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/[email protected]/
Fixes: 1f4aace ("fs/seq_file.c: simplify seq_file iteration code and interface")
Signed-off-by: NeilBrown <[email protected]>
Reported-by: Sergei Turchanov <[email protected]>
Tested-by: Sergei Turchanov <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Markus Elfring <[email protected]>
Cc: <[email protected]>	[4.19+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/seq_file.c

@@ -119,14 +119,14 @@ 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;
 			break;
 		}
 		pos += m->count;
 		m->count = 0;
-		p = m->op->next(m, p, &m->index);
 		if (pos == offset)
 			break;
 	}