[patch 13/18] seq_file: fix problem when seeking mid-record

[patch 13/18] seq_file: fix problem when seeking mid-record

[akpm]

From: NeilBrown <neilb@suse.com>
Subject: seq_file: fix problem when seeking mid-record

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@suse.com>
Reported-by: Sergei Turchanov <turchanov@farpost.com>
Tested-by: Sergei Turchanov <turchanov@farpost.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Markus Elfring <Markus.Elfring@web.de>
Cc: <stable@vger.kernel.org>	[4.19+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/fs/seq_file.c~seq_file-fix-problem-when-seeking-mid-record
+++ a/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;
 	}
_