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

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Date:   Fri, 29 Apr 2022 15:56:36 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, 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 02:00:44PM +0200, Peter Zijlstra wrote:
> 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?

Same as for any other generated code, the unwinder will try to fall back
to frame pointers, and if that doesn't work, the unwind stops.

That commit didn't change anything since it was already not being
directly executed anyway, but rather used to generate code on the fly.

And before that commit it was being ignored by ORC anyway, thanks to
STACK_FRAME_NON_STANDARD.  Which can now be removed since this code is
now data and objtool will no longer try to understand it.

-- 
Josh