netdev - Re: [PATCH bpf-next 0/3] libbpf: auto-resize relocatable LOAD/STORE instructions

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Date:   Thu, 8 Oct 2020 10:59:44 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Luka Perkov <luka.perkov@...tura.hr>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Tony Ambardar <tony.ambardar@...il.com>,
        Juraj Vijtiuk <juraj.vijtiuk@...tura.hr>,
        Luka Oreskovic <luka.oreskovic@...tura.hr>,
        Sven Fijan <sven.fijan@...tura.hr>,
        David Marcinkovic <david.marcinkovic@...tura.hr>,
        Jakov Petrina <jakov.petrina@...tura.hr>
Subject: Re: [PATCH bpf-next 0/3] libbpf: auto-resize relocatable LOAD/STORE instructions

On Thu, Oct 8, 2020 at 3:34 AM Luka Perkov <luka.perkov@...tura.hr> wrote:
>
> Hello Andrii,
>
> On Wed, Oct 7, 2020 at 8:01 PM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Wed, Oct 7, 2020 at 10:56 AM Luka Perkov <luka.perkov@...tura.hr> wrote:
> > >
> > > Hello Andrii,
> > >
> > > On Fri, Oct 2, 2020 at 3:09 AM Andrii Nakryiko <andriin@...com> wrote:
> > > > Patch set implements logic in libbpf to auto-adjust memory size (1-, 2-, 4-,
> > > > 8-bytes) of load/store (LD/ST/STX) instructions which have BPF CO-RE field
> > > > offset relocation associated with it. In practice this means transparent
> > > > handling of 32-bit kernels, both pointer and unsigned integers. Signed
> > > > integers are not relocatable with zero-extending loads/stores, so libbpf
> > > > poisons them and generates a warning. If/when BPF gets support for sign-extending
> > > > loads/stores, it would be possible to automatically relocate them as well.
> > > >
> > > > All the details are contained in patch #1 comments and commit message.
> > > > Patch #2 is a simple change in libbpf to make advanced testing with custom BTF
> > > > easier. Patch #3 validates correct uses of auto-resizable loads, as well as
> > > > check that libbpf fails invalid uses.
> > > >
> > > > I'd really appreciate folks that use BPF on 32-bit architectures to test this
> > > > out with their BPF programs and report if there are any problems with the
> > > > approach.
> > > >
> > > > Cc: Luka Perkov <luka.perkov@...tura.hr>
> > >
> > > First, thank you for the support and sending this series. It took us a
> > > bit longer to run the tests as our target hardware still did not fully
> > > get complete mainline support and we had to rebase our patches. These
> > > are not related to BPF.
> > >
> > > Related to this patch, we have tested various BPF programs with this
> > > patch, and can confirm that it fixed previous issues with pointer
> > > offsets that we had and reported at:
> > >
> > > https://lore.kernel.org/r/CA+XBgLU=8PFkP8S32e4gpst0=R4MFv8rZA5KaO+cEPYSnTRYYw@mail.gmail.com/.
> > >
> > > Most of our programs now work and we are currently debugging other
> > > programs that still aren't working. We are still not sure if the
> > > remaining issues are related to this or not, but will let you know
> > > sometime this week after further and more detailed investigation.
> > >
> >
> > Ok, great, thanks for the update.
>
> Just to update you that we have identified that the problem was a
> known issue with JIT as we had enabled the BPF_JIT_ALWAYS_ON.
>
> That said, it would be great to see this series included in 5.10 :)

This is purely a libbpf feature, completely agnostic to kernel
versions. So you'll get this with upcoming libbpf 0.2.0 release.

>
> Thanks,
> Luka