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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Sun, 27 Nov 2022 23:04:53 +0100
To:     "Chen, Hu1" <hu1.chen@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Jiri Olsa <olsajiri@gmail.com>, jpoimboe@kernel.org,
        memxor@gmail.com, bpf@vger.kernel.org,
        Pengfei Xu <pengfei.xu@intel.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org
Subject: Re: [PATCH bpf v2] selftests/bpf: Fix "missing ENDBR" BUG for
 destructor kfunc
Message-ID: <Y4PfBZpdZL00tDMu@krava>
References: <20221122073244.21279-1-hu1.chen@intel.com>
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 <d6555f27-95bf-5474-3006-6f8d399ab556@intel.com>
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On Fri, Nov 25, 2022 at 09:44:29PM +0800, Chen, Hu1 wrote:
> On 11/22/2022 10:14 PM, Peter Zijlstra wrote:
> > On Tue, Nov 22, 2022 at 02:48:07PM +0100, Jiri Olsa wrote:
> >> On Mon, Nov 21, 2022 at 11:32:43PM -0800, Chen Hu wrote:
> >>> With CONFIG_X86_KERNEL_IBT enabled, the test_verifier triggers the
> >>> following BUG:
> >>>
> >>>   traps: Missing ENDBR: bpf_kfunc_call_test_release+0x0/0x30
> >>>   ------------[ cut here ]------------
> >>>   kernel BUG at arch/x86/kernel/traps.c:254!
> >>>   invalid opcode: 0000 [#1] PREEMPT SMP
> >>>   <TASK>
> >>>    asm_exc_control_protection+0x26/0x50
> >>>   RIP: 0010:bpf_kfunc_call_test_release+0x0/0x30
> >>>   Code: 00 48 c7 c7 18 f2 e1 b4 e8 0d ca 8c ff 48 c7 c0 00 f2 e1 b4 c3
> >>> 	0f 1f 44 00 00 66 0f 1f 00 0f 1f 44 00 00 0f 0b 31 c0 c3 66 90
> >>>        <66> 0f 1f 00 0f 1f 44 00 00 48 85 ff 74 13 4c 8d 47 18 b8 ff ff ff
> >>>    bpf_map_free_kptrs+0x2e/0x70
> >>>    array_map_free+0x57/0x140
> >>>    process_one_work+0x194/0x3a0
> >>>    worker_thread+0x54/0x3a0
> >>>    ? rescuer_thread+0x390/0x390
> >>>    kthread+0xe9/0x110
> >>>    ? kthread_complete_and_exit+0x20/0x20
> >>>
> >>> This is because there are no compile-time references to the destructor
> >>> kfuncs, bpf_kfunc_call_test_release() for example. So objtool marked
> >>> them sealable and ENDBR in the functions were sealed (converted to NOP)
> >>> by apply_ibt_endbr().
> > 
> > If there is no compile time reference to it, what stops an LTO linker
> > from throwing it out in the first place?
> >
> 
> Ah, my stupid.
> 
> The only references to this function from kernel space are:
>     $ grep -r bpf_kfunc_call_test_release
>     net/bpf/test_run.c:noinline void bpf_kfunc_call_test_release(struct prog_test_ref_kfunc *p)
>     net/bpf/test_run.c:BTF_ID_FLAGS(func, bpf_kfunc_call_test_release, KF_RELEASE)
>     net/bpf/test_run.c:BTF_ID(func, bpf_kfunc_call_test_release)
> 
> Macro BTF_ID_... puts the function names to .BTF_ids section. It looks
> like:
> __BTF_ID__func__bpf_kfunc_call_test_release__692

bpf_kfunc_call_test_release test function called bpf program as kfunc
(check tools/testing/selftests/bpf/progs/*.c)

it's placed in BTF ID lists so verifier can validate its ID when called
from bpf program.. it has no other caller from kernel side

jirka

> 
> When running, it uses kallsyms_lookup_name() to find the function
> address via names in .BTF_ids section.
> 
> 
> Hi jirka,
> Please kindly correct me if my understanding of BTF_ids is wrong.