Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention mechanism

[Yafang Shao]

On Tue, Apr 25, 2023 at 5:40 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Wed, 19 Apr 2023 15:46:34 -0700
> Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
>
> > No. Just one prog at entry into any of the kernel functions
> > and another prog at entry of funcs that 1st bpf prog called indirectly.
> > Like one prog is tracing networking events while another
> > is focusing on mm. They should not conflict.
>
> You mean that you have:
>
> function start:
>   __bpf_prog_enter_recur()
>     bpf_program1()
>       __bpf_prog_enter_recur()
>         bpf_program2();
>       __bpf_prog_exit_recur()
>   __bpf_prog_exit_recur()
>
>   rest of function
>
> That is, a bpf program can be called within another bpf pogram between
> the prog_enter and prog_exit(), that is in the same context (normal,
> softirq, irq, etc)?
>

Right, that can happen per my verification. Below is a simple bpf
program to verify it.

struct {
    __uint(type, BPF_MAP_TYPE_LPM_TRIE);
    __type(key, __u64);
    __type(value, __u64);
    __uint(max_entries, 1024);
    __uint(map_flags, BPF_F_NO_PREALLOC);
} write_map SEC(".maps");

__u64 key;

SEC("fentry/kernel_clone")
int program1()
{
    __u64 value = 1;

    bpf_printk("before update");
    // It will call trie_update_elem and thus trigger program2.
    bpf_map_update_elem(&write_map, &key, &value, BPF_ANY);
    __sync_fetch_and_add(&key, 1);
    bpf_printk("after update");
    return 0;
}

SEC("fentry/trie_update_elem")
int program2()
{
    bpf_printk("trie_update_elem");
    return 0;
}

The result as follows,

         kubelet-203203  [018] ....1  9579.862862:
__bpf_prog_enter_recur: __bpf_prog_enter_recur
         kubelet-203203  [018] ...11  9579.862869: bpf_trace_printk:
before update
         kubelet-203203  [018] ....2  9579.862869:
__bpf_prog_enter_recur: __bpf_prog_enter_recur
         kubelet-203203  [018] ...12  9579.862870: bpf_trace_printk:
trie_update_elem
         kubelet-203203  [018] ....2  9579.862870:
__bpf_prog_exit_recur: __bpf_prog_exit_recur
         kubelet-203203  [018] ...11  9579.862870: bpf_trace_printk:
after update
         kubelet-203203  [018] ....1  9579.862871:
__bpf_prog_exit_recur: __bpf_prog_exit_recur

Note that we can't trace __bpf_prog_enter_recur and
__bpf_prog_exit_recur, so we have to modify the kernel to print them.

> The protection is on the trampoline where the bpf program is called.
> Not sure how ftrace can stop BPF or BPF stop ftrace, unless bpf is
> tracing a ftrace callback, or ftrace is tracing a bpf function.
>
> -- Steve



-- 
Regards
Yafang