netdev - Re: [PATCH bpf-next 04/15] bpf: Support bpf program calling kernel function

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Message-ID: <70f08dbd-94d2-79ed-d764-0849d45f662b@fb.com>
Date:   Thu, 18 Mar 2021 18:51:49 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Martin KaFai Lau <kafai@...com>
CC:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 04/15] bpf: Support bpf program calling kernel
 function

On 3/18/21 6:03 PM, Andrii Nakryiko wrote:
>> +       desc->imm = BPF_CAST_CALL(addr) - __bpf_call_base;
> Is this difference guaranteed to always fit within s32?
> 

we have this restriction in many places: JIT, dispatcher, trampoline,
and bpf interpreter.
Modules and kernel .text are in the same 4G for the same reason: 
performance.