linux-kernel - Re: [PATCH v7 8/8] [DO NOT MERGE] x86/kexec: Add CFI type information to relocate

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Message-ID: <7tq4tti5pv7fjboiapuglkcsodl4nsk53rj36skg4xb2bkysei@ncoz2ztiddm7>
Date: Fri, 14 Mar 2025 10:52:53 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: David Woodhouse <dwmw2@...radead.org>
Cc: kexec@...ts.infradead.org, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, 
	Nikolay Borisov <nik.borisov@...e.com>, linux-kernel@...r.kernel.org, Simon Horman <horms@...nel.org>, 
	Dave Young <dyoung@...hat.com>, Peter Zijlstra <peterz@...radead.org>, bsz@...zon.de
Subject: Re: [PATCH v7 8/8] [DO NOT MERGE] x86/kexec: Add CFI type
 information to relocate_kernel()

On Fri, Mar 14, 2025 at 05:23:15PM +0000, David Woodhouse wrote:
> ISTR this version is OK with Clang and CONFIG_CFI_CLANG but with GCC I
> get this:
> 
> vmlinux.o: warning: objtool: relocate_kernel+0x69: unsupported stack register modification
> 
>         /* setup a new stack at the end of the physical control page */
>         lea     PAGE_SIZE(%rsi), %rsp
>   79:   48 8d a6 00 10 00 00    lea    0x1000(%rsi),%rsp
> 
> 
> Maybe the answer is to put the UNWIND_HINT_FUNC into #ifdef
> CONFIG_CFI_CLANG but that seems wrong.

The UNWIND_HINT_FUNC definitely looks wrong, why would Clang need it?

> I'll have another look at putting it in the data section, and see if I
> can remember why I didn't want to do that before (and if that's still
> relevant now).

IIRC, the reasons were the patched alternative, and also you wanted to
disassemble (but note that's still possible with gdb).

Here was a patch to make it work:

  https://lore.kernel.org/20241218212326.44qff3i5n6cxuu5d@jpoimboe

-- 
Josh