Date:   Sat, 28 Dec 2019 01:47:17 +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 06/11] bpf: Introduce BPF_MAP_TYPE_STRUCT_OPS

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)?

> 
> > +                               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.

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

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