Re: [PATCH v4 bpf-next 02/13] bpf: Enable bpf_skc_to_* sock casting helper to networking prog type

[Martin KaFai Lau <kafai@fb.com>]

On Fri, Sep 25, 2020 at 09:26:36AM +0100, Lorenz Bauer wrote:
> On Fri, 25 Sep 2020 at 01:04, Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > There is a constant need to add more fields into the bpf_tcp_sock
> > for the bpf programs running at tc, sock_ops...etc.
> >
> > A current workaround could be to use bpf_probe_read_kernel().  However,
> > other than making another helper call for reading each field and missing
> > CO-RE, it is also not as intuitive to use as directly reading
> > "tp->lsndtime" for example.  While already having perfmon cap to do
> > bpf_probe_read_kernel(), it will be much easier if the bpf prog can
> > directly read from the tcp_sock.
> >
> > This patch tries to do that by using the existing casting-helpers
> > bpf_skc_to_*() whose func_proto returns a btf_id.  For example, the
> > func_proto of bpf_skc_to_tcp_sock returns the btf_id of the
> > kernel "struct tcp_sock".
> >
> > These helpers are also added to is_ptr_cast_function().
> > It ensures the returning reg (BPF_REF_0) will also carries the ref_obj_id.
> > That will keep the ref-tracking works properly.
> >
> > The bpf_skc_to_* helpers are made available to most of the bpf prog
> > types in filter.c. The bpf_skc_to_* helpers will be limited by
> > perfmon cap.
> >
> > This patch adds a ARG_PTR_TO_BTF_ID_SOCK_COMMON.  The helper accepting
> > this arg can accept a btf-id-ptr (PTR_TO_BTF_ID + &btf_sock_ids[BTF_SOCK_TYPE_SOCK_COMMON])
> > or a legacy-ctx-convert-skc-ptr (PTR_TO_SOCK_COMMON).  The bpf_skc_to_*()
> > helpers are changed to take ARG_PTR_TO_BTF_ID_SOCK_COMMON such that
> > they will accept pointer obtained from skb->sk.
> >
> > Instead of specifying both arg_type and arg_btf_id in the same func_proto
> > which is how the current ARG_PTR_TO_BTF_ID does, the arg_btf_id of
> > the new ARG_PTR_TO_BTF_ID_SOCK_COMMON is specified in the
> > compatible_reg_types[] in verifier.c.  The reason is the arg_btf_id is
> > always the same.  Discussion in this thread:
> > https://lore.kernel.org/bpf/20200922070422.1917351-1-kafai@fb.com/
> >
> > The ARG_PTR_TO_BTF_ID_ part gives a clear expectation that the helper is
> > expecting a PTR_TO_BTF_ID which could be NULL.  This is the same
> > behavior as the existing helper taking ARG_PTR_TO_BTF_ID.
> >
> > The _SOCK_COMMON part means the helper is also expecting the legacy
> > SOCK_COMMON pointer.
> >
> > By excluding the _OR_NULL part, the bpf prog cannot call helper
> > with a literal NULL which doesn't make sense in most cases.
> > e.g. bpf_skc_to_tcp_sock(NULL) will be rejected.  All PTR_TO_*_OR_NULL
> > reg has to do a NULL check first before passing into the helper or else
> > the bpf prog will be rejected.  This behavior is nothing new and
> > consistent with the current expectation during bpf-prog-load.
> >
> > [ ARG_PTR_TO_BTF_ID_SOCK_COMMON will be used to replace
> >   ARG_PTR_TO_SOCK* of other existing helpers later such that
> >   those existing helpers can take the PTR_TO_BTF_ID returned by
> >   the bpf_skc_to_*() helpers.
> >
> >   The only special case is bpf_sk_lookup_assign() which can accept a
> >   literal NULL ptr.  It has to be handled specially in another follow
> >   up patch if there is a need (e.g. by renaming ARG_PTR_TO_SOCKET_OR_NULL
> >   to ARG_PTR_TO_BTF_ID_SOCK_COMMON_OR_NULL). ]
> >
> > [ When converting the older helpers that take ARG_PTR_TO_SOCK* in
> >   the later patch, if the kernel does not support BTF,
> >   ARG_PTR_TO_BTF_ID_SOCK_COMMON will behave like ARG_PTR_TO_SOCK_COMMON
> >   because no reg->type could have PTR_TO_BTF_ID in this case.
> >
> >   It is not a concern for the newer-btf-only helper like the bpf_skc_to_*()
> >   here though because these helpers must require BTF vmlinux to begin
> >   with. ]
> >
> > Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> > ---
> >  include/linux/bpf.h   |  1 +
> >  kernel/bpf/verifier.c | 34 +++++++++++++++++++--
> >  net/core/filter.c     | 69 ++++++++++++++++++++++++++++++-------------
> >  3 files changed, 82 insertions(+), 22 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index fc5c901c7542..d0937f1d2980 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -292,6 +292,7 @@ enum bpf_arg_type {
> >         ARG_PTR_TO_ALLOC_MEM,   /* pointer to dynamically allocated memory */
> >         ARG_PTR_TO_ALLOC_MEM_OR_NULL,   /* pointer to dynamically allocated memory or NULL */
> >         ARG_CONST_ALLOC_SIZE_OR_ZERO,   /* number of allocated bytes requested */
> > +       ARG_PTR_TO_BTF_ID_SOCK_COMMON,  /* pointer to in-kernel sock_common or bpf-mirrored bpf_sock */
> >         __BPF_ARG_TYPE_MAX,
> >  };
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 945fa2b4d096..d4ba29fb17a6 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -486,7 +486,12 @@ static bool is_acquire_function(enum bpf_func_id func_id,
> >  static bool is_ptr_cast_function(enum bpf_func_id func_id)
> >  {
> >         return func_id == BPF_FUNC_tcp_sock ||
> > -               func_id == BPF_FUNC_sk_fullsock;
> > +               func_id == BPF_FUNC_sk_fullsock ||
> > +               func_id == BPF_FUNC_skc_to_tcp_sock ||
> > +               func_id == BPF_FUNC_skc_to_tcp6_sock ||
> > +               func_id == BPF_FUNC_skc_to_udp6_sock ||
> > +               func_id == BPF_FUNC_skc_to_tcp_timewait_sock ||
> > +               func_id == BPF_FUNC_skc_to_tcp_request_sock;
> >  }
> >
> >  /* string representation of 'enum bpf_reg_type' */
> > @@ -3953,6 +3958,7 @@ static int resolve_map_arg_type(struct bpf_verifier_env *env,
> >
> >  struct bpf_reg_types {
> >         const enum bpf_reg_type types[10];
> > +       u32 *btf_id;
> >  };
> >
> >  static const struct bpf_reg_types map_key_value_types = {
> > @@ -3973,6 +3979,17 @@ static const struct bpf_reg_types sock_types = {
> >         },
> >  };
> >
> > +static const struct bpf_reg_types btf_id_sock_common_types = {
> > +       .types = {
> > +               PTR_TO_SOCK_COMMON,
> > +               PTR_TO_SOCKET,
> > +               PTR_TO_TCP_SOCK,
> > +               PTR_TO_XDP_SOCK,
> > +               PTR_TO_BTF_ID,
> > +       },
> > +       .btf_id = &btf_sock_ids[BTF_SOCK_TYPE_SOCK_COMMON],
> > +};
> > +
> >  static const struct bpf_reg_types mem_types = {
> >         .types = {
> >                 PTR_TO_STACK,
> > @@ -4014,6 +4031,7 @@ static const struct bpf_reg_types *compatible_reg_types[__BPF_ARG_TYPE_MAX] = {
> >         [ARG_PTR_TO_CTX]                = &context_types,
> >         [ARG_PTR_TO_CTX_OR_NULL]        = &context_types,
> >         [ARG_PTR_TO_SOCK_COMMON]        = &sock_types,
> > +       [ARG_PTR_TO_BTF_ID_SOCK_COMMON] = &btf_id_sock_common_types,
> >         [ARG_PTR_TO_SOCKET]             = &fullsock_types,
> >         [ARG_PTR_TO_SOCKET_OR_NULL]     = &fullsock_types,
> >         [ARG_PTR_TO_BTF_ID]             = &btf_ptr_types,
> > @@ -4059,6 +4077,14 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> >
> >  found:
> >         if (type == PTR_TO_BTF_ID) {
> > +               if (!arg_btf_id) {
> > +                       if (!compatible->btf_id) {
> > +                               verbose(env, "verifier internal error: missing arg compatible BTF ID\n");
> > +                               return -EFAULT;
> > +                       }
> > +                       arg_btf_id = compatible->btf_id;
> > +               }
> > +
> >                 if (!btf_struct_ids_match(&env->log, reg->off, reg->btf_id,
> >                                           *arg_btf_id)) {
> >                         verbose(env, "R%d is of type %s but %s is expected\n",
> > @@ -4575,10 +4601,14 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
> >  {
> >         int i;
> >
> > -       for (i = 0; i < ARRAY_SIZE(fn->arg_type); i++)
> > +       for (i = 0; i < ARRAY_SIZE(fn->arg_type); i++) {
> >                 if (fn->arg_type[i] == ARG_PTR_TO_BTF_ID && !fn->arg_btf_id[i])
> >                         return false;
> >
> > +               if (fn->arg_type[i] != ARG_PTR_TO_BTF_ID && fn->arg_btf_id[i])
> > +                       return false;
> > +       }
> > +
> 
> This is a hold over from the previous patchset?
hmm... what do you mean?