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Date: Fri, 25 Sep 2020 14:50:44 +0100
From: Lorenz Bauer <lmb@...udflare.com>
To: Martin KaFai Lau <kafai@...com>
Cc: bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>,
Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 bpf-next 02/13] bpf: Enable bpf_skc_to_* sock casting
helper to networking prog type
On Fri, 25 Sep 2020 at 14:18, Martin KaFai Lau <kafai@...com> wrote:
>
> 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@...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@...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?
Sorry, I misread the patch!
--
Lorenz Bauer | Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK
www.cloudflare.com
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