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Message-ID: <1f9cae15-979c-c049-78a9-f89d5cd1b53e@bytedance.com>
Date: Thu, 14 Sep 2023 16:56:31 +0800
From: Chuyi Zhou <zhouchuyi@...edance.com>
To: bpf@...r.kernel.org,
Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
martin.lau@...nel.org, tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 5/6] bpf: teach the verifier to enforce
css_iter and process_iter in RCU CS
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.
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