netdev - Re: [PATCH bpf] libbpf: fix CO-RE relocs against .text section

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Date:   Tue, 23 Jun 2020 17:40:53 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf] libbpf: fix CO-RE relocs against .text section

On Sat, Jun 20, 2020 at 12:06 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 6/19/20 4:04 PM, Andrii Nakryiko wrote:
> > bpf_object__find_program_by_title(), used by CO-RE relocation code, doesn't
> > return .text "BPF program", if it is a function storage for sub-programs.
> > Because of that, any CO-RE relocation in helper non-inlined functions will
> > fail. Fix this by searching for .text-corresponding BPF program manually.
> >
> > Adjust one of bpf_iter selftest to exhibit this pattern.
> >
> > Reported-by: Yonghong Song <yhs@...com>
> > Fixes: ddc7c3042614 ("libbpf: implement BPF CO-RE offset relocation algorithm")
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
>
> Acked-by: Yonghong Song <yhs@...com>
>
> But the fix here only fixed the issue for interpreter mode.
> For jit only mode, we still have issues. The following patch can fix
> the jit mode issue,
>
> =============
>
>  From 4d66814513ec45b86a30a1231b8a000d4bfc6f1a Mon Sep 17 00:00:00 2001
> From: Yonghong Song <yhs@...com>
> Date: Fri, 19 Jun 2020 23:26:13 -0700
> Subject: [PATCH bpf] bpf: set the number of exception entries properly for
>   subprograms
>
> Currently, if a bpf program has more than one subprograms, each
> program will be jitted separately. For tracing problem, the
> prog->aux->num_exentries is not setup properly. For example,
> with bpf_iter_netlink.c modified to force one function not inlined,
> and with proper libbpf fix, with CONFIG_BPF_JIT_ALWAYS_ON,
> we will have error like below:
>    $ ./test_progs -n 3/3
>    ...
>    libbpf: failed to load program 'iter/netlink'
>    libbpf: failed to load object 'bpf_iter_netlink'
>    libbpf: failed to load BPF skeleton 'bpf_iter_netlink': -4007
>    test_netlink:FAIL:bpf_iter_netlink__open_and_load skeleton
> open_and_load failed
>    #3/3 netlink:FAIL
> The dmesg shows the following errors:
>    ex gen bug
> which is triggered by the following code in arch/x86/net/bpf_jit_comp.c:
>    if (excnt >= bpf_prog->aux->num_exentries) {
>      pr_err("ex gen bug\n");
>      return -EFAULT;
>    }
>
> If the program has more than one subprograms, num_exentries is actually
> 0 since it is not setup.
>
> This patch fixed the issue by setuping proper num_exentries for
> each subprogram before calling jit function.
>
> Signed-off-by: Yonghong Song <yhs@...com>

Thanks for fixing. Applied both to bpf tree.
Yonghong, next time please submit the patch properly.
It was very awkward to copy-paste it manually from the thread.
I've edited the commit log a bit.