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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Fri, Dec 20, 2019 at 10:26 PM Martin KaFai Lau <kafai@fb.com> 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@fb.com>
> ---
>  include/linux/bpf.h               |  30 +++++++
>  include/linux/bpf_types.h         |   4 +
>  include/linux/btf.h               |  34 ++++++++
>  include/uapi/linux/bpf.h          |   1 +
>  kernel/bpf/Makefile               |   2 +-
>  kernel/bpf/bpf_struct_ops.c       | 122 +++++++++++++++++++++++++++
>  kernel/bpf/bpf_struct_ops_types.h |   4 +
>  kernel/bpf/btf.c                  |  88 ++++++++++++++------
>  kernel/bpf/syscall.c              |  17 ++--
>  kernel/bpf/verifier.c             | 134 +++++++++++++++++++++++-------
>  10 files changed, 372 insertions(+), 64 deletions(-)
>  create mode 100644 kernel/bpf/bpf_struct_ops.c
>  create mode 100644 kernel/bpf/bpf_struct_ops_types.h
>

All looks good, apart from the concern with partially-initialized
bpf_struct_ops.

[...]

> +const struct bpf_prog_ops bpf_struct_ops_prog_ops = {
> +};
> +
> +void bpf_struct_ops_init(struct btf *_btf_vmlinux)

this is always get passed vmlinux's btf, so why not call it short and
sweet "btf"? _btf_vmlinux is kind of ugly and verbose.

> +{
> +       const struct btf_member *member;
> +       struct bpf_struct_ops *st_ops;
> +       struct bpf_verifier_log log = {};
> +       const struct btf_type *t;
> +       const char *mname;
> +       s32 type_id;
> +       u32 i, j;
> +

[...]

> +static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
> +{
> +       const struct btf_type *t, *func_proto;
> +       const struct bpf_struct_ops *st_ops;
> +       const struct btf_member *member;
> +       struct bpf_prog *prog = env->prog;
> +       u32 btf_id, member_idx;
> +       const char *mname;
> +
> +       btf_id = prog->aux->attach_btf_id;
> +       st_ops = bpf_struct_ops_find(btf_id);

if struct_ops initialization fails, type will be NULL and type_id will
be 0, which we rely on here to not get partially-initialized
bpf_struct_ops, right? Small comment mentioning this would be helpful.


> +       if (!st_ops) {
> +               verbose(env, "attach_btf_id %u is not a supported struct\n",
> +                       btf_id);
> +               return -ENOTSUPP;
> +       }
> +

[...]

>  static int check_attach_btf_id(struct bpf_verifier_env *env)
>  {
>         struct bpf_prog *prog = env->prog;
> @@ -9520,6 +9591,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
>         long addr;
>         u64 key;
>
> +       if (prog->type == BPF_PROG_TYPE_STRUCT_OPS)
> +               return check_struct_ops_btf_id(env);
> +

There is a btf_id == 0 check below, you need to check that for
STRUCT_OPS as well, otherwise you can get partially-initialized
bpf_struct_ops struct in check_struct_ops_btf_id.

>         if (prog->type != BPF_PROG_TYPE_TRACING)
>                 return 0;
>
> --
> 2.17.1
>