenbo Zhang <ethercflow@gmail.com>]

> On Sat, Nov 23, 2019 at 05:35:14AM +0000, Al Viro wrote:
> > On Fri, Nov 22, 2019 at 09:19:21PM -0800, Alexei Starovoitov wrote:
> >
> > > hard to tell. It will be run out of bpf prog that attaches to kprobe or
> > > tracepoint. What is the concern about locking?
> > > d_path() doesn't take any locks and doesn't depend on any locks. Above 'if'
> > > checks that plain d_path() is used and not some specilized callback with
> > > unknown logic.
> >
> > It sure as hell does.  It might end up taking rename_lock and/or mount_lock
> > spinlock components.  It'll try not to, but if the first pass ends up with
> > seqlock mismatch, it will just grab the spinlock the second time around.

> ohh. got it. I missed _or_lock() part in there.
> The need_seqretry() logic is tricky. afaics there is no way for the checks
> outside of prepend_path() to prevent spin_lock to happen. And adding a flag to
> prepend_path() to return early if retry is needed is too ugly. So this helper
> won't be safe to be run out of kprobe. But if we allow it for tracepoints only
> it should be ok. I think. There are no tracepoints in inner guts of vfs and I
> don't think they will ever be. So running in tracepoint->bpf_prog->d_path we
> will be sure that rename_lock+mount_lock can be safely spinlocked. Am I missing
> something?

Hi Alexei,

Would you please give me an example of a deadlock condition under kprobe+bpf?
I'm not familiar with this detail and want to learn more.

> Above 'if's are not enough to make sure that it won't dead lock.
> Allowing it in tracing_func_proto() means that it's available to kprobe too.
> Hence deadlock is possible. Please see previous email thread.
> This helper is safe in tracepoint+bpf only.

So I should move it to `tp_prog_prog_func_proto` and `raw_tp_prog_func_prog`
right? Is raw tracepoint+bpf safe?

Thank you.

Alexei Starovoitov <alexei.starovoitov@gmail.com> 于2019年12月5日周四 下午3:19写道：
>
> On Wed, Dec 04, 2019 at 11:20:35PM -0500, Wenbo Zhang wrote:
> >
> > +BPF_CALL_3(bpf_get_file_path, char *, dst, u32, size, int, fd)
> > +{
> > +     struct file *f;
> > +     char *p;
> > +     int ret = -EBADF;
> > +
> > +     /* Ensure we're in user context which is safe for the helper to
> > +      * run. This helper has no business in a kthread.
> > +      */
> > +     if (unlikely(in_interrupt() ||
> > +                  current->flags & (PF_KTHREAD | PF_EXITING))) {
> > +             ret = -EPERM;
> > +             goto error;
> > +     }
> > +
> > +     /* Use fget_raw instead of fget to support O_PATH, and it doesn't
> > +      * have any sleepable code, so it's ok to be here.
> > +      */
> > +     f = fget_raw(fd);
> > +     if (!f)
> > +             goto error;
> > +
> > +     /* For unmountable pseudo filesystem, it seems to have no meaning
> > +      * to get their fake paths as they don't have path, and to be no
> > +      * way to validate this function pointer can be always safe to call
> > +      * in the current context.
> > +      */
> > +     if (f->f_path.dentry->d_op && f->f_path.dentry->d_op->d_dname) {
> > +             ret = -EINVAL;
> > +             fput(f);
> > +             goto error;
> > +     }
> > +
> > +     /* After filter unmountable pseudo filesytem, d_path won't call
> > +      * dentry->d_op->d_name(), the normally path doesn't have any
> > +      * sleepable code, and despite it uses the current macro to get
> > +      * fs_struct (current->fs), we've already ensured we're in user
> > +      * context, so it's ok to be here.
> > +      */
> > +     p = d_path(&f->f_path, dst, size);
>
> Above 'if's are not enough to make sure that it won't dead lock.
> Allowing it in tracing_func_proto() means that it's available to kprobe too.
> Hence deadlock is possible. Please see previous email thread.
> This helper is safe in tracepoint+bpf only.
>