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Date:   Fri, 27 Dec 2019 18:24:41 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin Lau <kafai@...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 06/11] bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS

On Fri, Dec 27, 2019 at 5:47 PM Martin Lau <kafai@...com> wrote:
>
> On Mon, Dec 23, 2019 at 03:05:08PM -0800, Andrii Nakryiko wrote:
> > On Fri, Dec 20, 2019 at 10:26 PM Martin KaFai Lau <kafai@...com> wrote:
> > >
> > > The patch introduces BPF_MAP_TYPE_STRUCT_OPS.  The map value
> > > is a kernel struct with its func ptr implemented in bpf prog.
> > > This new map is the interface to register/unregister/introspect
> > > a bpf implemented kernel struct.
> > >
> > > The kernel struct is actually embedded inside another new struct
> > > (or called the "value" struct in the code).  For example,
> > > "struct tcp_congestion_ops" is embbeded in:
> > > struct bpf_struct_ops_tcp_congestion_ops {
> > >         refcount_t refcnt;
> > >         enum bpf_struct_ops_state state;
> > >         struct tcp_congestion_ops data;  /* <-- kernel subsystem struct here */
> > > }
> > > The map value is "struct bpf_struct_ops_tcp_congestion_ops".
> > > The "bpftool map dump" will then be able to show the
> > > state ("inuse"/"tobefree") and the number of subsystem's refcnt (e.g.
> > > number of tcp_sock in the tcp_congestion_ops case).  This "value" struct
> > > is created automatically by a macro.  Having a separate "value" struct
> > > will also make extending "struct bpf_struct_ops_XYZ" easier (e.g. adding
> > > "void (*init)(void)" to "struct bpf_struct_ops_XYZ" to do some
> > > initialization works before registering the struct_ops to the kernel
> > > subsystem).  The libbpf will take care of finding and populating the
> > > "struct bpf_struct_ops_XYZ" from "struct XYZ".
> > >
> > > Register a struct_ops to a kernel subsystem:
> > > 1. Load all needed BPF_PROG_TYPE_STRUCT_OPS prog(s)
> > > 2. Create a BPF_MAP_TYPE_STRUCT_OPS with attr->btf_vmlinux_value_type_id
> > >    set to the btf id "struct bpf_struct_ops_tcp_congestion_ops" of the
> > >    running kernel.
> > >    Instead of reusing the attr->btf_value_type_id,
> > >    btf_vmlinux_value_type_id s added such that attr->btf_fd can still be
> > >    used as the "user" btf which could store other useful sysadmin/debug
> > >    info that may be introduced in the furture,
> > >    e.g. creation-date/compiler-details/map-creator...etc.
> > > 3. Create a "struct bpf_struct_ops_tcp_congestion_ops" object as described
> > >    in the running kernel btf.  Populate the value of this object.
> > >    The function ptr should be populated with the prog fds.
> > > 4. Call BPF_MAP_UPDATE with the object created in (3) as
> > >    the map value.  The key is always "0".
> > >
> > > During BPF_MAP_UPDATE, the code that saves the kernel-func-ptr's
> > > args as an array of u64 is generated.  BPF_MAP_UPDATE also allows
> > > the specific struct_ops to do some final checks in "st_ops->init_member()"
> > > (e.g. ensure all mandatory func ptrs are implemented).
> > > If everything looks good, it will register this kernel struct
> > > to the kernel subsystem.  The map will not allow further update
> > > from this point.
> > >
> > > Unregister a struct_ops from the kernel subsystem:
> > > BPF_MAP_DELETE with key "0".
> > >
> > > Introspect a struct_ops:
> > > BPF_MAP_LOOKUP_ELEM with key "0".  The map value returned will
> > > have the prog _id_ populated as the func ptr.
> > >
> > > The map value state (enum bpf_struct_ops_state) will transit from:
> > > INIT (map created) =>
> > > INUSE (map updated, i.e. reg) =>
> > > TOBEFREE (map value deleted, i.e. unreg)
> > >
> > > The kernel subsystem needs to call bpf_struct_ops_get() and
> > > bpf_struct_ops_put() to manage the "refcnt" in the
> > > "struct bpf_struct_ops_XYZ".  This patch uses a separate refcnt
> > > for the purose of tracking the subsystem usage.  Another approach
> > > is to reuse the map->refcnt and then "show" (i.e. during map_lookup)
> > > the subsystem's usage by doing map->refcnt - map->usercnt to filter out
> > > the map-fd/pinned-map usage.  However, that will also tie down the
> > > future semantics of map->refcnt and map->usercnt.
> > >
> > > The very first subsystem's refcnt (during reg()) holds one
> > > count to map->refcnt.  When the very last subsystem's refcnt
> > > is gone, it will also release the map->refcnt.  All bpf_prog will be
> > > freed when the map->refcnt reaches 0 (i.e. during map_free()).
> > >
> > > Here is how the bpftool map command will look like:
> > > [root@...h-fb-vm1 bpf]# bpftool map show
> > > 6: struct_ops  name dctcp  flags 0x0
> > >         key 4B  value 256B  max_entries 1  memlock 4096B
> > >         btf_id 6
> > > [root@...h-fb-vm1 bpf]# bpftool map dump id 6
> > > [{
> > >         "value": {
> > >             "refcnt": {
> > >                 "refs": {
> > >                     "counter": 1
> > >                 }
> > >             },
> > >             "state": 1,
> > >             "data": {
> > >                 "list": {
> > >                     "next": 0,
> > >                     "prev": 0
> > >                 },
> > >                 "key": 0,
> > >                 "flags": 2,
> > >                 "init": 24,
> > >                 "release": 0,
> > >                 "ssthresh": 25,
> > >                 "cong_avoid": 30,
> > >                 "set_state": 27,
> > >                 "cwnd_event": 28,
> > >                 "in_ack_event": 26,
> > >                 "undo_cwnd": 29,
> > >                 "pkts_acked": 0,
> > >                 "min_tso_segs": 0,
> > >                 "sndbuf_expand": 0,
> > >                 "cong_control": 0,
> > >                 "get_info": 0,
> > >                 "name": [98,112,102,95,100,99,116,99,112,0,0,0,0,0,0,0
> > >                 ],
> > >                 "owner": 0
> > >             }
> > >         }
> > >     }
> > > ]
> > >
> > > Misc Notes:
> > > * bpf_struct_ops_map_sys_lookup_elem() is added for syscall lookup.
> > >   It does an inplace update on "*value" instead returning a pointer
> > >   to syscall.c.  Otherwise, it needs a separate copy of "zero" value
> > >   for the BPF_STRUCT_OPS_STATE_INIT to avoid races.
> > >
> > > * The bpf_struct_ops_map_delete_elem() is also called without
> > >   preempt_disable() from map_delete_elem().  It is because
> > >   the "->unreg()" may requires sleepable context, e.g.
> > >   the "tcp_unregister_congestion_control()".
> > >
> > > * "const" is added to some of the existing "struct btf_func_model *"
> > >   function arg to avoid a compiler warning caused by this patch.
> > >
> > > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > > ---
> >
> > LGTM! Few questions below to improve my understanding.
> >
> > Acked-by: Andrii Nakryiko <andriin@...com>
> >
> > >  arch/x86/net/bpf_jit_comp.c |  11 +-
> > >  include/linux/bpf.h         |  49 +++-
> > >  include/linux/bpf_types.h   |   3 +
> > >  include/linux/btf.h         |  13 +
> > >  include/uapi/linux/bpf.h    |   7 +-
> > >  kernel/bpf/bpf_struct_ops.c | 468 +++++++++++++++++++++++++++++++++++-
> > >  kernel/bpf/btf.c            |  20 +-
> > >  kernel/bpf/map_in_map.c     |   3 +-
> > >  kernel/bpf/syscall.c        |  49 ++--
> > >  kernel/bpf/trampoline.c     |   5 +-
> > >  kernel/bpf/verifier.c       |   5 +
> > >  11 files changed, 593 insertions(+), 40 deletions(-)
> > >
> >
> > [...]
> >
> > > +               /* All non func ptr member must be 0 */
> > > +               if (!btf_type_resolve_func_ptr(btf_vmlinux, member->type,
> > > +                                              NULL)) {
> > > +                       u32 msize;
> > > +
> > > +                       mtype = btf_resolve_size(btf_vmlinux, mtype,
> > > +                                                &msize, NULL, NULL);
> > > +                       if (IS_ERR(mtype)) {
> > > +                               err = PTR_ERR(mtype);
> > > +                               goto reset_unlock;
> > > +                       }
> > > +
> > > +                       if (memchr_inv(udata + moff, 0, msize)) {
> >
> >
> > just double-checking: we are ok with having non-zeroed padding in a
> > struct, is that right?
> Sorry for the delay.
>
> You meant the end-padding of the kernel side struct (i.e. kdata (or kvalue))
> could be non-zero?  The btf's struct size (i.e. vt->size) should include
> the padding and the whole vt->size is init to 0.
>
> or you meant the user passed in udata (or uvalue)?

The latter, udata. You check member-by-member, but if there is padding
between fields or at the end of a struct, nothing is currently
checking it for zeroes. So probably safer to check those paddings
inbetween as well.

>
> >
> > > +                               err = -EINVAL;
> > > +                               goto reset_unlock;
> > > +                       }
> > > +
> > > +                       continue;
> > > +               }
> > > +
> >
> > [...]
> >
> > > +
> > > +               err = arch_prepare_bpf_trampoline(image,
> > > +                                                 &st_ops->func_models[i], 0,
> > > +                                                 &prog, 1, NULL, 0, NULL);
> > > +               if (err < 0)
> > > +                       goto reset_unlock;
> > > +
> > > +               *(void **)(kdata + moff) = image;
> > > +               image += err;
> >
> > are there any alignment requirements on image pointer for trampoline?
> Not that I know of from reading arch_prepare_bpf_trampoline()
> which also can generate codes to call multiple bpf_prog.

Yeah, I think you are right, at least for x86. Variable-sized x86
instructions pretty much rule out any alignment. It might not be the
case for fixed-sized instructions architectures, but we should cross
that bridge when we get to it.

>
> >
> > > +
> > > +               /* put prog_id to udata */
> > > +               *(unsigned long *)(udata + moff) = prog->aux->id;
> > > +       }
> > > +
> >
> > [...]

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