bpf: map_seq_next should always increase position index

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

diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
index e11059b..bd2fd8e 100644 (file)
--- a/kernel/bpf/inode.c
+++ b/kernel/bpf/inode.c
@@ -196,6 +196,7 @@ static void *map_seq_next(struct seq_file *m, void *v, loff_t *pos)
        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_file *m, void *v, loff_t *pos)
                map_iter(m)->done = true;
                return NULL;
        }
-
-       ++(*pos);
        return key;
 }