[RFC bpf-next 3/3] selftests/bpf: test BPF_F_COMPARE compare-and-delete

[Gyutae Bae <gyutae.opensource@navercorp.com>]

From: Gyutae Bae <gyutae.bae@navercorp.com>

Add a test for compare-and-delete on a hash map. There is no libbpf
wrapper yet, so it issues the request through the raw bpf() syscall and
compares against a revision field stored in the value.

A matching expected value deletes the element, a stale one leaves it in
place and returns -EBUSY, and deleting an already-removed key returns
-ENOENT. A compare window that reaches past value_size is rejected with
-EINVAL, as are compare fields passed without BPF_F_COMPARE, so a dropped
flag cannot silently turn into an unconditional delete. Running the same
call on an array map, which has no compare-and-delete handler, returns
-EOPNOTSUPP.

Co-developed-by: Minsu Jeon <minsu.jeon@navercorp.com>
Signed-off-by: Minsu Jeon <minsu.jeon@navercorp.com>
Co-developed-by: Siwan Kim <siwan.kim@navercorp.com>
Signed-off-by: Siwan Kim <siwan.kim@navercorp.com>
Co-developed-by: Jonghyeon Kim <jong-hyeon.kim@navercorp.com>
Signed-off-by: Jonghyeon Kim <jong-hyeon.kim@navercorp.com>
Signed-off-by: Gyutae Bae <gyutae.bae@navercorp.com>
---
 .../selftests/bpf/prog_tests/map_cmp_delete.c | 106 ++++++++++++++++++
 1 file changed, 106 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/map_cmp_delete.c

diff --git a/tools/testing/selftests/bpf/prog_tests/map_cmp_delete.c b/tools/testing/selftests/bpf/prog_tests/map_cmp_delete.c
new file mode 100644
index 000000000000..6d6df13adcc4
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/map_cmp_delete.c
@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright (c) 2026 NAVER Corp.
+ *
+ * Author: Gyutae Bae <gyutae.bae@navercorp.com>
+ */
+
+#include <test_progs.h>
+#include <sys/syscall.h>
+
+struct val {
+	__u64 revision;	/* synchronization field we compare on */
+	__u64 payload;
+};
+
+/* libbpf has no wrapper that passes the compare-and-delete fields, so
+ * issue bpf() directly. 'flags' is a parameter so tests can also
+ * exercise the no-BPF_F_COMPARE case.
+ */
+static int cmp_delete_flags(int fd, const void *key, const void *compare,
+			    __u32 off, __u32 size, __u64 flags)
+{
+	union bpf_attr attr;
+	int ret;
+
+	memset(&attr, 0, sizeof(attr));
+	attr.map_fd = fd;
+	attr.key = (__u64)(unsigned long)key;
+	attr.flags = flags;
+	attr.compare = (__u64)(unsigned long)compare;
+	attr.compare_offset = off;
+	attr.compare_size = size;
+	ret = syscall(__NR_bpf, BPF_MAP_DELETE_ELEM, &attr, sizeof(attr));
+	return ret < 0 ? -errno : ret;
+}
+
+static int cmp_delete(int fd, const void *key, const void *compare,
+		      __u32 off, __u32 size)
+{
+	return cmp_delete_flags(fd, key, compare, off, size, BPF_F_COMPARE);
+}
+
+static int hash_map(void)
+{
+	return bpf_map_create(BPF_MAP_TYPE_HASH, "cmp_hash",
+			      sizeof(__u32), sizeof(struct val), 64, NULL);
+}
+
+static void test_contract(void)
+{
+	const __u32 off = offsetof(struct val, revision);
+	const __u32 sz = sizeof(((struct val *)0)->revision);
+	struct val v = { .revision = 10, .payload = 0xabc };
+	struct val snap = v;
+	__u32 k = 1;
+	int fd = hash_map();
+
+	if (!ASSERT_GE(fd, 0, "map_create"))
+		return;
+	ASSERT_OK(bpf_map_update_elem(fd, &k, &v, BPF_ANY), "insert");
+
+	/* mismatch -> -EBUSY, entry preserved */
+	snap.revision = 9;
+	ASSERT_EQ(cmp_delete(fd, &k, &snap, off, sz), -EBUSY, "mismatch_ebusy");
+	ASSERT_OK(bpf_map_lookup_elem(fd, &k, &v), "still_present");
+
+	/* compare fields set but BPF_F_COMPARE omitted -> -EINVAL, entry preserved
+	 * (a dropped flag must not silently become an unconditional delete)
+	 */
+	ASSERT_EQ(cmp_delete_flags(fd, &k, &snap, off, sz, 0), -EINVAL,
+		  "cmp_fields_no_flag_einval");
+	ASSERT_OK(bpf_map_lookup_elem(fd, &k, &v), "preserved_no_flag");
+
+	/* match -> 0, entry gone */
+	snap.revision = 10;
+	ASSERT_OK(cmp_delete(fd, &k, &snap, off, sz), "match_deletes");
+	ASSERT_EQ(bpf_map_lookup_elem(fd, &k, &v), -ENOENT, "gone");
+
+	/* absent -> -ENOENT */
+	ASSERT_EQ(cmp_delete(fd, &k, &snap, off, sz), -ENOENT, "absent_enoent");
+
+	/* bad window -> -EINVAL */
+	ASSERT_EQ(cmp_delete(fd, &k, &snap, 0, sizeof(struct val) + 8), -EINVAL,
+		  "oversize_einval");
+
+	/* unsupported map type (no map_delete_elem_cmp op) -> -EOPNOTSUPP */
+	{
+		int afd = bpf_map_create(BPF_MAP_TYPE_ARRAY, "cmp_arr",
+					 sizeof(__u32), sizeof(struct val), 4, NULL);
+		__u32 ak = 0;
+
+		if (ASSERT_GE(afd, 0, "array_create")) {
+			ASSERT_EQ(cmp_delete(afd, &ak, &snap, off, sz),
+				  -EOPNOTSUPP, "array_eopnotsupp");
+			close(afd);
+		}
+	}
+	close(fd);
+}
+
+void test_map_cmp_delete(void)
+{
+	if (test__start_subtest("contract"))
+		test_contract();
+}
-- 
2.39.5 (Apple Git-154)