linux-kernel - Re: [PATCH bpf-next v2 2/2] bpf: Make BPF_JIT_DEFAULT

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Date:   Mon, 21 Feb 2022 18:14:20 -0800
From:   Song Liu <song@...nel.org>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/2] bpf: Make BPF_JIT_DEFAULT_ON selectable
 in Kconfig

On Fri, Feb 18, 2022 at 7:15 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> Currently, only x86, arm64 and s390 select ARCH_WANT_DEFAULT_BPF_JIT,
> the other archs do not select ARCH_WANT_DEFAULT_BPF_JIT. On the archs
> without ARCH_WANT_DEFAULT_BPF_JIT, if we want to set bpf_jit_enable to
> 1 by default, the only way is to enable CONFIG_BPF_JIT_ALWAYS_ON, then
> the users can not change it to 0 or 2, it seems bad for some users. We
> can select ARCH_WANT_DEFAULT_BPF_JIT for those archs if it is proper,
> but at least for now, make BPF_JIT_DEFAULT_ON selectable can give them
> a chance.
>
> Additionally, with this patch, under !BPF_JIT_ALWAYS_ON, we can disable
> BPF_JIT_DEFAULT_ON on the archs with ARCH_WANT_DEFAULT_BPF_JIT when make
> menuconfig, it seems flexible for some developers.
>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>  kernel/bpf/Kconfig | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
> index cbf3f65..461ac60 100644
> --- a/kernel/bpf/Kconfig
> +++ b/kernel/bpf/Kconfig
> @@ -54,6 +54,7 @@ config BPF_JIT
>  config BPF_JIT_ALWAYS_ON
>         bool "Permanently enable BPF JIT and remove BPF interpreter"
>         depends on BPF_SYSCALL && HAVE_EBPF_JIT && BPF_JIT
> +       select BPF_JIT_DEFAULT_ON
>         help
>           Enables BPF JIT and removes BPF interpreter to avoid speculative
>           execution of BPF instructions by the interpreter.
> @@ -63,8 +64,16 @@ config BPF_JIT_ALWAYS_ON
>           in failure.
>
>  config BPF_JIT_DEFAULT_ON
> -       def_bool ARCH_WANT_DEFAULT_BPF_JIT || BPF_JIT_ALWAYS_ON
> -       depends on HAVE_EBPF_JIT && BPF_JIT
> +       bool "Defaultly enable BPF JIT and remove BPF interpreter"

I think "remove BPF interpreter" is not accurate. I guess we can just say
"Enable BPF JIT by default". (also "defaultly" sounds weird to me).

> +       default y if ARCH_WANT_DEFAULT_BPF_JIT
> +       depends on BPF_SYSCALL && HAVE_EBPF_JIT && BPF_JIT
> +       help
> +         Enables BPF JIT and removes BPF interpreter to avoid speculative
> +         execution of BPF instructions by the interpreter.
> +
> +         When CONFIG_BPF_JIT_DEFAULT_ON is enabled but CONFIG_BPF_JIT_ALWAYS_ON
> +         is disabled, /proc/sys/net/core/bpf_jit_enable is set to 1 by default
> +         and can be changed to 0 or 2.
>
>  config BPF_UNPRIV_DEFAULT_OFF
>         bool "Disable unprivileged BPF by default"
> --
> 2.1.0
>