Re: [PATCH bpf v2] selftests/bpf: Fix "missing ENDBR" BUG for destructor kfunc

[Chen, Hu1]

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

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.