[PATCH bpf v1 2/8] selftests/bpf: Test killing a loader during instruction rewrites

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Exercise fatal-signal handling after verifier exploration has completed. Load
a 32768-instruction control program to estimate the time needed for linear
verification, then load the same-sized program made of unconditional jumps by
zero in a child process.

The latter reaches the quadratic bpf_opt_remove_nops() rewrite. Send SIGKILL
after four control-load durations and require the child to be reaped within
one second. Without cancellation points in the rewrite helpers, the killed
child stays in BPF_PROG_LOAD until all no-ops have been removed.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../bpf/prog_tests/prog_load_signal.c         | 131 ++++++++++++++++++
 1 file changed, 131 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/prog_load_signal.c

diff --git a/tools/testing/selftests/bpf/prog_tests/prog_load_signal.c b/tools/testing/selftests/bpf/prog_tests/prog_load_signal.c
new file mode 100644
index 000000000000..0f78db3bccc7
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/prog_load_signal.c
@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+#include <test_progs.h>
+
+#define NOP_CNT		32768
+#define MIN_KILL_DELAY_NS	(100ULL * 1000 * 1000)
+#define REAP_TIMEOUT_NS		(1000ULL * 1000 * 1000)
+
+static __u64 monotonic_ns(void)
+{
+	struct timespec ts;
+
+	clock_gettime(CLOCK_MONOTONIC, &ts);
+	return ts.tv_sec * 1000000000ULL + ts.tv_nsec;
+}
+
+static void sleep_ns(__u64 duration)
+{
+	struct timespec ts = {
+		.tv_sec = duration / 1000000000ULL,
+		.tv_nsec = duration % 1000000000ULL,
+	};
+
+	while (nanosleep(&ts, &ts) && errno == EINTR)
+		;
+}
+
+static int waitpid_timeout(pid_t pid, int *status, __u64 timeout)
+{
+	__u64 deadline = monotonic_ns() + timeout;
+	int ret;
+
+	do {
+		ret = waitpid(pid, status, WNOHANG);
+		if (ret)
+			return ret;
+		usleep(1000);
+	} while (monotonic_ns() < deadline);
+
+	return 0;
+}
+
+void test_prog_load_signal(void)
+{
+	struct bpf_insn *insns = NULL;
+	__u64 start, control_time, kill_delay;
+	int pipefd[2] = { -1, -1 };
+	int prog_fd = -1, status = 0;
+	pid_t pid = -1;
+	char byte;
+	int i, ret;
+
+	insns = calloc(NOP_CNT + 2, sizeof(*insns));
+	if (!ASSERT_OK_PTR(insns, "calloc"))
+		return;
+
+	for (i = 0; i < NOP_CNT; i++)
+		insns[i] = BPF_MOV64_REG(BPF_REG_1, BPF_REG_1);
+	insns[NOP_CNT] = BPF_MOV64_IMM(BPF_REG_0, 0);
+	insns[NOP_CNT + 1] = BPF_EXIT_INSN();
+
+	start = monotonic_ns();
+	prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, NULL, "GPL",
+				insns, NOP_CNT + 2, NULL);
+	control_time = monotonic_ns() - start;
+	if (!ASSERT_GE(prog_fd, 0, "control_prog_load"))
+		goto cleanup;
+	close(prog_fd);
+	prog_fd = -1;
+
+	for (i = 0; i < NOP_CNT; i++)
+		insns[i] = BPF_JMP_IMM(BPF_JA, 0, 0, 0);
+
+	if (!ASSERT_OK(pipe(pipefd), "pipe"))
+		goto cleanup;
+
+	pid = fork();
+	if (!ASSERT_GE(pid, 0, "fork"))
+		goto cleanup;
+	if (!pid) {
+		close(pipefd[0]);
+		if (write(pipefd[1], "x", 1) != 1)
+			_exit(1);
+		close(pipefd[1]);
+		prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, NULL, "GPL",
+					insns, NOP_CNT + 2, NULL);
+		if (prog_fd >= 0)
+			close(prog_fd);
+		_exit(prog_fd < 0);
+	}
+
+	close(pipefd[1]);
+	pipefd[1] = -1;
+	ret = read(pipefd[0], &byte, 1);
+	if (!ASSERT_EQ(ret, 1, "child_ready"))
+		goto cleanup;
+
+	/*
+	 * Allow linear verification to finish before sending SIGKILL. The nop
+	 * removal pass is quadratic, so four control-load times still leaves a
+	 * wide window in which an affected kernel is rewriting instructions.
+	 */
+	kill_delay = MAX(control_time * 4, MIN_KILL_DELAY_NS);
+	sleep_ns(kill_delay);
+	if (!ASSERT_OK(kill(pid, SIGKILL), "kill"))
+		goto cleanup;
+
+	start = monotonic_ns();
+	ret = waitpid_timeout(pid, &status, REAP_TIMEOUT_NS);
+	if (!ASSERT_EQ(ret, pid, "prog_load_killable")) {
+		fprintf(stderr, "control load %llu us, child still alive %llu us after SIGKILL\n",
+			control_time / 1000, (monotonic_ns() - start) / 1000);
+		goto cleanup;
+	}
+	pid = -1;
+	ASSERT_TRUE(WIFSIGNALED(status) && WTERMSIG(status) == SIGKILL,
+		    "killed_by_sigkill");
+
+cleanup:
+	if (prog_fd >= 0)
+		close(prog_fd);
+	if (pipefd[0] >= 0)
+		close(pipefd[0]);
+	if (pipefd[1] >= 0)
+		close(pipefd[1]);
+	if (pid > 0) {
+		kill(pid, SIGKILL);
+		waitpid(pid, &status, 0);
+	}
+	free(insns);
+}
-- 
2.53.0