bpf: Lift hashtab key_size limit

Currently key_size of hashtab is limited to MAX_BPF_STACK.
As the key of hashtab can also be a value from a per cpu map it can be
larger than MAX_BPF_STACK.

The use-case for this patch originates to implement allow/disallow
lists for files and file paths. The maximum length of file paths is
defined by PATH_MAX with 4096 chars including nul.
This limit exceeds MAX_BPF_STACK.

Changelog:

v3:
 - Rebase

v2:
 - Add a test for bpf side

Signed-off-by: Florian Lehner <[email protected]>

Author: florianl

kernel/bpf/hashtab.c

@@ -390,17 +390,11 @@ static int htab_map_alloc_check(union bpf_attr *attr)
 	    attr->value_size == 0)
 		return -EINVAL;
 
-	if (attr->key_size > MAX_BPF_STACK)
-		/* eBPF programs initialize keys on stack, so they cannot be
-		 * larger than max stack size
-		 */
-		return -E2BIG;
-
-	if (attr->value_size >= KMALLOC_MAX_SIZE -
-	    MAX_BPF_STACK - sizeof(struct htab_elem))
-		/* if value_size is bigger, the user space won't be able to
-		 * access the elements via bpf syscall. This check also makes
-		 * sure that the elem_size doesn't overflow and it's
+	if ((attr->key_size + attr->value_size) >= KMALLOC_MAX_SIZE -
+	    sizeof(struct htab_elem))
+		/* if key_size + value_size is bigger, the user space won't be
+		 * able to access the elements via bpf syscall. This check
+		 * also makes sure that the elem_size doesn't overflow and it's
 		 * kmalloc-able later in htab_map_update_elem()
 		 */
 		return -E2BIG;

tools/testing/selftests/bpf/prog_tests/hash_large_key.c

@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2020 Florian Lehner
+
+#include <test_progs.h>
+
+void test_hash_large_key(void)
+{
+	const char *file = "./test_hash_large_key.o";
+	int prog_fd, map_fd[2];
+	struct bpf_object *obj = NULL;
+	int err = 0;
+
+	err = bpf_prog_load(file, BPF_PROG_TYPE_CGROUP_SKB, &obj, &prog_fd);
+	if (CHECK_FAIL(err)) {
+		printf("test_hash_large_key: bpf_prog_load errno %d", errno);
+		goto close_prog;
+	}
+
+	map_fd[0] = bpf_find_map(__func__, obj, "hash_map");
+	if (CHECK_FAIL(map_fd[0] < 0))
+		goto close_prog;
+	map_fd[1] = bpf_find_map(__func__, obj, "key_map");
+	if (CHECK_FAIL(map_fd[1] < 0))
+		goto close_prog;
+
+close_prog:
+	bpf_object__close(obj);
+}

tools/testing/selftests/bpf/progs/test_hash_large_key.c

@@ -0,0 +1,45 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2020 Florian Lehner
+
+#include <linux/bpf.h>
+#include <linux/version.h>
+#include <bpf/bpf_helpers.h>
+
+struct bigelement {
+	int a;
+	char b[4096];
+	long long c;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 1);
+	__type(key, struct bigelement);
+	__type(value, __u32);
+} hash_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, __u32);
+	__type(value, struct bigelement);
+} key_map SEC(".maps");
+
+SEC("hash_large_key_demo")
+int bpf_hash_large_key_test(struct __sk_buf *skb)
+{
+	int zero = 0, err = 1, value = 42;
+	struct bigelement *key;
+
+	key = bpf_map_lookup_elem(&key_map, &zero);
+	if (!key)
+		goto err;
+
+	if (bpf_map_update_elem(&hash_map, key, &value, BPF_ANY))
+		goto err;
+	err = 0;
+err:
+	return err;
+}
+
+char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/test_maps.c

@@ -1225,7 +1225,7 @@ static void test_map_large(void)
 {
 	struct bigkey {
 		int a;
-		char b[116];
+		char b[4096];
 		long long c;
 	} key;
 	int fd, i, value;