bpf, lpm: fix lookup bug in map_delete_elem [Linux 5.0]

bpf, lpm: fix lookup bug in map_delete_elem [Linux 5.0]

This Linux kernel change "bpf, lpm: fix lookup bug in map_delete_elem" is included in the Linux 5.0 release. This change is authored by Alban Crequy <alban [at] kinvolk.io> on Fri Feb 22 14:19:08 2019 +0100. The commit for this change in Linux stable tree is 7c0cdf0 (patch).

bpf, lpm: fix lookup bug in map_delete_elem

trie_delete_elem() was deleting an entry even though it was not matching
if the prefixlen was correct. This patch adds a check on matchlen.

Reproducer:

$ sudo bpftool map create /sys/fs/bpf/mylpm type lpm_trie key 8 value 1 entries 128 name mylpm flags 1
$ sudo bpftool map update pinned /sys/fs/bpf/mylpm key hex 10 00 00 00 aa bb cc dd value hex 01
$ sudo bpftool map dump pinned /sys/fs/bpf/mylpm
key: 10 00 00 00 aa bb cc dd  value: 01
Found 1 element
$ sudo bpftool map delete pinned /sys/fs/bpf/mylpm key hex 10 00 00 00 ff ff ff ff
$ echo $?
0
$ sudo bpftool map dump pinned /sys/fs/bpf/mylpm
Found 0 elements

A similar reproducer is added in the selftests.

Without the patch:

$ sudo ./tools/testing/selftests/bpf/test_lpm_map
test_lpm_map: test_lpm_map.c:485: test_lpm_delete: Assertion `bpf_map_delete_elem(map_fd, key) == -1 && errno == ENOENT' failed.
Aborted

With the patch: test_lpm_map runs without errors.

Fixes: e454cf595853 ("bpf: Implement map_delete_elem for BPF_MAP_TYPE_LPM_TRIE")
Cc: Craig Gallek <[email protected]>
Signed-off-by: Alban Crequy <[email protected]>
Acked-by: Craig Gallek <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

There are 11 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/bpf/lpm_trie.c                      |  1 +
 tools/testing/selftests/bpf/test_lpm_map.c | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
index abf1002..93a5cbb 100644
--- a/kernel/bpf/lpm_trie.c
+++ b/kernel/bpf/lpm_trie.c
@@ -471,6 +471,7 @@ static int trie_delete_elem(struct bpf_map *map, void *_key)
    }

    if (!node || node->prefixlen != key->prefixlen ||
+       node->prefixlen != matchlen ||
        (node->flags & LPM_TREE_NODE_FLAG_IM)) {
        ret = -ENOENT;
        goto out;
diff --git a/tools/testing/selftests/bpf/test_lpm_map.c b/tools/testing/selftests/bpf/test_lpm_map.c
index 147e34c..02d7c87 100644
--- a/tools/testing/selftests/bpf/test_lpm_map.c
+++ b/tools/testing/selftests/bpf/test_lpm_map.c
@@ -474,6 +474,16 @@ static void test_lpm_delete(void)
    assert(bpf_map_lookup_elem(map_fd, key, &value) == -1 &&
        errno == ENOENT);

+   key->prefixlen = 30; // unused prefix so far
+   inet_pton(AF_INET, "192.255.0.0", key->data);
+   assert(bpf_map_delete_elem(map_fd, key) == -1 &&
+       errno == ENOENT);
+
+   key->prefixlen = 16; // same prefix as the root node
+   inet_pton(AF_INET, "192.255.0.0", key->data);
+   assert(bpf_map_delete_elem(map_fd, key) == -1 &&
+       errno == ENOENT);
+
    /* assert initial lookup */
    key->prefixlen = 32;
    inet_pton(AF_INET, "192.168.0.1", key->data);

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