netdev - Re: [PATCH v2 bpf-next 09/12] bpf: add support for BTF pointers to x86 JIT

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Date:   Fri, 11 Oct 2019 11:48:19 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>, x86@...nel.org,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 09/12] bpf: add support for BTF pointers to
 x86 JIT

On Wed, Oct 9, 2019 at 9:15 PM Alexei Starovoitov <ast@...nel.org> wrote:
>
> Pointer to BTF object is a pointer to kernel object or NULL.
> Such pointers can only be used by BPF_LDX instructions.
> The verifier changed their opcode from LDX|MEM|size
> to LDX|PROBE_MEM|size to make JITing easier.
> The number of entries in extable is the number of BPF_LDX insns
> that access kernel memory via "pointer to BTF type".
> Only these load instructions can fault.
> Since x86 extable is relative it has to be allocated in the same
> memory region as JITed code.
> Allocate it prior to last pass of JITing and let the last pass populate it.
> Pointer to extable in bpf_prog_aux is necessary to make page fault
> handling fast.
> Page fault handling is done in two steps:
> 1. bpf_prog_kallsyms_find() finds BPF program that page faulted.
>    It's done by walking rb tree.
> 2. then extable for given bpf program is binary searched.
> This process is similar to how page faulting is done for kernel modules.
> The exception handler skips over faulting x86 instruction and
> initializes destination register with zero. This mimics exact
> behavior of bpf_probe_read (when probe_kernel_read faults dest is zeroed).
>
> JITs for other architectures can add support in similar way.
> Until then they will reject unknown opcode and fallback to interpreter.
>
> Since extable should be aligned and placed near JITed code
> make bpf_jit_binary_alloc() return 4 byte aligned image offset,
> so that extable aligning formula in bpf_int_jit_compile() doesn't need
> to rely on internal implementation of bpf_jit_binary_alloc().
> On x86 gcc defaults to 16-byte alignment for regular kernel functions
> due to better performance. JITed code may be aligned to 16 in the future,
> but it will use 4 in the meantime.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---

Acked-by: Andrii Nakryiko <andriin@...com>

>  arch/x86/net/bpf_jit_comp.c | 97 +++++++++++++++++++++++++++++++++++--
>  include/linux/bpf.h         |  3 ++
>  include/linux/extable.h     | 10 ++++
>  kernel/bpf/core.c           | 20 +++++++-
>  kernel/bpf/verifier.c       |  1 +
>  kernel/extable.c            |  2 +
>  6 files changed, 128 insertions(+), 5 deletions(-)
>

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