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: Florian Lehner <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: John Fastabend <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

1 files changed, 43 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/hash_large_key.c b/tools/testing/selftests/bpf/prog_tests/hash_large_key.c
new file mode 100644
index 000000000000..34684c0fc76d
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/hash_large_key.c
@@ -0,0 +1,43 @@
+// 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);
+}