Messages in this thread | | | Date | Fri, 15 Sep 2023 13:46:43 +0800 | Subject | Re: [External] Re: [PATCH bpf-next v2 5/6] bpf: teach the verifier to enforce css_iter and process_iter in RCU CS | From | Chuyi Zhou <> |
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Hello.
在 2023/9/15 07:26, Andrii Nakryiko 写道: > On Thu, Sep 14, 2023 at 1:56 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote: >> >> >> >> 在 2023/9/13 21:53, Chuyi Zhou 写道: >>> Hello. >>> >>> 在 2023/9/12 15:01, Chuyi Zhou 写道: >>>> css_iter and process_iter should be used in rcu section. Specifically, in >>>> sleepable progs explicit bpf_rcu_read_lock() is needed before use these >>>> iters. In normal bpf progs that have implicit rcu_read_lock(), it's OK to >>>> use them directly. >>>> >>>> This patch checks whether we are in rcu cs before we want to invoke >>>> bpf_iter_process_new and bpf_iter_css_{pre, post}_new in >>>> mark_stack_slots_iter(). If the rcu protection is guaranteed, we would >>>> let st->type = PTR_TO_STACK | MEM_RCU. is_iter_reg_valid_init() will >>>> reject if reg->type is UNTRUSTED. >>> >>> I use the following BPF Prog to test this patch: >>> >>> SEC("?fentry.s/" SYS_PREFIX "sys_getpgid") >>> int iter_task_for_each_sleep(void *ctx) >>> { >>> struct task_struct *task; >>> struct task_struct *cur_task = bpf_get_current_task_btf(); >>> >>> if (cur_task->pid != target_pid) >>> return 0; >>> bpf_rcu_read_lock(); >>> bpf_for_each(process, task) { >>> bpf_rcu_read_unlock(); >>> if (task->pid == target_pid) >>> process_cnt += 1; >>> bpf_rcu_read_lock(); >>> } >>> bpf_rcu_read_unlock(); >>> return 0; >>> } >>> >>> Unfortunately, we can pass the verifier. >>> >>> Then I add some printk-messages before setting/clearing state to help >>> debug: >>> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >>> index d151e6b43a5f..35f3fa9471a9 100644 >>> --- a/kernel/bpf/verifier.c >>> +++ b/kernel/bpf/verifier.c >>> @@ -1200,7 +1200,7 @@ static int mark_stack_slots_iter(struct >>> bpf_verifier_env *env, >>> __mark_reg_known_zero(st); >>> st->type = PTR_TO_STACK; /* we don't have dedicated reg >>> type */ >>> if (is_iter_need_rcu(meta)) { >>> + printk("mark reg_addr : %px", st); >>> if (in_rcu_cs(env)) >>> st->type |= MEM_RCU; >>> else >>> @@ -11472,8 +11472,8 @@ static int check_kfunc_call(struct >>> bpf_verifier_env *env, struct bpf_insn *insn, >>> return -EINVAL; >>> } else if (rcu_unlock) { >>> bpf_for_each_reg_in_vstate(env->cur_state, >>> state, reg, ({ >>> + printk("clear reg_addr : %px MEM_RCU : >>> %d PTR_UNTRUSTED : %d\n ", reg, reg->type & MEM_RCU, reg->type & >>> PTR_UNTRUSTED); >>> if (reg->type & MEM_RCU) { >>> - printk("clear reg addr : %lld", >>> reg); >>> reg->type &= ~(MEM_RCU | >>> PTR_MAYBE_NULL); >>> reg->type |= PTR_UNTRUSTED; >>> } >>> >>> >>> The demsg log: >>> >>> [ 393.705324] mark reg_addr : ffff88814e40e200 >>> >>> [ 393.706883] clear reg_addr : ffff88814d5f8000 MEM_RCU : 0 >>> PTR_UNTRUSTED : 0 >>> >>> [ 393.707353] clear reg_addr : ffff88814d5f8078 MEM_RCU : 0 >>> PTR_UNTRUSTED : 0 >>> >>> [ 393.708099] clear reg_addr : ffff88814d5f80f0 MEM_RCU : 0 >>> PTR_UNTRUSTED : 0 >>> .... >>> .... >>> >>> I didn't see ffff88814e40e200 is cleared as expected because >>> bpf_for_each_reg_in_vstate didn't find it. >>> >>> It seems when we are doing bpf_read_unlock() in the middle of iteration >>> and want to clearing state through bpf_for_each_reg_in_vstate, we can >>> not find the previous reg which we marked MEM_RCU/PTR_UNTRUSTED in >>> mark_stack_slots_iter(). >>> >> >> bpf_get_spilled_reg will skip slots if they are not STACK_SPILL, but in >> mark_stack_slots_iter() we has marked the slots *STACK_ITER* >> >> With the following change, everything seems work OK. >> >> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h >> index a3236651ec64..83c5ecccadb4 100644 >> --- a/include/linux/bpf_verifier.h >> +++ b/include/linux/bpf_verifier.h >> @@ -387,7 +387,7 @@ struct bpf_verifier_state { >> >> #define bpf_get_spilled_reg(slot, frame) \ >> (((slot < frame->allocated_stack / BPF_REG_SIZE) && \ >> - (frame->stack[slot].slot_type[0] == STACK_SPILL)) \ >> + (frame->stack[slot].slot_type[0] == STACK_SPILL || >> frame->stack[slot].slot_type[0] == STACK_ITER)) \ >> ? &frame->stack[slot].spilled_ptr : NULL) >> >> I am not sure whether this would harm some logic implicitly when using >> bpf_get_spilled_reg/bpf_for_each_spilled_reg in other place. If so, >> maybe we should add a extra parameter to control the picking behaviour. >> >> #define bpf_get_spilled_reg(slot, frame, stack_type) >> \ >> (((slot < frame->allocated_stack / BPF_REG_SIZE) && \ >> (frame->stack[slot].slot_type[0] == stack_type)) \ >> ? &frame->stack[slot].spilled_ptr : NULL) >> >> Thanks. > > I don't think it's safe to just make bpf_get_spilled_reg, and > subsequently bpf_for_each_reg_in_vstate and bpf_for_each_spilled_reg > just suddenly start iterating iterator states and/or dynptrs. At least > some of existing uses of those assume they are really working just > with registers.
IIUC, when we are doing bpf_rcu_unlock, we do need to clear the state of reg including STACK_ITER.
Maybe here we only need change the logic when using bpf_for_each_reg_in_vstate to clear state in bpf_rcu_unlock and keep everything else unchanged ?
Thanks.
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