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From:   Song Liu <song@kernel.org>
Date:   Mon, 24 Jan 2022 23:21:17 -0800
Message-ID: <CAPhsuW4AXLirZwjH4YfJLYj1VUU2muQx1wTkkUpeBBH9kvw2Ag@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 Mon, Jan 24, 2022 at 9:21 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Mon, Jan 24, 2022 at 10:27 AM Song Liu <songliubraving@fb.com> wrote:
> > >
> > > Are arches expected to allocate rw buffers in different ways? If not,
> > > I would consider putting this into the common code as well. Then
> > > arch-specific code would do something like
> > >
> > >  header = bpf_jit_binary_alloc_pack(size, &prg_buf, &prg_addr, ...);
> > >  ...
> > >  /*
> > >   * Generate code into prg_buf, the code should assume that its first
> > >   * byte is located at prg_addr.
> > >   */
> > >  ...
> > >  bpf_jit_binary_finalize_pack(header, prg_buf);
> > >
> > > where bpf_jit_binary_finalize_pack() would copy prg_buf to header and
> > > free it.
>
> It feels right, but bpf_jit_binary_finalize_pack() sounds 100% arch
> dependent. The only thing it will do is perform a copy via text_poke.
> What else?
>
> > I think this should work.
> >
> > We will need an API like: bpf_arch_text_copy, which uses text_poke_copy()
> > for x86_64 and s390_kernel_write() for x390. We will use bpf_arch_text_copy
> > to
> >   1) write header->size;
> >   2) do finally copy in bpf_jit_binary_finalize_pack().
>
> we can combine all text_poke operations into one.
>
> Can we add an 'image' pointer into struct bpf_binary_header ?

There is a 4-byte hole in bpf_binary_header. How about we put
image_offset there? Actually we only need 2 bytes for offset.

> Then do:
> int bpf_jit_binary_alloc_pack(size, &ro_hdr, &rw_hdr);
>
> ro_hdr->image would be the address used to compute offsets by JIT.

If we only do one text_poke(), we cannot write ro_hdr->image yet. We
can use ro_hdr + rw_hdr->image_offset instead.

> rw_hdr->image would point to kvmalloc-ed area for emitting insns.
> rw_hdr->size would already be populated.
>
> The JITs would write insns into rw_hdr->image including 'int 3' insns.
> At the end the JIT will do text_poke_copy(ro_hdr, rw_hdr, rw_hdr->size);
> That would be the only copy that will transfer everything into final
> location.
> Then kvfree(rw_hdr)

The only problem is the asymmetry of allocating rw_hdr from bpf/core.c,
and freeing it from arch/bpf_jit_comp.c. But it doesn't bother me too much.

Thanks,
Song