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Date: Wed, 9 Sep 2020 20:58:38 +0200
From: Jiri Olsa <jolsa@...hat.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Eelco Chaudron <echaudro@...hat.com>, netdev@...r.kernel.org,
bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
Andrii Nakryiko <andriin@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...omium.org>,
Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Fix context type resolving for
extension programs
On Wed, Sep 09, 2020 at 05:54:58PM +0200, Toke Høiland-Jørgensen wrote:
> Jiri Olsa <jolsa@...nel.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@...hat.com>
> > Signed-off-by: Jiri Olsa <jolsa@...nel.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?
thanks,
jirka
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