On Mon, Mar 17, 2025 at 5:18 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote: > > If we attach fexit/fmod_ret to __noreturn functions, 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() with fexit 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. That 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 > > In this patch, all functions annotated with __noreturn are rejected, except > for the following cases: > - Functions that result in a system reboot, such as panic, > machine_real_restart and rust_begin_unwind > - Functions that are never executed by tasks, such as rest_init and > cpu_startup_entry > - Functions implemented in assembly, such as rewind_stack_and_make_dead and > xen_cpu_bringup_again, lack an associated BTF ID. > > 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 -- > Attaching fexit/fmod_ret to __noreturn functions is rejected. > > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx> > --- > kernel/bpf/verifier.c | 48 +++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 48 insertions(+) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 9971c03adfd5..b7d7d5c4989f 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -22841,6 +22841,49 @@ BTF_ID(func, __rcu_read_unlock) > #endif > BTF_SET_END(btf_id_deny) > > +/* fexit and fmod_ret can't be used to attach to __noreturn functions. > + * Currently, we must manually list all __noreturn functions here. Once a more > + * robust solution is implemented, this workaround can be removed. > + */ > +BTF_SET_START(noreturn_deny) > +#define NORETURN(fn) BTF_ID(func, fn) no need for extra macro. Just use BTF_ID(...) below. > +#ifdef CONFIG_IA32_EMULATION > +NORETURN(__ia32_sys_exit) > +NORETURN(__ia32_sys_exit_group) > +#endif > +#ifdef CONFIG_KUNIT > +NORETURN(__kunit_abort) > +NORETURN(kunit_try_catch_throw) > +#endif > +#ifdef CONFIG_MODULES > +NORETURN(__module_put_and_kthread_exit) > +#endif > +#ifdef CONFIG_X86_64 > +NORETURN(__x64_sys_exit) > +NORETURN(__x64_sys_exit_group) > +#endif > +#ifdef CONFIG_XEN_PV_SMP > +NORETURN(cpu_bringup_and_idle) > +#endif it's called during bringup. bpf doesn't exist at that time. Drop it. > +NORETURN(do_exit) > +NORETURN(do_group_exit) > +#if defined(CONFIG_X86) && defined(CONFIG_SMP) > +NORETURN(hlt_play_dead) > +#endif This one is similar to panic. Drop it. > +#ifdef CONFIG_HYPERV > +NORETURN(hv_ghcb_terminate) > +#endif Also does 'hlt'. Drop it. > +NORETURN(kthread_complete_and_exit) > +NORETURN(kthread_exit) > +NORETURN(make_task_dead) > +#ifdef CONFIG_AMD_MEM_ENCRYPT > +NORETURN(sev_es_terminate) > +NORETURN(snp_abort) drop both for the same reason as above. > +#endif > +NORETURN(stop_this_cpu) and this one as well. Pls make sure to resend as series of 2 patches otherwise bpf CI will complain. pw-bot: cr
