Hi, Neil,
Thanks for reviewing, more below.
On Fri, Jan 29, 2021 at 6:56 AM NeilBrown <neilb@xxxxxxx> wrote:
>
> On Fri, Jan 22 2021, Xin Long wrote:
>
> > In commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code
> > and interface"), it broke a behavior: op show() is always called when op
> > next() returns an available obj.
>
> Interesting. I was not aware that some callers assumed this guarantee.
> If we are going to support it (which seems reasonable) we should add a
> statement of this guarantee to the documentation -
> Documentation/filesystems/seq_file.rst.
> Maybe a new paragraph after "Finally, the show() function ..."
>
> Note that show() will *always* be called after a successful start()
> or next() call, so that it can release any resources (such as
> ref-counts) that was acquired by those calls.
OK, that's good, will add it.
>
>
> >
> > This caused a refcnt leak in net/sctp/proc.c, as of the seq_operations
> > sctp_assoc_ops, transport obj is held in op next() and released in op
> > show().
> >
> > Here fix it by moving count check against iov_iter_count after calling
> > op show() so that op show() can still be called when op next() returns
> > an available obj.
> >
> > Note that m->index needs to increase so that op start() could go fetch
> > the next obj in the next round.
>
> This is certainly wrong.
> As the introduction in my patch said:
>
> A large part of achieving this is to *always* call ->next after ->show
> has successfully stored all of an entry in the buffer. Never just
> increment the index instead.
Understand.
>
> Incrementing ->index in common seq_file code is wrong.
>
> As we are no longer calling ->next after a successful ->show, we need to
> make that ->show appear unsuccessful so that it will be retried. This
> is done be setting "m->count = offs".
> So the moved code below becomes
>
> if (m->count >= iov_iter_count(iter)) {
> /* That record is more than we want, so discard it */
> m->count = offs;
> break;
> }
But I'm not sure if this's a better way, as discarding it means the last
show() call is just a waste, next time it has to call show() for that
obj again. Note that this is a different case from [1] (show() call
actually failed) and [2](the buffer overflowed), and it makes sense
to call show() again due to [1] and [2] next time.
if (err > 0) { <---[1]
m->count = offs;
} else if (err || seq_has_overflowed(m)) { <--- [2]
m->count = offs;
break;
}
if (m->count >= iov_iter_count(iter)) { <---[3]
But for this one [3], all it needs is just enter into seq_read again and
do the copying, no need to discard it.
>
> Possibly that can be merged into the preceding 'if'.
>
> Also the traverse() function contains a call to ->next that is not
> reliably followed by a call to ->show, even when successful. That needs
> to be fixed too.
Right, But I don't see a way here other than Incrementing m->index in
traverse():
@@ -114,16 +114,19 @@ static int traverse(struct seq_file *m, loff_t offset)
}
if (seq_has_overflowed(m))
goto Eoverflow;
- p = m->op->next(m, p, &m->index);
if (pos + m->count > offset) {
m->from = offset - pos;
m->count -= m->from;
+ m->index++;
break;
}
pos += m->count;
m->count = 0;
- if (pos == offset)
+ if (pos == offset) {
+ m->index++;
break;
+ }
+ p = m->op->next(m, p, &m->index);
}
>
> Thanks,
> NeilBrown
>
>
>
> >
> > Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface")
> > Reported-by: Prijesh <prpatel@xxxxxxxxxx>
> > Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
> > ---
> > fs/seq_file.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/seq_file.c b/fs/seq_file.c
> > index 03a369c..da304f7 100644
> > --- a/fs/seq_file.c
> > +++ b/fs/seq_file.c
> > @@ -264,8 +264,6 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
> > }
> > if (!p || IS_ERR(p)) // no next record for us
> > break;
> > - if (m->count >= iov_iter_count(iter))
> > - break;
> > err = m->op->show(m, p);
> > if (err > 0) { // ->show() says "skip it"
> > m->count = offs;
> > @@ -273,6 +271,10 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
> > m->count = offs;
> > break;
> > }
> > + if (m->count >= iov_iter_count(iter)) {
> > + m->index++;
> > + break;
> > + }
> > }
> > m->op->stop(m, p);
> > n = copy_to_iter(m->buf, m->count, iter);
> > --
> > 2.1.0
