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From:   Song Liu <song@kernel.org>
Date:   Tue, 25 Jan 2022 17:28:53 -0800
Message-ID: <CAPhsuW5gVLZeKznY0U2ccYbSzDht5K4fz2-9ScT15WBBJXeUJw@mail.gmail.com>
Subject: Re: [PATCH v6 bpf-next 6/7] bpf: introduce bpf_prog_pack allocator
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Song Liu <songliubraving@fb.com>,
        Ilya Leoshkevich <iii@linux.ibm.com>,
        bpf <bpf@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>
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On Tue, Jan 25, 2022 at 5:20 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Jan 25, 2022 at 4:50 PM Song Liu <song@kernel.org> wrote:
> >
> > On Tue, Jan 25, 2022 at 4:38 PM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
[...]
> > > >
> > > > In bpf_jit_binary_hdr(), we calculate header as image & PAGE_MASK.
> > > > If we want s/PAGE_MASK/63 for x86_64, we will have different versions
> > > > of bpf_jit_binary_hdr(). It is not on any hot path, so we can use __weak for
> > > > it. Other than this, I think the solution works fine.
> > >
> > > I think it can stay generic.
> > >
> > > The existing bpf_jit_binary_hdr() will do & PAGE_MASK
> > > while bpf_jit_binary_hdr_pack() will do & 63.
> >
> > The problem with this approach is that we need bpf_prog_ksym_set_addr
> > to be smart to pick bpf_jit_binary_hdr() or bpf_jit_binary_hdr_pack().
>
> We can probably add a true JIT image size to bpf_prog_aux.
> bpf_prog_ksym_set_addr() is approximating the end:
> prog->aux->ksym.end   = addr + hdr->pages * PAGE_SIZE
> which doesn't have to include all the 'int 3' padding after the end.
>
> Or add a flag to bpf_prog_aux.
> Ideally bpf_jit_free() would stay generic too.

Both ideas sound promising. Let me try to implement them and see
which is better (or maybe we get both).

Thanks for the suggestions!
Song