Subject: Re: [PATCH bpf-next 1/2] bpf: Fix context type resolving for extension programs

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 11 Sep 2020 12:52:17 +0200

Alexei Starovoitov <[email-masked]> writes:

> On Wed, Sep 9, 2020 at 8:11 AM Jiri Olsa <[email-masked]> wrote:
>>
>> Eelco reported we can't properly access arguments if the tracing
>> program is attached to extension program.
>>
>> Having following program:
>>
>>   SEC("classifier/test_pkt_md_access")
>>   int test_pkt_md_access(struct __sk_buff *skb)
>>
>> with its extension:
>>
>>   SEC("freplace/test_pkt_md_access")
>>   int test_pkt_md_access_new(struct __sk_buff *skb)
>>
>> and tracing that extension with:
>>
>>   SEC("fentry/test_pkt_md_access_new")
>>   int BPF_PROG(fentry, struct sk_buff *skb)
>>
>> It's not possible to access skb argument in the fentry program,
>> with following error from verifier:
>>
>>   ; int BPF_PROG(fentry, struct sk_buff *skb)
>>   0: (79) r1 = *(u64 *)(r1 +0)
>>   invalid bpf_context access off=0 size=8
>>
>> The problem is that btf_ctx_access gets the context type for the
>> traced program, which is in this case the extension.
>>
>> But when we trace extension program, we want to get the context
>> type of the program that the extension is attached to, so we can
>> access the argument properly in the trace program.
>>
>> Reported-by: Eelco Chaudron <[email-masked]>
>> Signed-off-by: Jiri Olsa <[email-masked]>
>> ---
>>  kernel/bpf/btf.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index f9ac6935ab3c..37ad01c32e5a 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -3859,6 +3859,14 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>         }
>>
>>         info->reg_type = PTR_TO_BTF_ID;
>> +
>> +       /* When we trace extension program, we want to get the context
>> +        * type of the program that the extension is attached to, so
>> +        * we can access the argument properly in the trace program.
>> +        */
>> +       if (tgt_prog && tgt_prog->type == BPF_PROG_TYPE_EXT)
>> +               tgt_prog = tgt_prog->aux->linked_prog;
>> +
>>         if (tgt_prog) {
>>                 ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type, arg);
>
> I think it would be cleaner to move resolve_prog_type() from verifier.c
> and use that helper function here.

FYI, I've added a different version of this patch to my freplace
multi-attach series (since the approach here was incompatible with
that).

-Toke