selftests/bpf: Add torn write detection test for htab BPF_F_LOCK

Add a consistency subtest to htab_reuse that detects torn writes
caused by the BPF_F_LOCK lockless update racing with element
reallocation in alloc_htab_elem().

The test uses three thread roles started simultaneously via a pipe:
 - locked updaters: BPF_F_LOCK|BPF_EXIST in-place updates
 - delete+update workers: delete then BPF_ANY|BPF_F_LOCK insert
 - locked readers: BPF_F_LOCK lookup checking value consistency

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260401-bpf_map_torn_writes-v1-2-782d071c55e7@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -59,7 +59,7 @@ static void *htab_update_fn(void *arg)
 	return NULL;
 }
 
-void test_htab_reuse(void)
+static void test_htab_reuse_basic(void)
 {
 	unsigned int i, wr_nr = 1, rd_nr = 4;
 	pthread_t tids[wr_nr + rd_nr];
@@ -99,3 +99,170 @@ void test_htab_reuse(void)
 	}
 	htab_reuse__destroy(skel);
 }
+
+/*
+ * Writes consistency test for BPF_F_LOCK update
+ *
+ * The race:
+ *   1. Thread A: BPF_F_LOCK|BPF_EXIST update
+ *   2. Thread B: delete element then update it with BPF_ANY
+ */
+
+struct htab_val_large {
+	struct bpf_spin_lock lock;
+	__u32 seq;
+	__u64 data[256];
+};
+
+struct consistency_ctx {
+	int fd;
+	int start_fd;
+	int loop;
+	volatile bool torn_write;
+};
+
+static void wait_for_start(int fd)
+{
+	char buf;
+
+	read(fd, &buf, 1);
+}
+
+static void *locked_update_fn(void *arg)
+{
+	struct consistency_ctx *ctx = arg;
+	struct htab_val_large value;
+	unsigned int key = 1;
+	int i;
+
+	memset(&value, 0xAA, sizeof(value));
+	wait_for_start(ctx->start_fd);
+
+	for (i = 0; i < ctx->loop; i++) {
+		value.seq = i;
+		bpf_map_update_elem(ctx->fd, &key, &value,
+				    BPF_F_LOCK | BPF_EXIST);
+	}
+
+	return NULL;
+}
+
+/* Delete + update: removes the element then re-creates it with BPF_ANY. */
+static void *delete_update_fn(void *arg)
+{
+	struct consistency_ctx *ctx = arg;
+	struct htab_val_large value;
+	unsigned int key = 1;
+	int i;
+
+	memset(&value, 0xBB, sizeof(value));
+
+	wait_for_start(ctx->start_fd);
+
+	for (i = 0; i < ctx->loop; i++) {
+		value.seq = i;
+		bpf_map_delete_elem(ctx->fd, &key);
+		bpf_map_update_elem(ctx->fd, &key, &value, BPF_ANY | BPF_F_LOCK);
+	}
+
+	return NULL;
+}
+
+static void *locked_lookup_fn(void *arg)
+{
+	struct consistency_ctx *ctx = arg;
+	struct htab_val_large value;
+	unsigned int key = 1;
+	int i, j;
+
+	wait_for_start(ctx->start_fd);
+
+	for (i = 0; i < ctx->loop && !ctx->torn_write; i++) {
+		if (bpf_map_lookup_elem_flags(ctx->fd, &key, &value, BPF_F_LOCK))
+			continue;
+
+		for (j = 0; j < 256; j++) {
+			if (value.data[j] != value.data[0]) {
+				ctx->torn_write = true;
+				return NULL;
+			}
+		}
+	}
+
+	return NULL;
+}
+
+static void test_htab_reuse_consistency(void)
+{
+	int threads_total = 6, threads = 2;
+	pthread_t tids[threads_total];
+	struct consistency_ctx ctx;
+	struct htab_val_large seed;
+	struct htab_reuse *skel;
+	unsigned int key = 1, i;
+	int pipefd[2];
+	int err;
+
+	skel = htab_reuse__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "htab_reuse__open_and_load"))
+		return;
+
+	if (!ASSERT_OK(pipe(pipefd), "pipe"))
+		goto out;
+
+	ctx.fd = bpf_map__fd(skel->maps.htab_lock_consistency);
+	ctx.start_fd = pipefd[0];
+	ctx.loop = 100000;
+	ctx.torn_write = false;
+
+	/* Seed the element so locked updaters have something to find */
+	memset(&seed, 0xBB, sizeof(seed));
+	err = bpf_map_update_elem(ctx.fd, &key, &seed, BPF_ANY);
+	if (!ASSERT_OK(err, "seed_element"))
+		goto close_pipe;
+
+	memset(tids, 0, sizeof(tids));
+	for (i = 0; i < threads; i++) {
+		err = pthread_create(&tids[i], NULL, locked_update_fn, &ctx);
+		if (!ASSERT_OK(err, "pthread_create"))
+			goto stop;
+	}
+	for (i = 0; i < threads; i++) {
+		err = pthread_create(&tids[threads + i], NULL, delete_update_fn, &ctx);
+		if (!ASSERT_OK(err, "pthread_create"))
+			goto stop;
+	}
+	for (i = 0; i < threads; i++) {
+		err = pthread_create(&tids[threads * 2 + i], NULL, locked_lookup_fn, &ctx);
+		if (!ASSERT_OK(err, "pthread_create"))
+			goto stop;
+	}
+
+	/* Release all threads simultaneously */
+	close(pipefd[1]);
+	pipefd[1] = -1;
+
+stop:
+	for (i = 0; i < threads_total; i++) {
+		if (!tids[i])
+			continue;
+		pthread_join(tids[i], NULL);
+	}
+
+	ASSERT_FALSE(ctx.torn_write, "no torn writes detected");
+
+close_pipe:
+	if (pipefd[1] >= 0)
+		close(pipefd[1]);
+	close(pipefd[0]);
+out:
+	htab_reuse__destroy(skel);
+}
+
+void test_htab_reuse(void)
+{
+	if (test__start_subtest("basic"))
+		test_htab_reuse_basic();
+	if (test__start_subtest("consistency"))
+		test_htab_reuse_consistency();
+}

tools/testing/selftests/bpf/progs/htab_reuse.c

@@ -17,3 +17,19 @@ struct {
 	__type(value, struct htab_val);
 	__uint(map_flags, BPF_F_NO_PREALLOC);
 } htab SEC(".maps");
+
+#define HTAB_NDATA 256
+
+struct htab_val_large {
+	struct bpf_spin_lock lock;
+	__u32 seq;
+	__u64 data[HTAB_NDATA];
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 8);
+	__type(key, unsigned int);
+	__type(value, struct htab_val_large);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+} htab_lock_consistency SEC(".maps");