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

[Peter Zijlstra]

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?