Jiri Olsa <jolsa@xxxxxxxxxx> writes:
> On Wed, Sep 09, 2020 at 05:54:58PM +0200, Toke Høiland-Jørgensen wrote:
>> Jiri Olsa <jolsa@xxxxxxxxxx> writes:
>>
>> > 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 <echaudro@xxxxxxxxxx>
>> > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
>> > ---
>> > 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;
>> > +
>>
>> In the discussion about multi-attach for freplace we kinda concluded[0]
>> that this linked_prog pointer was going away after attach. I have this
>> basically working, but need to test a bit more before posting it (see
>> [1] for current status).
>
> ok, feel free to use the test case from patch 2 ;-)
>
>>
>> But with this I guess we'll need to either do something different? Maybe
>> go chase down the target via the bpf_link or something?
>
> I'll check, could you please CC me on your next post?
Sure, will do!
-Toke
