Incorrect BPF stats accounting for fentry on arm64

[Xu Kuohai]

On 4/12/2024 2:09 AM, Ivan Babrou wrote:
> Hello,
> 
> We're seeing incorrect data for bpf runtime stats on arm64. Here's an example:
> 
> $ sudo bpftool prog show id 693110
> 693110: tracing  name __tcp_retransmit_skb  tag e37be2fbe8be4726  gpl
> run_time_ns 2493581964213176 run_cnt 1133532 recursion_misses 1
>      loaded_at 2024-04-10T22:33:09+0000  uid 62727
>      xlated 312B  jited 344B  memlock 4096B  map_ids 8550445,8550441
>      btf_id 8726522
>      pids prometheus-ebpf(2224907)
> 
> According to bpftool, this program reported 66555800ns of runtime at
> one point and then it jumped to 2493581675247416ns just 53s later when
> we looked at it again. This is happening only on arm64 nodes in our
> fleet on both v6.1.82 and v6.6.25.
> 
> We have two services that are involved:
> 
> * ebpf_exporter attaches bpf programs to the kernel and exports
> prometheus metrics and opentelementry traces driven by its probes
> * bpf_stats_exporter runs bpftool every 53s to capture bpf runtime metrics
> 
> The problematic fentry is attached to __tcp_retransmit_skb, but an
> identical one is also attached to tcp_send_loss_probe, which does not
> exhibit the same issue:
> 
> SEC("fentry/__tcp_retransmit_skb")
> int BPF_PROG(__tcp_retransmit_skb, struct sock *sk)
> {
>    return handle_sk((struct pt_regs *) ctx, sk, sk_kind_tcp_retransmit_skb);
> }
> 
> SEC("fentry/tcp_send_loss_probe")
> int BPF_PROG(tcp_send_loss_probe, struct sock *sk)
> {
>    return handle_sk((struct pt_regs *) ctx, sk, sk_kind_tcp_send_loss_probe);
> }
> 
> In handle_sk we do a map lookup and an optional ringbuf push. There is
> no sleeping (I don't think it's even allowed on v6.1). It's
> interesting that it only happens for the retransmit, but not for the
> loss probe.
> 
> The issue manifests some time after we restart ebpf_exporter and
> reattach the probes. It doesn't happen immediately, as we need to
> capture metrics 53s apart to produce a visible spike in metrics.
> 
> There is no corresponding spike in execution count, only in execution time.
> 
> It doesn't happen deterministically. Some ebpf_exporter restarts show
> it, some don't.
> 
> It doesn't keep happening after ebpf_exporter restart. It happens once
> and that's it.
> 
> Maybe recursion_misses plays a role here? We see none for
> tcp_send_loss_probe. We do see some for inet_sk_error_report
> tracepoint, but it doesn't spike like __tcp_retransmit_skb does.
> 
> The biggest smoking gun is that it only happens on arm64.
> 
> I'm happy to try out patches to figure this one out.
> 

I guess the issue is caused by the not setting of x20 register
when __bpf_prog_enter(prog) returns zero.

The following patch may help:

--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -1905,15 +1905,15 @@ static void invoke_bpf_prog(struct jit_ctx *ctx, struct bpf_tramp_link *l,

         emit_call(enter_prog, ctx);

+       /* save return value to callee saved register x20 */
+       emit(A64_MOV(1, A64_R(20), A64_R(0)), ctx);
+
         /* if (__bpf_prog_enter(prog) == 0)
          *         goto skip_exec_of_prog;
          */
         branch = ctx->image + ctx->idx;
         emit(A64_NOP, ctx);

-       /* save return value to callee saved register x20 */
-       emit(A64_MOV(1, A64_R(20), A64_R(0)), ctx);
-
         emit(A64_ADD_I(1, A64_R(0), A64_SP, args_off), ctx);
         if (!p->jited)
                 emit_addr_mov_i64(A64_R(1), (const u64)p->insnsi, ctx);