bpf: map_get_next_key to return first key on NULL

When iterating through a map, we need to find a key that does not exist
in the map so map_get_next_key will give us the first key of the map.
This often requires a lot of guessing in production systems.

This patch makes map_get_next_key return the first key when the key
pointer in the parameter is NULL.

Signed-off-by: Teng Qin <qinteng@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 8 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index b89288e2b589..13642c73dca0 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -536,14 +536,18 @@ static int map_get_next_key(union bpf_attr *attr)
         if (IS_ERR(map))
                 return PTR_ERR(map);
 
-        err = -ENOMEM;
-        key = kmalloc(map->key_size, GFP_USER);
-        if (!key)
-                goto err_put;
-
-        err = -EFAULT;
-        if (copy_from_user(key, ukey, map->key_size) != 0)
-                goto free_key;
+        if (ukey) {
+                err = -ENOMEM;
+                key = kmalloc(map->key_size, GFP_USER);
+                if (!key)
+                        goto err_put;
+
+                err = -EFAULT;
+                if (copy_from_user(key, ukey, map->key_size) != 0)
+                        goto free_key;
+        } else {
+                key = NULL;
+        }
 
         err = -ENOMEM;
         next_key = kmalloc(map->key_size, GFP_USER);