selftests/bpf: Add test for libbpf_sha256()

Test that libbpf_sha256() calculates SHA-256 digests correctly.

Tested with:
    make -C tools/testing/selftests/bpf/
    ./tools/testing/selftests/bpf/test_progs -t sha256 -v

Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Eric Biggers

tools/testing/selftests/bpf/prog_tests/sha256.c

@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright 2025 Google LLC */
+
+#include <test_progs.h>
+#include "bpf/libbpf_internal.h"
+
+#define MAX_LEN 4096
+
+/* Test libbpf_sha256() for all lengths from 0 to MAX_LEN inclusively. */
+void test_sha256(void)
+{
+	/*
+	 * The correctness of this value was verified by running this test with
+	 * libbpf_sha256() replaced by OpenSSL's SHA256().
+	 */
+	static const __u8 expected_digest_of_digests[SHA256_DIGEST_LENGTH] = {
+		0x62, 0x30, 0x0e, 0x1d, 0xea, 0x7f, 0xc4, 0x74,
+		0xfd, 0x8e, 0x64, 0x0b, 0xd8, 0x5f, 0xea, 0x04,
+		0xf3, 0xef, 0x77, 0x42, 0xc2, 0x01, 0xb8, 0x90,
+		0x6e, 0x19, 0x91, 0x1b, 0xca, 0xb3, 0x28, 0x42,
+	};
+	__u64 seed = 0;
+	__u8 *data = NULL, *digests = NULL;
+	__u8 digest_of_digests[SHA256_DIGEST_LENGTH];
+	size_t i;
+
+	data = malloc(MAX_LEN);
+	if (!ASSERT_OK_PTR(data, "malloc"))
+		goto out;
+	digests = malloc((MAX_LEN + 1) * SHA256_DIGEST_LENGTH);
+	if (!ASSERT_OK_PTR(digests, "malloc"))
+		goto out;
+
+	/* Generate MAX_LEN bytes of "random" data deterministically. */
+	for (i = 0; i < MAX_LEN; i++) {
+		seed = (seed * 25214903917 + 11) & ((1ULL << 48) - 1);
+		data[i] = (__u8)(seed >> 16);
+	}
+
+	/* Calculate a digest for each length 0 through MAX_LEN inclusively. */
+	for (i = 0; i <= MAX_LEN; i++)
+		libbpf_sha256(data, i, &digests[i * SHA256_DIGEST_LENGTH]);
+
+	/* Calculate and verify the digest of all the digests. */
+	libbpf_sha256(digests, (MAX_LEN + 1) * SHA256_DIGEST_LENGTH,
+		      digest_of_digests);
+	ASSERT_MEMEQ(digest_of_digests, expected_digest_of_digests,
+		     SHA256_DIGEST_LENGTH, "digest_of_digests");
+out:
+	free(data);
+	free(digests);
+}