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From:   Song Liu <song@kernel.org>
Date:   Tue, 25 Jan 2022 15:09:18 -0800
Message-ID: <CAPhsuW7AzQL5y+4stw_MZCg2sR3e5qe1YS0L1evxhCvfTWF5+Q@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 2:48 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Jan 25, 2022 at 2:25 PM Song Liu <song@kernel.org> wrote:
> >
> > On Tue, Jan 25, 2022 at 12:00 PM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Mon, Jan 24, 2022 at 11:21 PM Song Liu <song@kernel.org> wrote:
> > > >
> > > > 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.
> > >
> > > Good points.
> > > Maybe let's go back to Ilya's suggestion and return 4 pointers
> > > from bpf_jit_binary_alloc_pack ?
> >
> > How about we use image_offset, like:
> >
> > struct bpf_binary_header {
> >         u32 size;
> >         u32 image_offset;
> >         u8 image[] __aligned(BPF_IMAGE_ALIGNMENT);
> > };
> >
> > Then we can use
> >
> > image = (void *)header + header->image_offset;
>
> I'm not excited about it, since it leaks header details into JITs.
> Looks like we don't need JIT to be aware of it.
> How about we do random() % roundup(sizeof(struct bpf_binary_header), 64)
> to pick the image start and populate
> image-sizeof(struct bpf_binary_header) range
> with 'int 3'.
> This way we can completely hide binary_header inside generic code.
> The bpf_jit_binary_alloc_pack() would return ro_image and rw_image only.
> And JIT would pass them back into bpf_jit_binary_finalize_pack().
> From the image pointer it would be trivial to get to binary_header with &63.
> The 128 byte offset that we use today was chosen arbitrarily.
> We were burning the whole page for a single program, so 128 bytes zone
> at the front was ok.
> Now we will be packing progs rounded up to 64 bytes, so it's better
> to avoid wasting those 128 bytes regardless.

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.

Thanks,
Song