Re: [PATCH v6 bpf-next 6/7] bpf: introduce bpf_prog_pack allocator

[Alexei Starovoitov]

On Mon, Jan 24, 2022 at 11:21 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> On Mon, Jan 24, 2022 at 9:21 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Mon, Jan 24, 2022 at 10:27 AM Song Liu <songliubraving@xxxxxx> 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 ?

> > 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.

Indeed. Asymmetry needs to be fixed.
Let's then pass 4 pointers back into
bpf_jit_binary_finalize_pack()
which will call arch dependent weak function to do text_poke_copy
or use default __weak function that returns eopnotsupp
and then kvfree the rw_hdr ?
I'd like to avoid callbacks. imo __weak is easier to follow.