Re: [PATCH v4 bpf-next 17/20] bpf: Support attaching tracing BPF program to other BPF programs

From: Song Liu <songliubraving@fb.com>
To: Alexei Starovoitov <ast@kernel.org>
Cc: David Miller <davem@davemloft.net>,
	"daniel@iogearbox.net" <daniel@iogearbox.net>,
	"x86@kernel.org" <x86@kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH v4 bpf-next 17/20] bpf: Support attaching tracing BPF program to other BPF programs
Date: Thu, 14 Nov 2019 23:30:29 +0000	[thread overview]
Message-ID: <7F2E3D6E-C85C-44E1-9A99-4B28B0B60931@fb.com> (raw)
In-Reply-To: <20191114185720.1641606-18-ast@kernel.org>

> On Nov 14, 2019, at 10:57 AM, Alexei Starovoitov <ast@kernel.org> wrote:
> 
> Allow FENTRY/FEXIT BPF programs to attach to other BPF programs of any type
> including their subprograms. This feature allows snooping on input and output
> packets in XDP, TC programs including their return values. In order to do that
> the verifier needs to track types not only of vmlinux, but types of other BPF
> programs as well. The verifier also needs to translate uapi/linux/bpf.h types
> used by networking programs into kernel internal BTF types used by FENTRY/FEXIT
> BPF programs. In some cases LLVM optimizations can remove arguments from BPF
> subprograms without adjusting BTF info that LLVM backend knows. When BTF info
> disagrees with actual types that the verifiers sees the BPF trampoline has to
> fallback to conservative and treat all arguments as u64. The FENTRY/FEXIT
> program can still attach to such subprograms, but it won't be able to recognize
> pointer types like 'struct sk_buff *' and it won't be able to pass them to
> bpf_skb_output() for dumping packets to user space. The FENTRY/FEXIT program
> would need to use bpf_probe_read_kernel() instead.
> 
> The BPF_PROG_LOAD command is extended with attach_prog_fd field. When it's set
> to zero the attach_btf_id is one vmlinux BTF type ids. When attach_prog_fd
> points to previously loaded BPF program the attach_btf_id is BTF type id of
> main function or one of its subprograms.
> 
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Acked-by: Song Liu <songliubraving@fb.com>

[...]

> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 4620267b186e..40efde5eedcb 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3530,6 +3530,20 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, struct btf *btf,
> 	return ctx_type;
> }
> 

nit: maybe add a comment here or where we call btf_translate_to_vmlinux(). 

> +static int btf_translate_to_vmlinux(struct bpf_verifier_log *log,
> +				     struct btf *btf,
> +				     const struct btf_type *t,
> +				     enum bpf_prog_type prog_type)
> +{