linux-kernel - Re: [PATCH v2 0/2] bpf: permit JIT allocations to be served outside the module region

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Date:   Wed, 21 Nov 2018 19:48:07 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "eric.dumazet@...il.com" <eric.dumazet@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "arnd@...db.de" <arnd@...db.de>
Subject: Re: [PATCH v2 0/2] bpf: permit JIT allocations to be served outside
 the module region

On Wed, 2018-11-21 at 14:17 +0100, Ard Biesheuvel wrote:
> On arm64, modules are allocated from a 128 MB window which is close to
> the core kernel, so that relative direct branches are guaranteed to be
> in range (except in some KASLR configurations). Also, module_alloc()
> is in charge of allocating KASAN shadow memory when running with KASAN
> enabled.
> 
> This means that the way BPF reuses module_alloc()/module_memfree() is
> undesirable on arm64 (and potentially other architectures as well),
> and so this series refactors BPF's use of those functions to permit
> architectures to change this behavior.
> 
Hi Ard,

I am looking at adding optional BPF JIT in vmalloc functionality for x86 that
would use this refactor. In fact I have done the same thing with just different
names.

My implementation intends to use the module space until a usage limit is reached
and then overflow into vmalloc, so it would be an additional knob like
"bpf_jit_limit". Wondering if that should be a cross-arch concept that connects
to this. Does it fit in with what you are trying to do for arm64 here?

Thanks,

Rick