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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 10 May 2022 11:10:35 -0700
Message-ID: <CAADnVQKNkEX-caBjozegRaOb67g1HNOHn1e-enRk_s-7Gtt=gg@mail.gmail.com>
Subject: Re: [PATCH bpf] bpf_trace: bail out from bpf_kprobe_multi_link_attach
 when in compat
To:     Eugene Syromiatnikov <esyr@redhat.com>
Cc:     Jiri Olsa <jolsa@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
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On Fri, May 6, 2022 at 7:22 AM Eugene Syromiatnikov <esyr@redhat.com> wrote:
>
> Since bpf_kprobe_multi_link_attach doesn't support 32-bit kernels
> for whatever reason,

Jiri,
why did you add this restriction?

> having it enabled for compat processes on 64-bit
> kernels makes even less sense due to discrepances in the type sizes
> that it does not handle.

I don't follow this logic.
bpf progs are always 64-bit. Even when user space is 32-bit.
Jiri's check is for the kernel.

> Fixes: 0dcac272540613d4 ("bpf: Add multi kprobe link")
> Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
> ---
>  kernel/trace/bpf_trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index d8553f4..9560af6 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2410,7 +2410,7 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
>         int err;
>
>         /* no support for 32bit archs yet */
> -       if (sizeof(u64) != sizeof(void *))
> +       if (sizeof(u64) != sizeof(void *) || in_compat_syscall())
>                 return -EOPNOTSUPP;
>
>         if (prog->expected_attach_type != BPF_TRACE_KPROBE_MULTI)
> --
> 2.1.4
>