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

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

On Mon, Dec 23, 2019 at 2:30 PM Martin Lau <kafai@fb.com> wrote:
>
> On Mon, Dec 23, 2019 at 12:29:37PM -0800, Andrii Nakryiko wrote:
> > 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.
> This btf_id == 0 check is done at the beginning of bpf_struct_ops_find().
> Hence, bpf_struct_ops_find() won't try to search if btf_id is 0.
>

Ah right, I missed that check. Then yeah, it's not a concern. I still
don't like _btf_vmlinux name, but that's just a nit.

Acked-by: Andrii Nakryiko <andriin@fb.com>

> st_ops fields is only set when everything passed, so individual st_ops
> will not be partially initialized.
>
>
> >
> > >         if (prog->type != BPF_PROG_TYPE_TRACING)
> > >                 return 0;
> > >
> > > --
> > > 2.17.1
> > >