netdev - Re: [PATCH rfc bpf-next 6/8] bpf: add poke dependency tracking for prog array maps

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Date:   Mon, 18 Nov 2019 19:39:32 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH rfc bpf-next 6/8] bpf: add poke dependency tracking for
 prog array maps

On 11/18/19 6:39 PM, Andrii Nakryiko wrote:
> On Thu, Nov 14, 2019 at 5:04 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>
>> This work adds program tracking to prog array maps. This is needed such
>> that upon prog array updates/deletions we can fix up all programs which
>> make use of this tail call map. We add ops->map_poke_{un,}track() helpers
>> to maps to maintain the list of programs and ops->map_poke_run() for
>> triggering the actual update. bpf_array_aux is extended to contain the
>> list head and poke_mutex in order to serialize program patching during
>> updates/deletions. bpf_free_used_maps() will untrack the program shortly
>> before dropping the reference to the map.
>>
>> The prog_array_map_poke_run() is triggered during updates/deletions and
>> walks the maintained prog list. It checks in their poke_tabs whether the
>> map and key is matching and runs the actual bpf_arch_text_poke() for
>> patching in the nop or new jmp location. Depending on the type of update,
>> we use one of BPF_MOD_{NOP_TO_JUMP,JUMP_TO_NOP,JUMP_TO_JUMP}.
>>
>> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
>> ---
>>   include/linux/bpf.h   |  36 +++++++++++++
>>   kernel/bpf/arraymap.c | 120 +++++++++++++++++++++++++++++++++++++++++-
>>   kernel/bpf/core.c     |   9 +++-
>>   3 files changed, 162 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
>> index 0ff06a0d0058..62a369fb8d98 100644
>> --- a/include/linux/bpf.h
>> +++ b/include/linux/bpf.h
>> @@ -21,6 +21,7 @@ struct bpf_verifier_env;
>>   struct bpf_verifier_log;
>>   struct perf_event;
>>   struct bpf_prog;
>> +struct bpf_prog_aux;
>>   struct bpf_map;
>>   struct sock;
>>   struct seq_file;
>> @@ -63,6 +64,12 @@ struct bpf_map_ops {
>>                               const struct btf_type *key_type,
>>                               const struct btf_type *value_type);
>>
>> +       /* Prog poke tracking helpers. */
>> +       int (*map_poke_track)(struct bpf_map *map, struct bpf_prog_aux *aux);
>> +       void (*map_poke_untrack)(struct bpf_map *map, struct bpf_prog_aux *aux);
>> +       void (*map_poke_run)(struct bpf_map *map, u32 key, struct bpf_prog *old,
>> +                            struct bpf_prog *new);
> 
> You are passing bpf_prog_aux for track/untrack, but bpf_prog itself
> for run. Maybe stick to just bpf_prog everywhere?

This needs to be bpf_prog_aux as prog itself is not stable yet and can still
change, but aux itself is stable.

>> +
>>          /* Direct value access helpers. */
>>          int (*map_direct_value_addr)(const struct bpf_map *map,
>>                                       u64 *imm, u32 off);
>> @@ -584,6 +591,9 @@ struct bpf_array_aux {
>>           */
>>          enum bpf_prog_type type;
>>          bool jited;
>> +       /* Programs with direct jumps into programs part of this array. */
>> +       struct list_head poke_progs;
>> +       struct mutex poke_mutex;
>>   };
>>
> 
> [...]
>