Re: [PATCH bpf v3 2/2] selftests/bpf: Add test to ensure kprobe_multi is not sleepable

Kumar Kartikeya Dwivedi
2026-04-01T22:50:51Z

On Wed, 1 Apr 2026 at 21:11, Varun R Mallya <varunrmallya@gmail.com> wrote:
>
> Add a selftest to ensure that kprobe_multi programs cannot be attached
> using the BPF_F_SLEEPABLE flag. This test succeeds when the kernel
> rejects attachment of kprobe_multi when the BPF_F_SLEEPABLE flag is set.
>
> Signed-off-by: Varun R Mallya <varunrmallya@gmail.com>
> ---

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

> .../bpf/prog_tests/kprobe_multi_test.c | 41 +++++++++++++++++++
> .../bpf/progs/kprobe_multi_sleepable.c | 13 ++++++
> 2 files changed, 54 insertions(+)
> create mode 100644 tools/testing/selftests/bpf/progs/kprobe_multi_sleepable.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
> index 78c974d4ea33..f02fec2b6fda 100644
> --- a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
> +++ b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
> @@ -10,6 +10,7 @@
> #include "kprobe_multi_session_cookie.skel.h"
> #include "kprobe_multi_verifier.skel.h"
> #include "kprobe_write_ctx.skel.h"
> +#include "kprobe_multi_sleepable.skel.h"
> #include "bpf/libbpf_internal.h"
> #include "bpf/hashmap.h"
>
> @@ -633,6 +634,44 @@ static void test_attach_write_ctx(void)
> }
> #endif
>
> +static void test_attach_multi_sleepable(void)
> +{
> + struct kprobe_multi_sleepable *skel;
> + int err;
> +
> + skel = kprobe_multi_sleepable__open();
> + if (!ASSERT_OK_PTR(skel, "kprobe_multi_sleepable__open"))
> + return;
> +
> + err = bpf_program__set_flags(skel->progs.handle_kprobe_multi_sleepable,
> + BPF_F_SLEEPABLE);
> + if (!ASSERT_OK(err, "bpf_program__set_flags"))
> + goto cleanup;
> +
> + /* Load should succeed even with BPF_F_SLEEPABLE for KPROBE types */
> + err = kprobe_multi_sleepable__load(skel);
> + if (!ASSERT_OK(err, "kprobe_multi_sleepable__load"))
> + goto cleanup;
> +
> + /* Attachment must fail for kprobe.multi + BPF_F_SLEEPABLE.
> + * Also chosen a stable symbol to send into opts
> + */
> + LIBBPF_OPTS(bpf_kprobe_multi_opts, opts);
> + const char *sym = "vfs_read";
> +
> + opts.syms = &sym;
> + opts.cnt = 1;
> +
> + skel->links.handle_kprobe_multi_sleepable =
> + bpf_program__attach_kprobe_multi_opts(skel->progs.handle_kprobe_multi_sleepable,
> + NULL, &opts);
> + ASSERT_ERR_PTR(skel->links.handle_kprobe_multi_sleepable,
> + "bpf_program__attach_kprobe_multi_opts");

Nit: While vfs_read will likely remain stable, the check could
probably be stronger to distinguish an attach error from -EINVAL?
I added a typo to vfs_read and it still passed, because it failed to
attach instead of getting rejected on unfixed kernel.
May not be a big deal since vfs_read is unlikely to break.
I verified it works by adding bpf_copy_from_user to the program and
attaching to SYS_PREFIX sys_getpid and invoking the splat though, so
LGTM otherwise.

> +
> +cleanup:
> + kprobe_multi_sleepable__destroy(skel);
> +}
> +
> …