linux-kernel - [PATCH 4.19 02/85] seq_file: fix problem when seeking mid-record

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Date:   Thu, 22 Aug 2019 10:18:35 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, NeilBrown <neilb@...e.com>,
        Sergei Turchanov <turchanov@...post.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Markus Elfring <Markus.Elfring@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.19 02/85] seq_file: fix problem when seeking mid-record

From: NeilBrown <neilb@...e.com>

commit 6a2aeab59e97101b4001bac84388fc49a992f87e upstream.

If you use lseek or similar (e.g.  pread) to access a location in a
seq_file file that is within a record, rather than at a record boundary,
then the first read will return the remainder of the record, and the
second read will return the whole of that same record (instead of the
next record).  When seeking to a record boundary, the next record is
correctly returned.

This bug was introduced by a recent patch (identified below).  Before
that patch, seq_read() would increment m->index when the last of the
buffer was returned (m->count == 0).  After that patch, we rely on
->next to increment m->index after filling the buffer - but there was
one place where that didn't happen.

Link: https://lkml.kernel.org/lkml/877e7xl029.fsf@notabene.neil.brown.name/
Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface")
Signed-off-by: NeilBrown <neilb@...e.com>
Reported-by: Sergei Turchanov <turchanov@...post.com>
Tested-by: Sergei Turchanov <turchanov@...post.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Markus Elfring <Markus.Elfring@....de>
Cc: <stable@...r.kernel.org>	[4.19+]
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/seq_file.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -119,6 +119,7 @@ static int traverse(struct seq_file *m,
 		}
 		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;
@@ -126,7 +127,6 @@ static int traverse(struct seq_file *m,
 		}
 		pos += m->count;
 		m->count = 0;
-		p = m->op->next(m, p, &m->index);
 		if (pos == offset)
 			break;
 	}