netdev - Re: [PATCH bpf-next v2 4/6] bpf, libbpf: add bpf_tail_call

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Message-ID: <CAEf4BzYxSkjJzPVzOkOQkOVPUKri9aa69QGFrUdGjAf7f9Uf=w@mail.gmail.com>
Date:   Mon, 28 Sep 2020 10:39:55 -0700
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>,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf-next v2 4/6] bpf, libbpf: add bpf_tail_call_static
 helper for bpf programs

On Mon, Sep 28, 2020 at 7:39 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> Port of tail_call_static() helper function from Cilium's BPF code base [0]
> to libbpf, so others can easily consume it as well. We've been using this
> in production code for some time now. The main idea is that we guarantee
> that the kernel's BPF infrastructure and JIT (here: x86_64) can patch the
> JITed BPF insns with direct jumps instead of having to fall back to using
> expensive retpolines. By using inline asm, we guarantee that the compiler
> won't merge the call from different paths with potentially different
> content of r2/r3.
>
> We're also using Cilium's __throw_build_bug() macro (here as: __bpf_unreachable())
> in different places as a neat trick to trigger compilation errors when
> compiler does not remove code at compilation time. This works for the BPF
> back end as it does not implement the __builtin_trap().
>
>   [0] https://github.com/cilium/cilium/commit/f5537c26020d5297b70936c6b7d03a1e412a1035
>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Cc: Andrii Nakryiko <andriin@...com>
> ---

few optional nits below, but looks good to me:

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

>  tools/lib/bpf/bpf_helpers.h | 46 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 46 insertions(+)
>

[...]

> +/*
> + * Helper function to perform a tail call with a constant/immediate map slot.
> + */
> +static __always_inline void
> +bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)

nit: const void *ctx would work here, right? would avoid users having
to do unnecessary casts in some cases

> +{
> +       if (!__builtin_constant_p(slot))
> +               __bpf_unreachable();
> +
> +       /*
> +        * Provide a hard guarantee that LLVM won't optimize setting r2 (map
> +        * pointer) and r3 (constant map index) from _different paths_ ending
> +        * up at the _same_ call insn as otherwise we won't be able to use the
> +        * jmpq/nopl retpoline-free patching by the x86-64 JIT in the kernel
> +        * given they mismatch. See also d2e4c1e6c294 ("bpf: Constant map key
> +        * tracking for prog array pokes") for details on verifier tracking.
> +        *
> +        * Note on clobber list: we need to stay in-line with BPF calling
> +        * convention, so even if we don't end up using r0, r4, r5, we need
> +        * to mark them as clobber so that LLVM doesn't end up using them
> +        * before / after the call.
> +        */
> +       asm volatile("r1 = %[ctx]\n\t"
> +                    "r2 = %[map]\n\t"
> +                    "r3 = %[slot]\n\t"
> +                    "call 12\n\t"

nit: it's weird to have tabs at the end of each string literal,
especially that r1 doesn't start with a tab...

> +                    :: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
> +                    : "r0", "r1", "r2", "r3", "r4", "r5");
> +}
> +
>  /*
>   * Helper structure used by eBPF C program
>   * to describe BPF map attributes to libbpf loader
> --
> 2.21.0
>