From: Vasily Averin <vvs@virtuozzo.com>
Date: Sat, 25 Jan 2020 12:10:02 +0300
Subject: bpf: map_seq_next should always increase position index
Patch-mainline: v5.6-rc1
Git-commit: 90435a7891a2259b0f74c5a1bc5600d0d64cba8f
References: bsc#1155518

If seq_file .next fuction does not change position index,
read after some lseek can generate an unexpected output.

See also: https://bugzilla.kernel.org/show_bug.cgi?id=206283

v1 -> v2: removed missed increment in end of function

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/eca84fdd-c374-a154-d874-6c7b55fc3bc4@virtuozzo.com
Acked-by: Gary Lin <glin@suse.com>
---
 kernel/bpf/inode.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -196,6 +196,7 @@ static void *map_seq_next(struct seq_fil
 	void *key = map_iter(m)->key;
 	void *prev_key;
 
+	(*pos)++;
 	if (map_iter(m)->done)
 		return NULL;
 
@@ -208,8 +209,6 @@ static void *map_seq_next(struct seq_fil
 		map_iter(m)->done = true;
 		return NULL;
 	}
-
-	++(*pos);
 	return key;
 }