netdev - Re: [PATCH 6/9] x86/bpf: Fix JIT frame pointer usage

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Date:   Thu, 13 Jun 2019 14:58:09 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>,
        Kairui Song <kasong@...hat.com>
Subject: Re: [PATCH 6/9] x86/bpf: Fix JIT frame pointer usage

On Thu, Jun 13, 2019 at 08:21:03AM -0500, Josh Poimboeuf wrote:
> The BPF JIT code clobbers RBP.  This breaks frame pointer convention and
> thus prevents the FP unwinder from unwinding through JIT generated code.
> 
> RBP is currently used as the BPF stack frame pointer register.  The
> actual register used is opaque to the user, as long as it's a
> callee-saved register.  Change it to use R12 instead.
> 
> Fixes: d15d356887e7 ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER")
> Reported-by: Song Liu <songliubraving@...com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 43 +++++++++++++++++++++----------------
>  1 file changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index e649f977f8e1..bb1968fea50a 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -100,9 +100,8 @@ static int bpf_size_to_x86_bytes(int bpf_size)
>  /*
>   * The following table maps BPF registers to x86-64 registers.
>   *
> - * x86-64 register R12 is unused, since if used as base address
> - * register in load/store instructions, it always needs an
> - * extra byte of encoding and is callee saved.
> + * RBP isn't used; it needs to be preserved to allow the unwinder to move
> + * through generated code stacks.

Extra register save/restore is kinda annoying just to fix ORC.
Also every stack access from bpf prog will be encoded via r12 and consume
extra byte of encoding. I really don't like this approach.
Can you teach ORC to understand JIT-ed frames instead?