netdev - Re: [PATCH bpf-next v2 05/11] bpf: Introduce BPF_PROG_TYPE_STRUCT

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Date:   Mon, 23 Dec 2019 19:33:40 +0000
From:   Yonghong Song <yhs@...com>
To:     Martin Lau <kafai@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Kernel Team <Kernel-team@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 05/11] bpf: Introduce BPF_PROG_TYPE_STRUCT_OPS



On 12/20/19 10:26 PM, Martin KaFai Lau wrote:
> This patch allows the kernel's struct ops (i.e. func ptr) to be
> implemented in BPF.  The first use case in this series is the
> "struct tcp_congestion_ops" which will be introduced in a
> latter patch.
> 
> This patch introduces a new prog type BPF_PROG_TYPE_STRUCT_OPS.
> The BPF_PROG_TYPE_STRUCT_OPS prog is verified against a particular
> func ptr of a kernel struct.  The attr->attach_btf_id is the btf id
> of a kernel struct.  The attr->expected_attach_type is the member
> "index" of that kernel struct.  The first member of a struct starts
> with member index 0.  That will avoid ambiguity when a kernel struct
> has multiple func ptrs with the same func signature.
> 
> For example, a BPF_PROG_TYPE_STRUCT_OPS prog is written
> to implement the "init" func ptr of the "struct tcp_congestion_ops".
> The attr->attach_btf_id is the btf id of the "struct tcp_congestion_ops"
> of the _running_ kernel.  The attr->expected_attach_type is 3.
> 
> The ctx of BPF_PROG_TYPE_STRUCT_OPS is an array of u64 args saved
> by arch_prepare_bpf_trampoline that will be done in the next
> patch when introducing BPF_MAP_TYPE_STRUCT_OPS.
> 
> "struct bpf_struct_ops" is introduced as a common interface for the kernel
> struct that supports BPF_PROG_TYPE_STRUCT_OPS prog.  The supporting kernel
> struct will need to implement an instance of the "struct bpf_struct_ops".
> 
> The supporting kernel struct also needs to implement a bpf_verifier_ops.
> During BPF_PROG_LOAD, bpf_struct_ops_find() will find the right
> bpf_verifier_ops by searching the attr->attach_btf_id.
> 
> A new "btf_struct_access" is also added to the bpf_verifier_ops such
> that the supporting kernel struct can optionally provide its own specific
> check on accessing the func arg (e.g. provide limited write access).
> 
> After btf_vmlinux is parsed, the new bpf_struct_ops_init() is called
> to initialize some values (e.g. the btf id of the supporting kernel
> struct) and it can only be done once the btf_vmlinux is available.
> 
> The R0 checks at BPF_EXIT is excluded for the BPF_PROG_TYPE_STRUCT_OPS prog
> if the return type of the prog->aux->attach_func_proto is "void".
> 
> Signed-off-by: Martin KaFai Lau <kafai@...com>

Acked-by: Yonghong Song <yhs@...com>