netdev - Re: [PATCH bpf-next 6/8] bpf: constant map key tracking for prog array pokes

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Message-ID: <CAEf4BzYZO8ytf4inRQA7+gWrNQzK-0C5e1_4b1K6y30Fcqa-RQ@mail.gmail.com>
Date:   Wed, 20 Nov 2019 11:02:49 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
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 bpf-next 6/8] bpf: constant map key tracking for prog
 array pokes

On Mon, Nov 18, 2019 at 5:38 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> Add tracking of constant keys into tail call maps. The signature of
> bpf_tail_call_proto is that arg1 is ctx, arg2 map pointer and arg3
> is a index key. The direct call approach for tail calls can be enabled
> if the verifier asserted that for all branches leading to the tail call
> helper invocation, the map pointer and index key were both constant
> and the same.
>
> Tracking of map pointers we already do from prior work via c93552c443eb
> ("bpf: properly enforce index mask to prevent out-of-bounds speculation")
> and 09772d92cd5a ("bpf: avoid retpoline for lookup/update/ delete calls
> on maps").
>
> Given the tail call map index key is not on stack but directly in the
> register, we can add similar tracking approach and later in fixup_bpf_calls()
> add a poke descriptor to the progs poke_tab with the relevant information
> for the JITing phase.
>
> We internally reuse insn->imm for the rewritten BPF_JMP | BPF_TAIL_CALL
> instruction in order to point into the prog's poke_tab, and keep insn->imm
> as 0 as indicator that current indirect tail call emission must be used.
>
> Future work can generalize and add similar approach to optimize plain
> array map lookups. Difference there is that we need to look into the key
> value that sits on stack. For clarity in bpf_insn_aux_data, map_state
> has been renamed into map_ptr_state, so we get map_{ptr,key}_state as
> trackers.
>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---

LGTM.

Acked-by: Andrii Nakryiko <andriin@...com>

>  include/linux/bpf_verifier.h |   3 +-
>  kernel/bpf/verifier.c        | 116 ++++++++++++++++++++++++++++++++---
>  2 files changed, 110 insertions(+), 9 deletions(-)
>

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