Date:   Mon, 23 Dec 2019 22:29:59 +0000
From:   Martin Lau <kafai@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        "Kernel Team" <Kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 05/11] bpf: Introduce BPF_PROG_TYPE_STRUCT_OPS

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@...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@...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.

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
> >

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