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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Tue, 3 Mar 2020 14:26:09 -0800
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Subject: Re: [PATCH bpf-next 2/7] bpf: JIT helpers for fmod_ret progs
To:     KP Singh <kpsingh@chromium.org>
Cc:     open list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Paul Turner <pjt@google.com>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>
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On Tue, Mar 3, 2020 at 6:13 AM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> * Split the invoke_bpf program to prepare for special handling of
>   fmod_ret programs introduced in a subsequent patch.
> * Move the definition of emit_cond_near_jump and emit_nops as they are
>   needed for fmod_ret.
> * Refactor branch target alignment into its own function
>   align16_branch_target.
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 158 ++++++++++++++++++++----------------
>  1 file changed, 90 insertions(+), 68 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 15c7d28bc05c..475e354c2e88 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1361,35 +1361,100 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
>                          -(stack_size - i * 8));
>  }
>

[...]

> +
> +/* From Intel 64 and IA-32 Architectures Optimization
> + * Reference Manual, 3.4.1.4 Code Alignment, Assembly/Compiler
> + * Coding Rule 11: All branch targets should be 16-byte
> + * aligned.
> + */
> +static void align16_branch_target(u8 **pprog)
> +{
> +       u8 *target, *prog = *pprog;
> +
> +       target = PTR_ALIGN(prog, 16);
> +       if (target != prog)
> +               emit_nops(&prog, target - prog);
> +       if (target != prog)
> +               pr_err("calcultion error\n");

this wasn't in the original code, do you feel like it's more important
to check this and print error?

also typo: calculation error, but then it's a bit brief and
uninformative message. So I don't know, maybe just drop it?

> +}
> +
> +static int emit_cond_near_jump(u8 **pprog, void *func, void *ip, u8 jmp_cond)
> +{
> +       u8 *prog = *pprog;
> +       int cnt = 0;
> +       s64 offset;
> +
> +       offset = func - (ip + 2 + 4);
> +       if (!is_simm32(offset)) {
> +               pr_err("Target %p is out of range\n", func);
> +               return -EINVAL;
> +       }
> +       EMIT2_off32(0x0F, jmp_cond + 0x10, offset);
> +       *pprog = prog;
> +       return 0;
> +}
> +

[...]