Re: [PATCH] seq_read: move count check against iov_iter_count after calling op show

[NeilBrown <neilb@suse.de>]

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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.


>
> 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.

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;
  }

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.

Thanks,
NeilBrown

        

>
> Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface")
> Reported-by: Prijesh <prpatel@redhat.com>
> Signed-off-by: Xin Long <lucien.xin@gmail.com>
> ---
>  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

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