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From: NeilBrown <neilb@suse.de>
To: Xin Long <lucien.xin@gmail.com>,
	linux-fsdevel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>
Cc: NeilBrown <neilb@suse.com>
Subject: Re: [PATCH] seq_read: move count check against iov_iter_count after calling op show
Date: Fri, 29 Jan 2021 09:56:17 +1100	[thread overview]
Message-ID: <87r1m4fz72.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <91568e002fed69425485c17de223bef0ff660f3a.1611313420.git.lucien.xin@gmail.com>

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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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  parent reply	other threads:[~2021-01-28 22:58 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-22 11:03 [PATCH] seq_read: move count check against iov_iter_count after calling op show Xin Long
2021-01-28  9:52 ` Xin Long
2021-01-28 22:56 ` NeilBrown [this message]
2021-01-29  6:57   ` Xin Long
2021-02-04  4:57     ` Xin Long
2021-02-04  5:46       ` NeilBrown
2021-02-04  5:53         ` Xin Long
2021-02-04  6:08 ` [seq_read] 03c44acf0b: xfstests.generic.589.fail kernel test robot

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