From 3858cca150c69ea8dbc1f1b03fd444d99fb747b7 Mon Sep 17 00:00:00 2001 From: NeilBrown Date: Tue, 13 Aug 2019 15:37:44 -0700 Subject: [PATCH] seq_file: fix problem when seeking mid-record 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 Reported-by: Sergei Turchanov Tested-by: Sergei Turchanov Cc: Alexander Viro Cc: Markus Elfring Cc: [4.19+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- 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 1dea7a8a5255..05e58b56f620 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; }