On Mon, Feb 10, 2025 at 6:34 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> If we attach fexit to a function annotated with __noreturn, it will
> cause an issue that the bpf trampoline image will be left over even if
> the bpf link has been destroyed. Take attaching do_exit() for example. The
> fexit works as follows,
>
> bpf_trampoline
> + __bpf_tramp_enter
> + percpu_ref_get(&tr->pcref);
>
> + call do_exit()
>
> + __bpf_tramp_exit
> + percpu_ref_put(&tr->pcref);
>
> Since do_exit() never returns, the refcnt of the trampoline image is
> never decremented, preventing it from being freed. This can be verified
> with as follows,
>
> $ bpftool link show <<<< nothing output
> $ grep "bpf_trampoline_[0-9]" /proc/kallsyms
> ffffffffc04cb000 t bpf_trampoline_6442526459 [bpf] <<<< leftover
>
> With this change, attaching fexit probes to functions like do_exit() will
> be rejected.
>
> $ ./fexit
> libbpf: prog 'fexit': BPF program load failed: -EINVAL
> libbpf: prog 'fexit': -- BEGIN PROG LOAD LOG --
> processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> -- END PROG LOAD LOG --
> libbpf: prog 'fexit': failed to load: -EINVAL
> libbpf: failed to load object 'fexit_bpf'
> libbpf: failed to load BPF skeleton 'fexit_bpf': -EINVAL
> failed to load BPF object -22
>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> ---
> kernel/bpf/verifier.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 9971c03adfd5..f7224fc61e0c 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -22841,6 +22841,13 @@ BTF_ID(func, __rcu_read_unlock)
> #endif
> BTF_SET_END(btf_id_deny)
>
> +/* The functions annotated with __noreturn are denied. */
> +BTF_SET_START(fexit_deny)
> +#define NORETURN(fn) BTF_ID(func, fn)
> +#include <linux/noreturns.h>
> +#undef NORETURN
> +BTF_SET_END(fexit_deny)
> +
> static bool can_be_sleepable(struct bpf_prog *prog)
> {
> if (prog->type == BPF_PROG_TYPE_TRACING) {
> @@ -22929,6 +22936,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
> } else if (prog->type == BPF_PROG_TYPE_TRACING &&
> btf_id_set_contains(&btf_id_deny, btf_id)) {
> return -EINVAL;
> + } else if (prog->expected_attach_type == BPF_TRACE_FEXIT &&
> + btf_id_set_contains(&fexit_deny, btf_id)) {
Please add a verifier log (with verbose()) here, so that the user knows
why the program failed to attach.
Thanks,
Song
> + return -EINVAL;
> }
>
> key = bpf_trampoline_compute_key(tgt_prog, prog->aux->attach_btf, btf_id);
> --
> 2.43.5
>
