Commit c6bde958 authored by Florian Lehner's avatar Florian Lehner Committed by Alexei Starovoitov
Browse files

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:

v5:
 - Fix cast overflow

v4:
 - Utilize BPF skeleton in tests
 - Rebase

v3:
 - Rebase

v2:
 - Add a test for bpf side

Signed-off-by: default avatarFlorian Lehner <dev@der-flo.net>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201029201442.596690-1-dev@der-flo.net
parent b6b466a8

kernel/bpf/hashtab.c

+5 −11
Original line number Diff line number Diff line
@@ -415,17 +415,11 @@ 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 ((u64)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 → 100644
+43 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0



#include <test_progs.h>

#include "test_hash_large_key.skel.h"



void test_hash_large_key(void)

{

	int err, value = 21, duration = 0, hash_map_fd;

	struct test_hash_large_key *skel;



	struct bigelement {

		int a;

		char b[4096];

		long long c;

	} key;

	bzero(&key, sizeof(key));



	skel = test_hash_large_key__open_and_load();

	if (CHECK(!skel, "skel_open_and_load", "skeleton open/load failed\n"))

		return;



	hash_map_fd = bpf_map__fd(skel->maps.hash_map);

	if (CHECK(hash_map_fd < 0, "bpf_map__fd", "failed\n"))

		goto cleanup;



	err = test_hash_large_key__attach(skel);

	if (CHECK(err, "attach_raw_tp", "err %d\n", err))

		goto cleanup;



	err = bpf_map_update_elem(hash_map_fd, &key, &value, BPF_ANY);

	if (CHECK(err, "bpf_map_update_elem", "errno=%d\n", errno))

		goto cleanup;



	key.c = 1;

	err = bpf_map_lookup_elem(hash_map_fd, &key, &value);

	if (CHECK(err, "bpf_map_lookup_elem", "errno=%d\n", errno))

		goto cleanup;



	CHECK_FAIL(value != 42);



cleanup:

	test_hash_large_key__destroy(skel);

}

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

0 → 100644
+44 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0



#include <linux/bpf.h>

#include <bpf/bpf_helpers.h>



char _license[] SEC("license") = "GPL";



struct {

	__uint(type, BPF_MAP_TYPE_HASH);

	__uint(max_entries, 2);

	__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");



struct bigelement {

	int a;

	char b[4096];

	long long c;

};



SEC("raw_tracepoint/sys_enter")

int bpf_hash_large_key_test(void *ctx)

{

	int zero = 0, err = 1, value = 42;

	struct bigelement *key;



	key = bpf_map_lookup_elem(&key_map, &zero);

	if (!key)

		return 0;



	key->c = 1;

	if (bpf_map_update_elem(&hash_map, key, &value, BPF_ANY))

		return 0;



	return 0;

}

tools/testing/selftests/bpf/test_maps.c

+2 −1
Original line number Diff line number Diff line
@@ -1223,9 +1223,10 @@ out_map_in_map: 


static void test_map_large(void)

{



	struct bigkey {

		int a;

		char b[116];

		char b[4096];

		long long c;

	} key;

	int fd, i, value;

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