Re: Trying the bpf trace a bpf xdp program

["Eelco Chaudron" <echaudro@redhat.com>]

On 4 Dec 2019, at 19:52, Eelco Chaudron wrote:

> On 4 Dec 2019, at 19:01, Yonghong Song wrote:
>
> <SNIP>
>
>>>> I’ve put my code on GitHub, maybe it’s just something stupid…
>>
>> Thanks for the test case. This indeed a kernel bug.
>> The following change fixed the issue:
>>
>>
>> -bash-4.4$ git diff
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index a0482e1c4a77..034ef81f935b 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -9636,7 +9636,10 @@ static int check_attach_btf_id(struct
>> bpf_verifier_env *env)
>>                                  ret = -EINVAL;
>>                                  goto out;
>>                          }
>> -                       addr = (long)
>> tgt_prog->aux->func[subprog]->bpf_func;
>> +                       if (subprog == 0)
>> +                               addr = (long) tgt_prog->bpf_func;
>> +                       else
>> +                               addr = (long)
>> tgt_prog->aux->func[subprog]->bpf_func;
>>                  } else {
>>                          addr = kallsyms_lookup_name(tname);
>>                          if (!addr) {
>> -bash-4.4$
>>
>> The reason is for a bpf program without any additional subprogram
>> (callees), tgt_prog->aux->func is not populated and is a NULL 
>> pointer,
>> so the access tgt_prog->aux->func[0]->bpf_func will segfault.
>>
>> With the above change, your test works properly.
>
> Thanks for the quick response, and as you mention the test passes with 
> the patch above.
>
> I will continue my experiments later this week, and let you know if I 
> run into any other problems.
>

With the following program I get some access errors:

#define bpf_debug(fmt, ...)                         \
{                                                   \
   char __fmt[] = fmt;                               \
   bpf_trace_printk(__fmt, sizeof(__fmt),            \
                    ##__VA_ARGS__);                  \
}

BPF_TRACE_2("fexit/xdp_prog_simple", trace_on_exit,
             struct xdp_md *, xdp, int, ret)
{
   __u32 rx_queue;

   __builtin_preserve_access_index(({
         rx_queue = xdp->rx_queue_index;
       }));

   bpf_debug("fexit: queue = %u, ret = %d\n", rx_queue, ret);

   return 0;
}

I assume the XDP context has not been vetted?

libbpf: -- BEGIN DUMP LOG ---
libbpf:
func#0 @0
BPF program ctx type is not a struct
Type info disagrees with actual arguments due to compiler optimizations
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
; BPF_TRACE_2("fexit/xdp_prog_simple", trace_on_exit,
0: (b7) r2 = 16
1: R1=ctx(id=0,off=0,imm=0) R2_w=inv16 R10=fp0
; BPF_TRACE_2("fexit/xdp_prog_simple", trace_on_exit,
1: (79) r3 = *(u64 *)(r1 +0)
2: R1=ctx(id=0,off=0,imm=0) R2_w=inv16 
R3_w=ptr_xdp_buff(id=0,off=0,imm=0) R10=fp0
2: (0f) r3 += r2
last_idx 2 first_idx 0
regs=4 stack=0 before 1: (79) r3 = *(u64 *)(r1 +0)
regs=4 stack=0 before 0: (b7) r2 = 16
3: R1=ctx(id=0,off=0,imm=0) R2_w=invP16 
R3_w=ptr_xdp_buff(id=0,off=16,imm=0) R10=fp0
; rx_queue = xdp->rx_queue_index;
3: (61) r3 = *(u32 *)(r3 +0)
cannot access ptr member data_meta with moff 16 in struct xdp_buff with 
off 16 size 4
verification time 102 usec
stack depth 0
processed 4 insns (limit 1000000) max_states_per_insn 0 total_states 0 
peak_states 0 mark_read 0
libbpf: -- END LOG --


Trying to use the helpers, passes verification, however, it’s dumping 
invalid content:

BPF_TRACE_2("fexit/xdp_prog_simple", trace_on_exit,
             struct xdp_md *, xdp, int, ret)
{
   __u32 rx_queue;

   bpf_probe_read_kernel(&rx_queue, sizeof(rx_queue),
                         __builtin_preserve_access_index(&xdp->rx_queue_index));

   bpf_debug("fexit: queue = %u, ret = %d\n", rx_queue, ret);
   return 0;
}

Debug output:

    ping6-2752  [004] ..s1 60763.917790: 0: SIMPLE: [ifindex = 4, queue 
=  0]
    ping6-2752  [004] ..s1 60763.917800: 0: fexit: queue = 2969379072, 
ret = 2
    ping6-2752  [004] ..s1 60764.941817: 0: SIMPLE: [ifindex = 4, queue 
=  0]
    ping6-2752  [004] ..s1 60764.941828: 0: fexit: queue = 2969379072, 
ret = 2
    ping6-2752  [004] ..s1 60765.965835: 0: SIMPLE: [ifindex = 4, queue 
=  0]


Tried the same with fentry for this function, but the same results as 
fexit.

Any hints?

Thanks,


Eelco