Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

[Nathan Chancellor]

On 6/9/2021 8:08 AM, Peter Zijlstra wrote:
> On Wed, Jun 09, 2021 at 02:23:28PM +0200, Lukasz Majczak wrote:
> > śr., 9 cze 2021 o 09:20 Peter Zijlstra <peterz@xxxxxxxxxxxxx> napisał(a):
> > > On Wed, Jun 09, 2021 at 09:11:18AM +0200, Lukasz Majczak wrote:
> > >
> > > > I'm sorry I was on vacation last week - do you still need the requested debugs?
> > >
> > > If the patch here:
> > >
> > >    https://lkml.kernel.org/r/YL3q1qFO9QIRL/BA@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > does not fix things for you (don't think it actually will), then yes,
> > > please send me the information requested.
> >
> > Ok, it didn't help. Peter, Josh I have sent you a private email with
> > requested information.
>
> OK, I think I've found it. Check this one:
>
>   5d5:   0f 85 00 00 00 00       jne    5db <cpuidle_reflect+0x22>       5d7: R_X86_64_PLT32     __x86_indirect_thunk_r11-0x4
>
>
> +Relocation section '.rela.altinstructions' at offset 0 contains 14 entries:
> +    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
>
> +0000000000000018  0000000200000002 R_X86_64_PC32          0000000000000000 .text + 5d5
> +000000000000001c  0000009200000002 R_X86_64_PC32          0000000000000000 __x86_indirect_alt_call_r11 + 0
>
> Apparently we get conditional branches to retpoline thunks and objtool
> completely messes that up. I'm betting this also explains the problems
> Nathan is having.

Yes, the below patch gets my kernel back to booting so it seems the root 
cause is the same.

> *groan*,.. not sure what to do about this, except return to having
> objtool generate code, which everybody hated on. For now I'll make it
> skip the conditional branches.
>
> I wonder if the compiler will also generate conditional tail calls, and
> what that does with static_call... now I have to check all that.
>
> ---

Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>

> diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> index 24295d39713b..523aa4157f80 100644
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -747,6 +747,10 @@ int arch_rewrite_retpolines(struct objtool_file *file)
>   
>   	list_for_each_entry(insn, &file->retpoline_call_list, call_node) {
>   
> +		if (insn->type != INSN_JUMP_DYNAMIC &&
> +		    insn->type != INSN_CALL_DYNAMIC)
> +			continue;
> +
>   		if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
>   			continue;
>