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

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Message-ID: <c2471b0a81ebd183d32e76f995a70b7912c1d4a1.camel@infradead.org>
Date: Fri, 14 Mar 2025 17:23:15 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.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, 2025-03-14 at 09:07 -0700, Josh Poimboeuf wrote:
> On Wed, Mar 12, 2025 at 02:34:20PM +0000, David Woodhouse wrote:
> > From: David Woodhouse <dwmw@...zon.co.uk>
> > 
> > A previous commit added __nocfi to machine_kexec() because it makes an
> > indirect call to relocate_kernel() which lacked CFI type information,
> > and caused the system to crash.
> > 
> > Use SYM_TYPED_FUNC_START() to ensure that the type information is
> > present, and remove the __nocfi tag.
> > 
> > I still can't make objtool happy with this in both GCC and Clang builds
> > at the same time, so not yet for merging; only included in this series
> > to nerd-snipe the objtool maintainers.
> 
> Again, I'd recommend just compiling the code in .data..relocate_kernel.
> Then objtool doesn't need to care.
> 
> Otherwise it would help to know what warnings you're getting with this
> patch, and the corresponding .config.

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.

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).

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