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; } _