Re: [PATCH bpf-next v1] bpf, iter: clean up bpf_seq_read().

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Aug 02, 2022 at 05:15:50PM -0700, Hao Luo wrote:
> On Tue, Aug 2, 2022 at 4:14 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Mon, Aug 01, 2022 at 01:50:39PM -0700, Hao Luo wrote:
> >
> > SNIP
> >
> > > +static int do_seq_show(struct seq_file *seq, void *p, size_t offs)
> > > +{
> > > +     int err;
> > > +
> > > +     WARN_ON(IS_ERR_OR_NULL(p));
> > > +
> > > +     err = seq->op->show(seq, p);
> > > +     if (err > 0) {
> > > +             /* object is skipped, decrease seq_num, so next
> > > +              * valid object can reuse the same seq_num.
> > > +              */
> > > +             bpf_iter_dec_seq_num(seq);
> > > +             seq->count = offs;
> > > +             return err;
> > > +     }
> > > +
> > > +     if (err < 0 || seq_has_overflowed(seq)) {
> > > +             seq->count = offs;
> > > +             return err ? err : -E2BIG;
> > > +     }
> > > +
> > > +     /* err == 0 and no overflow */
> > > +     return 0;
> > > +}
> > > +
> > > +/* do_seq_stop, stops at the given object 'p'. 'p' could be an ERR or NULL. If
> > > + * 'p' is an ERR or there was an overflow, reset seq->count to 'offs' and
> > > + * returns error. Returns 0 otherwise.
> > > + */
> > > +static int do_seq_stop(struct seq_file *seq, void *p, size_t offs)
> > > +{
> > > +     if (IS_ERR(p)) {
> > > +             seq->op->stop(seq, NULL);
> > > +             seq->count = offs;
> >
> > should we set seq->count to 0 in case of error?
> >
> 
> Thanks Jiri. To be honest, I don't know. There are two paths that may
> lead to an error "p".
> 
> First, seq->op->start() could return ERR, in that case, '"offs'" is
> zero and we set it to zero already. This is fine.

ah right, offs is zero at that time, looks good then

> 
> The other one, seq->op->next() could return ERR. This is a case where
> bpf_seq_read() fails to handle right now, so I am not sure what to do.

but maybe we don't need to set seq->count in here, like:

	static int do_seq_stop(struct seq_file *seq, void *p, size_t offs)
	{
		if (IS_ERR(p)) {
			seq->op->stop(seq, NULL);
			return PTR_ERR(p);
		}

because it's already set by error path of do_seq_show

> 
> Based on my understanding reading the code, if seq->count isn't
> zeroed, the current read() will not copy data, but the next read()
> will copy data (see the "if (seq->count)" at the beginning of
> bpf_seq_read). If seq->count is zeroed, the data in buffer will be
> discarded. I don't know what is right.

I think we should return the buffer up to the point there's an error,
that's why we set seq->count to previous offs value after failed
show callback

jirka