selftests/bpf: Add fixed vs rotating verifier log tests

Add selftests validating BPF_LOG_FIXED behavior, which used to be the
only behavior, and now default rotating BPF verifier log, which returns
just up to last N bytes of full verifier log, instead of returning
-ENOSPC.

To stress test correctness of in-kernel verifier log logic, we force it
to truncate program's verifier log to all lengths from 1 all the way to
its full size (about 450 bytes today). This was a useful stress test
while developing the feature.

For both fixed and rotating log modes we expect -ENOSPC if log contents
doesn't fit in user-supplied log buffer.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/bpf/20230406234205.323208-7-andrii@kernel.org

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include <bpf/btf.h>
+
+#include "test_log_buf.skel.h"
+
+
+static bool check_prog_load(int prog_fd, bool expect_err, const char *tag)
+{
+	if (expect_err) {
+		if (!ASSERT_LT(prog_fd, 0, tag)) {
+			close(prog_fd);
+			return false;
+		}
+	} else /* !expect_err */ {
+		if (!ASSERT_GT(prog_fd, 0, tag))
+			return false;
+	}
+	return true;
+}
+
+static void verif_log_subtest(const char *name, bool expect_load_error, int log_level)
+{
+	LIBBPF_OPTS(bpf_prog_load_opts, opts);
+	struct {
+		/* strategically placed before others to avoid accidental modification by kernel */
+		char filler[1024];
+		char buf[1024];
+		/* strategically placed after buf[] to catch more accidental corruptions */
+		char reference[1024];
+	} logs;
+	char *exp_log, prog_name[16], op_name[32];
+	struct test_log_buf *skel;
+	struct bpf_program *prog;
+	const struct bpf_insn *insns;
+	size_t insn_cnt, fixed_log_sz;
+	int i, mode, err, prog_fd;
+
+	skel = test_log_buf__open();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return;
+
+	bpf_object__for_each_program(prog, skel->obj) {
+		if (strcmp(bpf_program__name(prog), name) == 0)
+			bpf_program__set_autoload(prog, true);
+		else
+			bpf_program__set_autoload(prog, false);
+	}
+
+	err = test_log_buf__load(skel);
+	if (!expect_load_error && !ASSERT_OK(err, "unexpected_load_failure"))
+		goto cleanup;
+	if (expect_load_error && !ASSERT_ERR(err, "unexpected_load_success"))
+		goto cleanup;
+
+	insns = bpf_program__insns(skel->progs.good_prog);
+	insn_cnt = bpf_program__insn_cnt(skel->progs.good_prog);
+
+	opts.log_buf = logs.reference;
+	opts.log_size = sizeof(logs.reference);
+	opts.log_level = log_level | 8 /* BPF_LOG_FIXED */;
+	prog_fd = bpf_prog_load(BPF_PROG_TYPE_RAW_TRACEPOINT, "log_fixed",
+				"GPL", insns, insn_cnt, &opts);
+	if (!check_prog_load(prog_fd, expect_load_error, "fixed_buf_prog_load"))
+		goto cleanup;
+	close(prog_fd);
+
+	fixed_log_sz = strlen(logs.reference) + 1;
+	if (!ASSERT_GT(fixed_log_sz, 50, "fixed_log_sz"))
+		goto cleanup;
+	memset(logs.reference + fixed_log_sz, 0, sizeof(logs.reference) - fixed_log_sz);
+
+	/* validate BPF_LOG_FIXED works as verifier log used to work, that is:
+	 * we get -ENOSPC and beginning of the full verifier log. This only
+	 * works for log_level 2 and log_level 1 + failed program. For log
+	 * level 2 we don't reset log at all. For log_level 1 + failed program
+	 * we don't get to verification stats output. With log level 1
+	 * for successful program  final result will be just verifier stats.
+	 * But if provided too short log buf, kernel will NULL-out log->ubuf
+	 * and will stop emitting further log. This means we'll never see
+	 * predictable verifier stats.
+	 * Long story short, we do the following -ENOSPC test only for
+	 * predictable combinations.
+	 */
+	if (log_level >= 2 || expect_load_error) {
+		opts.log_buf = logs.buf;
+		opts.log_level = log_level | 8; /* fixed-length log */
+		opts.log_size = 25;
+
+		prog_fd = bpf_prog_load(BPF_PROG_TYPE_RAW_TRACEPOINT, "log_fixed50",
+					"GPL", insns, insn_cnt, &opts);
+		if (!ASSERT_EQ(prog_fd, -ENOSPC, "unexpected_log_fixed_prog_load_result")) {
+			if (prog_fd >= 0)
+				close(prog_fd);
+			goto cleanup;
+		}
+		if (!ASSERT_EQ(strlen(logs.buf), 24, "log_fixed_25"))
+			goto cleanup;
+		if (!ASSERT_STRNEQ(logs.buf, logs.reference, 24, op_name))
+			goto cleanup;
+	}
+
+	/* validate rolling verifier log logic: try all variations of log buf
+	 * length to force various truncation scenarios
+	 */
+	opts.log_buf = logs.buf;
+
+	/* rotating mode, then fixed mode */
+	for (mode = 1; mode >= 0; mode--) {
+		/* prefill logs.buf with 'A's to detect any write beyond allowed length */
+		memset(logs.filler, 'A', sizeof(logs.filler));
+		logs.filler[sizeof(logs.filler) - 1] = '\0';
+		memset(logs.buf, 'A', sizeof(logs.buf));
+		logs.buf[sizeof(logs.buf) - 1] = '\0';
+
+		for (i = 1; i < fixed_log_sz; i++) {
+			opts.log_size = i;
+			opts.log_level = log_level | (mode ? 0 : 8 /* BPF_LOG_FIXED */);
+
+			snprintf(prog_name, sizeof(prog_name),
+				 "log_%s_%d", mode ? "roll" : "fixed", i);
+			prog_fd = bpf_prog_load(BPF_PROG_TYPE_RAW_TRACEPOINT, prog_name,
+						"GPL", insns, insn_cnt, &opts);
+
+			snprintf(op_name, sizeof(op_name),
+				 "log_%s_prog_load_%d", mode ? "roll" : "fixed", i);
+			if (!ASSERT_EQ(prog_fd, -ENOSPC, op_name)) {
+				if (prog_fd >= 0)
+					close(prog_fd);
+				goto cleanup;
+			}
+
+			snprintf(op_name, sizeof(op_name),
+				 "log_%s_strlen_%d", mode ? "roll" : "fixed", i);
+			ASSERT_EQ(strlen(logs.buf), i - 1, op_name);
+
+			if (mode)
+				exp_log = logs.reference + fixed_log_sz - i;
+			else
+				exp_log = logs.reference;
+
+			snprintf(op_name, sizeof(op_name),
+				 "log_%s_contents_%d", mode ? "roll" : "fixed", i);
+			if (!ASSERT_STRNEQ(logs.buf, exp_log, i - 1, op_name)) {
+				printf("CMP:%d\nS1:'%s'\nS2:'%s'\n",
+					strncmp(logs.buf, exp_log, i - 1),
+					logs.buf, exp_log);
+				goto cleanup;
+			}
+
+			/* check that unused portions of logs.buf is not overwritten */
+			snprintf(op_name, sizeof(op_name),
+				 "log_%s_unused_%d", mode ? "roll" : "fixed", i);
+			if (!ASSERT_STREQ(logs.buf + i, logs.filler + i, op_name)) {
+				printf("CMP:%d\nS1:'%s'\nS2:'%s'\n",
+					strcmp(logs.buf + i, logs.filler + i),
+					logs.buf + i, logs.filler + i);
+				goto cleanup;
+			}
+		}
+	}
+
+cleanup:
+	test_log_buf__destroy(skel);
+}
+
+void test_verifier_log(void)
+{
+	if (test__start_subtest("good_prog-level1"))
+		verif_log_subtest("good_prog", false, 1);
+	if (test__start_subtest("good_prog-level2"))
+		verif_log_subtest("good_prog", false, 2);
+	if (test__start_subtest("bad_prog-level1"))
+		verif_log_subtest("bad_prog", true, 1);
+	if (test__start_subtest("bad_prog-level2"))
+		verif_log_subtest("bad_prog", true, 2);
+}