[PATCH] seq_file: fix problem when seeking mid-record.

[NeilBrown]

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).
Whnn 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@xxxxxxxxxxxxxxxxxxxxxxxx/
Reported-by-tested-by: Sergei Turchanov <turchanov@xxxxxxxxxxx>
Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code
	and interface")
Cc: stable@xxxxxxxxxxxxxxx (v4.19+)
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---

Hi Andrew: as you applied the offending patch for me, maybe you could
queue up this fix too.
Thanks,
NeilBrown

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

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 04f09689cd6d..1600034a929b 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -119,6 +119,7 @@ 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;
@@ -126,7 +127,6 @@ static int traverse(struct seq_file *m, loff_t offset)
 		}
 		pos += m->count;
 		m->count = 0;
-		p = m->op->next(m, p, &m->index);
 		if (pos == offset)
 			break;
 	}
-- 
2.14.0.rc0.dirty

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