selftests/bpf: add file dynptr tests

Introducing selftests for validating file-backed dynptr works as
expected.
 * validate implementation supports dynptr slice and read operations
 * validate destructors should be paired with initializers
 * validate sleepable progs can page in.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251026203853.135105-11-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 145 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/file_reader.c b/tools/testing/selftests/bpf/progs/file_reader.c
new file mode 100644
index 000000000000..2585f83b0ce5
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/file_reader.c
@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <string.h>
+#include <stdbool.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+#include "errno.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} arrmap SEC(".maps");
+
+struct elem {
+	struct file *file;
+	struct bpf_task_work tw;
+};
+
+char user_buf[256000];
+char tmp_buf[256000];
+
+int pid = 0;
+int err, run_success = 0;
+
+static int validate_file_read(struct file *file);
+static int task_work_callback(struct bpf_map *map, void *key, void *value);
+
+SEC("lsm/file_open")
+int on_open_expect_fault(void *c)
+{
+	struct bpf_dynptr dynptr;
+	struct file *file;
+	int local_err = 1;
+	__u32 user_buf_sz = sizeof(user_buf);
+
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
+		return 0;
+
+	file = bpf_get_task_exe_file(bpf_get_current_task_btf());
+	if (!file)
+		return 0;
+
+	if (bpf_dynptr_from_file(file, 0, &dynptr))
+		goto out;
+
+	local_err = bpf_dynptr_read(tmp_buf, user_buf_sz, &dynptr, 0, 0);
+	if (local_err == -EFAULT) { /* Expect page fault */
+		local_err = 0;
+		run_success = 1;
+	}
+out:
+	bpf_dynptr_file_discard(&dynptr);
+	if (local_err)
+		err = local_err;
+	bpf_put_file(file);
+	return 0;
+}
+
+SEC("lsm/file_open")
+int on_open_validate_file_read(void *c)
+{
+	struct task_struct *task = bpf_get_current_task_btf();
+	struct elem *work;
+	int key = 0;
+
+	if (bpf_get_current_pid_tgid() >> 32 != pid)
+		return 0;
+
+	work = bpf_map_lookup_elem(&arrmap, &key);
+	if (!work) {
+		err = 1;
+		return 0;
+	}
+	bpf_task_work_schedule_signal(task, &work->tw, &arrmap, task_work_callback, NULL);
+	return 0;
+}
+
+/* Called in a sleepable context, read 256K bytes, cross check with user space read data */
+static int task_work_callback(struct bpf_map *map, void *key, void *value)
+{
+	struct task_struct *task = bpf_get_current_task_btf();
+	struct file *file = bpf_get_task_exe_file(task);
+
+	if (!file)
+		return 0;
+
+	err = validate_file_read(file);
+	if (!err)
+		run_success = 1;
+	bpf_put_file(file);
+	return 0;
+}
+
+static int verify_dynptr_read(struct bpf_dynptr *ptr, u32 off, char *user_buf, u32 len)
+{
+	int i;
+
+	if (bpf_dynptr_read(tmp_buf, len, ptr, off, 0))
+		return 1;
+
+	/* Verify file contents read from BPF is the same as the one read from userspace */
+	bpf_for(i, 0, len)
+	{
+		if (tmp_buf[i] != user_buf[i])
+			return 1;
+	}
+	return 0;
+}
+
+static int validate_file_read(struct file *file)
+{
+	struct bpf_dynptr dynptr;
+	int loc_err = 1, off;
+	__u32 user_buf_sz = sizeof(user_buf);
+
+	if (bpf_dynptr_from_file(file, 0, &dynptr))
+		goto cleanup;
+
+	loc_err = verify_dynptr_read(&dynptr, 0, user_buf, user_buf_sz);
+	off = 1;
+	loc_err = loc_err ?: verify_dynptr_read(&dynptr, off, user_buf + off, user_buf_sz - off);
+	off = user_buf_sz - 1;
+	loc_err = loc_err ?: verify_dynptr_read(&dynptr, off, user_buf + off, user_buf_sz - off);
+	/* Read file with random offset and length */
+	off = 4097;
+	loc_err = loc_err ?: verify_dynptr_read(&dynptr, off, user_buf + off, 100);
+
+	/* Adjust dynptr, verify read */
+	loc_err = loc_err ?: bpf_dynptr_adjust(&dynptr, off, off + 1);
+	loc_err = loc_err ?: verify_dynptr_read(&dynptr, 0, user_buf + off, 1);
+	/* Can't read more than 1 byte */
+	loc_err = loc_err ?: verify_dynptr_read(&dynptr, 0, user_buf + off, 2) == 0;
+	/* Can't read with far offset */
+	loc_err = loc_err ?: verify_dynptr_read(&dynptr, 1, user_buf + off, 1) == 0;
+
+cleanup:
+	bpf_dynptr_file_discard(&dynptr);
+	return loc_err;
+}