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

From: Jiri Olsa <[email-masked]>
Date: Mon, 21 Nov 2022 15:32:49 +0100

On Mon, Nov 21, 2022 at 12:51:13AM -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().

nice :) thanks for the fix, some suggestions below

> 
> This fix creates dummy compile-time references to destructor kfuncs so
> ENDBR stay there.
> 
> Signed-off-by: Chen Hu <[email-masked]>
> Tested-by: Pengfei Xu <[email-masked]>
> ---
>  include/linux/btf_ids.h | 7 +++++++
>  net/bpf/test_run.c      | 2 ++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> index 2aea877d644f..6c6b520ea58f 100644
> --- a/include/linux/btf_ids.h
> +++ b/include/linux/btf_ids.h
> @@ -266,4 +266,11 @@ MAX_BTF_TRACING_TYPE,
>  
>  extern u32 btf_tracing_ids[];
>  
> +#if defined(CONFIG_X86_KERNEL_IBT) && !defined(__DISABLE_EXPORTS)
> +#define BTF_IBT_NOSEAL(name)					\
> +	asm(IBT_NOSEAL(#name));
> +#else
> +#define BTF_IBT_NOSEAL(name)
> +#endif /* CONFIG_X86_KERNEL_IBT */

this is not BTF or BTF ID specific, instead should we add some generic macro like:

  FUNC_IBT_NOSEAL(...)

> +
>  #endif
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 13d578ce2a09..465952e5de11 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -1653,6 +1653,8 @@ BTF_ID(struct, prog_test_ref_kfunc)
>  BTF_ID(func, bpf_kfunc_call_test_release)
>  BTF_ID(struct, prog_test_member)
>  BTF_ID(func, bpf_kfunc_call_memb_release)
> +BTF_IBT_NOSEAL(bpf_kfunc_call_test_release)
> +BTF_IBT_NOSEAL(bpf_kfunc_call_memb_release)

same here, it looks like it's part of the list above, I think this would be better
after function body like:

  noinline void bpf_kfunc_call_memb_release(struct prog_test_member *p)
  {
  }
  FUNC_IBT_NOSEAL(bpf_kfunc_call_memb_release)

thanks,
jirka