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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Jiri Olsa <jolsa@redhat.com> writes:

> On Wed, Sep 09, 2020 at 05:54:58PM +0200, Toke Høiland-Jørgensen wrote:
>> Jiri Olsa <jolsa@kernel.org> 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@redhat.com>
>> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>> > ---
>> >  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