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

[Jiri Olsa <olsajiri@gmail.com>]

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.