Re: [RFC PATCH bpf-next 3/3] selftests/bpf: add ftrace_permanent test

On Tue, Jul 28, 2026 at 08:59:59PM -0400, Andrey Grodzovsky wrote:
> Cover the FTRACE_OPS_FL_PERMANENT fix on BPF trampolines and
> ftrace-based kprobes/kretprobes: attach a fentry, fexit, kprobe, and
> kretprobe program in turn and confirm kernel.ftrace_enabled=0 is
> refused with EBUSY while attached, then succeeds once detached. Also
> confirm a new fentry/kprobe attach is itself refused while
> ftrace_enabled=0 is already in effect.
> 
> kprobe.multi/kretprobe.multi/kprobe.session are intentionally not
> covered, matching the production change's scope.
> 
> Skip the kprobe/kretprobe subtests when CONFIG_KPROBES_ON_FTRACE is
> off (e.g. arm64), read back via skel->kconfig->CONFIG_KPROBES_ON_FTRACE
> (same __kconfig idiom as progs/test_fill_link_info.c).
> 
> Assisted-by: Claude:claude-sonnet-5
> Signed-off-by: Andrey Grodzovsky <[email protected]>
> ---

hi,
I have some links active already after boot which makes test to fail:

  [root@qemu-1 bpf]# echo 0 > /proc/sys/kernel/ftrace_enabled
  bash: echo: write error: Device or resource busy

  [root@qemu-1 bpf]# ./test_progs -t ftrace_permanent -v
  bpf_testmod.ko is already unloaded.
  Loading bpf_testmod.ko...
  Successfully loaded bpf_testmod.ko.
  test_ftrace_permanent:PASS:skel_open_and_load 0 nsec
  check_blocks_disable:PASS:attach_fentry 0 nsec
  check_blocks_disable:PASS:disable_refused 0 nsec
  check_blocks_disable:PASS:read_back 0 nsec
  check_blocks_disable:PASS:still_enabled 0 nsec
  check_blocks_disable:FAIL:disable_after_detach unexpected error: -16 (errno 16)
  check_blocks_disable:PASS:reenable 0 nsec
  #145/1   ftrace_permanent/fentry_blocks_disable:FAIL
  check_blocks_disable:PASS:attach_fexit 0 nsec
  check_blocks_disable:PASS:disable_refused 0 nsec
  check_blocks_disable:PASS:read_back 0 nsec
  check_blocks_disable:PASS:still_enabled 0 nsec
  check_blocks_disable:FAIL:disable_after_detach unexpected error: -16 (errno 16)
  check_blocks_disable:PASS:reenable 0 nsec
  #145/2   ftrace_permanent/fexit_blocks_disable:FAIL
  check_blocks_disable:PASS:attach_kprobe 0 nsec
  check_blocks_disable:PASS:disable_refused 0 nsec
  check_blocks_disable:PASS:read_back 0 nsec
  check_blocks_disable:PASS:still_enabled 0 nsec
  check_blocks_disable:FAIL:disable_after_detach unexpected error: -16 (errno 16)
  check_blocks_disable:PASS:reenable 0 nsec
  #145/3   ftrace_permanent/kprobe_blocks_disable:FAIL
  check_blocks_disable:PASS:attach_kretprobe 0 nsec
  check_blocks_disable:PASS:disable_refused 0 nsec
  check_blocks_disable:PASS:read_back 0 nsec
  check_blocks_disable:PASS:still_enabled 0 nsec
  check_blocks_disable:FAIL:disable_after_detach unexpected error: -16 (errno 16)
  check_blocks_disable:PASS:reenable 0 nsec
  #145/4   ftrace_permanent/kretprobe_blocks_disable:FAIL
  check_attach_while_disabled_refused:FAIL:disable unexpected error: -16 (errno 16)
  #145/5   ftrace_permanent/fentry_attach_while_disabled_refused:FAIL
  check_attach_while_disabled_refused:FAIL:disable unexpected error: -16 (errno 16)
  #145/6   ftrace_permanent/kprobe_attach_while_disabled_refused:FAIL
  #145     ftrace_permanent:FAIL
  Summary: 0/0 PASSED, 0 SKIPPED, 1 FAILED
  Successfully unloaded bpf_testmod.ko.

jirka


>  .../bpf/prog_tests/ftrace_permanent.c         | 144 ++++++++++++++++++
>  .../selftests/bpf/progs/ftrace_permanent.c    |  43 ++++++
>  2 files changed, 187 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/ftrace_permanent.c
>  create mode 100644 tools/testing/selftests/bpf/progs/ftrace_permanent.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/ftrace_permanent.c b/tools/testing/selftests/bpf/prog_tests/ftrace_permanent.c
> new file mode 100644
> index 000000000000..dbe78009a291
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/ftrace_permanent.c
> @@ -0,0 +1,144 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2026 CrowdStrike */
> +#include <test_progs.h>
> +#include "ftrace_permanent.skel.h"
> +
> +/*
> + * fentry/fexit/kprobe/kretprobe now carry FTRACE_OPS_FL_PERMANENT, so
> + * kernel.ftrace_enabled=0 must be refused with EBUSY while any of them
> + * is attached, and allowed again once detached.
> + *
> + * kprobe.multi/kretprobe.multi/kprobe.session are out of scope.
> + * kprobe/kretprobe subtests need CONFIG_KPROBES_ON_FTRACE (e.g. not on
> + * arm64) and are skipped otherwise.
> + */
> +
> +#define FTRACE_ENABLED_PATH "/proc/sys/kernel/ftrace_enabled"
> +
> +static int read_ftrace_enabled(int *val)
> +{
> +	char buf[16] = {};
> +	int fd, n;
> +
> +	fd = open(FTRACE_ENABLED_PATH, O_RDONLY);
> +	if (fd < 0)
> +		return -errno;
> +	n = read(fd, buf, sizeof(buf) - 1);
> +	close(fd);
> +	if (n <= 0)
> +		return -EIO;
> +	*val = atoi(buf);
> +	return 0;
> +}
> +
> +/* Returns 0 on success, or -errno on write failure. */
> +static int write_ftrace_enabled(int val)
> +{
> +	char buf[4];
> +	int fd, n, len, err = 0;
> +
> +	fd = open(FTRACE_ENABLED_PATH, O_WRONLY);
> +	if (fd < 0)
> +		return -errno;
> +	len = snprintf(buf, sizeof(buf), "%d", val);
> +	n = write(fd, buf, len);
> +	if (n < 0)
> +		err = -errno;
> +	close(fd);
> +	return err;
> +}
> +
> +/*
> + * Attach @prog, assert kernel.ftrace_enabled=0 is refused while attached
> + * and stays at 1, then detach and assert the disable now succeeds.
> + */
> +static void check_blocks_disable(struct bpf_program *prog, const char *name)
> +{
> +	struct bpf_link *link;
> +	int val, err;
> +
> +	link = bpf_program__attach(prog);
> +	if (!ASSERT_OK_PTR(link, name))
> +		return;
> +
> +	err = write_ftrace_enabled(0);
> +	ASSERT_EQ(err, -EBUSY, "disable_refused");
> +	if (!ASSERT_OK(read_ftrace_enabled(&val), "read_back"))
> +		goto detach;
> +	ASSERT_EQ(val, 1, "still_enabled");
> +
> +detach:
> +	bpf_link__destroy(link);
> +
> +	ASSERT_OK(write_ftrace_enabled(0), "disable_after_detach");
> +	ASSERT_OK(write_ftrace_enabled(1), "reenable");
> +}
> +
> +/* Attach @prog while ftrace_enabled=0 and assert it is refused. */
> +static void check_attach_while_disabled_refused(struct bpf_program *prog, const char *name)
> +{
> +	struct bpf_link *link;
> +
> +	if (!ASSERT_OK(write_ftrace_enabled(0), "disable"))
> +		return;
> +
> +	link = bpf_program__attach(prog);
> +	if (!ASSERT_ERR_PTR(link, name))
> +		bpf_link__destroy(link);
> +
> +	ASSERT_OK(write_ftrace_enabled(1), "reenable");
> +}
> +
> +void test_ftrace_permanent(void)
> +{
> +	struct ftrace_permanent *skel;
> +	bool kprobes_on_ftrace;
> +	int orig = 1;
> +
> +	/* Save and always restore ftrace_enabled. */
> +	if (read_ftrace_enabled(&orig)) {
> +		test__skip();
> +		return;
> +	}
> +
> +	skel = ftrace_permanent__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
> +		goto restore;
> +
> +	kprobes_on_ftrace = skel->kconfig->CONFIG_KPROBES_ON_FTRACE;
> +
> +	if (test__start_subtest("fentry_blocks_disable"))
> +		check_blocks_disable(skel->progs.test_fentry, "attach_fentry");
> +
> +	if (test__start_subtest("fexit_blocks_disable"))
> +		check_blocks_disable(skel->progs.test_fexit, "attach_fexit");
> +
> +	if (test__start_subtest("kprobe_blocks_disable")) {
> +		if (kprobes_on_ftrace)
> +			check_blocks_disable(skel->progs.test_kprobe, "attach_kprobe");
> +		else
> +			test__skip();
> +	}
> +
> +	if (test__start_subtest("kretprobe_blocks_disable")) {
> +		if (kprobes_on_ftrace)
> +			check_blocks_disable(skel->progs.test_kretprobe, "attach_kretprobe");
> +		else
> +			test__skip();
> +	}
> +
> +	if (test__start_subtest("fentry_attach_while_disabled_refused"))
> +		check_attach_while_disabled_refused(skel->progs.test_fentry, "attach_fentry");
> +
> +	if (test__start_subtest("kprobe_attach_while_disabled_refused")) {
> +		if (kprobes_on_ftrace)
> +			check_attach_while_disabled_refused(skel->progs.test_kprobe,
> +							    "attach_kprobe");
> +		else
> +			test__skip();
> +	}
> +
> +	ftrace_permanent__destroy(skel);
> +restore:
> +	write_ftrace_enabled(orig);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/ftrace_permanent.c b/tools/testing/selftests/bpf/progs/ftrace_permanent.c
> new file mode 100644
> index 000000000000..ca706d8edc23
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/ftrace_permanent.c
> @@ -0,0 +1,43 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2026 CrowdStrike */
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +#include <stdbool.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +extern bool CONFIG_KPROBES_ON_FTRACE __kconfig __weak;
> +
> +/* This function is here to have CONFIG_KPROBES_ON_FTRACE used and
> + * added to object BTF, so the userspace side can read it back via
> + * skel->kconfig->CONFIG_KPROBES_ON_FTRACE.
> + */
> +int unused(void)
> +{
> +	return CONFIG_KPROBES_ON_FTRACE ? 0 : 1;
> +}
> +
> +SEC("fentry/bpf_fentry_test1")
> +int BPF_PROG(test_fentry, int a)
> +{
> +	return 0;
> +}
> +
> +SEC("fexit/bpf_fentry_test2")
> +int BPF_PROG(test_fexit, int a, __u64 b)
> +{
> +	return 0;
> +}
> +
> +SEC("kprobe/bpf_fentry_test3")
> +int test_kprobe(struct pt_regs *ctx)
> +{
> +	return 0;
> +}
> +
> +SEC("kretprobe/bpf_fentry_test4")
> +int BPF_KRETPROBE(test_kretprobe)
> +{
> +	return 0;
> +}
> -- 
> 2.34.1
>