netdev - Re: [PATCH v6 bpf-next 6/7] bpf: introduce bpf_prog

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Date:   Tue, 25 Jan 2022 17:34:37 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Song Liu <song@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Song Liu <songliubraving@...com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH v6 bpf-next 6/7] bpf: introduce bpf_prog_pack allocator

On 1/25/22 5:31 PM, Song Liu wrote:
> On Tue, Jan 25, 2022 at 5:28 PM Song Liu <song@...nel.org> wrote:
>>
>> On Tue, Jan 25, 2022 at 5:20 PM Alexei Starovoitov
>> <alexei.starovoitov@...il.com> wrote:
>>>
>>> On Tue, Jan 25, 2022 at 4:50 PM Song Liu <song@...nel.org> wrote:
>>>>
>>>> On Tue, Jan 25, 2022 at 4:38 PM Alexei Starovoitov
>>>> <alexei.starovoitov@...il.com> wrote:
>>>>>
>> [...]
>>>>>>
>>>>>> 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.
>>>>>
>>>>> I think it can stay generic.
>>>>>
>>>>> The existing bpf_jit_binary_hdr() will do & PAGE_MASK
>>>>> while bpf_jit_binary_hdr_pack() will do & 63.
>>>>
>>>> The problem with this approach is that we need bpf_prog_ksym_set_addr
>>>> to be smart to pick bpf_jit_binary_hdr() or bpf_jit_binary_hdr_pack().
>>>
>>> We can probably add a true JIT image size to bpf_prog_aux.
>>> bpf_prog_ksym_set_addr() is approximating the end:
>>> prog->aux->ksym.end   = addr + hdr->pages * PAGE_SIZE
>>> which doesn't have to include all the 'int 3' padding after the end.
> 
> Actually, we can use prog->jited_len in bpf_prog_ksym_set_addr(), right?

Lol. Yeah. We should. Looks like somebody remembers their own
code in perf_event_bpf_emit_ksymbols() ;)