linux-kernel - Re: [PATCH] objtool: Fix STACK_FRAME_NON

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Date:   Fri, 29 Apr 2022 14:00:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, jpoimboe@...hat.com
Cc:     linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH] objtool: Fix STACK_FRAME_NON_STANDARD reloc type

On Fri, Apr 29, 2022 at 11:20:24AM +0200, Peter Zijlstra wrote:
> 
> STACK_FRAME_NON_STANDARD results in inconsistent relocation types
> depending on .c or .S usage:
> 
>   Relocation section '.rela.discard.func_stack_frame_non_standard' at offset 0x3c01090 contains 5 entries:
>   Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
>   0000000000000000  00020c2200000002 R_X86_64_PC32          0000000000047b40 do_suspend_lowlevel + 0
>   0000000000000008  0002461e00000001 R_X86_64_64            00000000000480a0 machine_real_restart + 0
>   0000000000000010  0000001400000001 R_X86_64_64            0000000000000000 .rodata + b3d4
>   0000000000000018  0002444600000002 R_X86_64_PC32          00000000000678a0 __efi64_thunk + 0
>   0000000000000020  0002659d00000001 R_X86_64_64            0000000000113160 __crash_kexec + 0

So that weird .rodata entry is optprobe_template_func.

It being in .rodata also means it's not validated and there is no ORC
data generated, is that all intentional? The changelog for:

  877b145f0f47 ("x86/kprobes: Move trampoline code into RODATA")

doesn't really say anything useful about any of that :/

I also don't see any kprobe/optprobe hooks in unwind.h, so what happens
if we hit an optprobe?