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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 09 Sep 2020 17:54:58 +0200

Jiri Olsa <[email-masked]> 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 <[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;
> +

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).

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?

-Toke

[0] https://lore.kernel.org/bpf/[email-masked]/
[1] https://git.kernel.org/pub/scm/linux/kernel/git/toke/linux.git/log/?h=bpf-freplace-multi-attach-alt-03