linux-kernel - Re: [PATCH v5 07/20] x86/kexec: Invoke copy of relocate

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Message-ID: <79ea9c10ae4f1dae16be8c05bb7b7e6b72512f10.camel@infradead.org>
Date: Mon, 16 Dec 2024 12:09:59 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Nathan Chancellor <nathan@...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>,
 jpoimboe@...nel.org, bsz@...zon.de
Subject: Re: [PATCH v5 07/20] x86/kexec: Invoke copy of relocate_kernel()
 instead of the original

On Sun, 2024-12-15 at 22:49 -0700, Nathan Chancellor wrote:
> On Sun, Dec 15, 2024 at 10:09:57AM +0000, David Woodhouse wrote:
> > On Sat, 2024-12-14 at 16:08 -0700, Nathan Chancellor wrote:
> > > 
> > > I guess this seems somewhat unavoidable because control_page is just a
> > > 'void *', perhaps machine_kexec() should just be marked as __nocfi? This
> > > diff resolves that issue for me.
> > 
> > The patch below seems to work too. I already wanted to deal with the
> 
> Can confirm, thanks for the quick fix. With your fix for the first issue
> I reported, the fix I sent for LTO, and this patch below, I can kexec on
> a CFI and LTO enabled kernel without any issues.

Argh, using SYM_TYPED_FUNC_START() leads to objtool having more opinions.

I have the Clang build working in my tree now, but the GCC build now complains

vmlinux.o: warning: objtool: relocate_kernel+0x0: unreachable instruction

It seems like adding UNWIND_HINT_FUNC ought to be the answer for that?
But then it complains about this instead:

vmlinux.o: warning: objtool: relocate_kernel+0x69: unsupported stack register modification

That's the lea instruction at
       lea     PAGE_SIZE(%rsi), %rsp
 79:   48 8d a6 00 10 00 00    lea    0x1000(%rsi),%rsp

I've pushed what I have to my kexec-debug branch; I was hoping to post
the fixes for the tip/x86/boot branch this morning but I've run out of
time and will be travelling for the rest of the week. Will try to get
something sent out this evening when I land.

I may resort to the __nocfi option for now. As noted on the typedef
patch and in IRC, the whole SYM_TYPED_FUNC_START() thing using the type
information from the *C* code which is actually doing the call anyway,
is a bit tautological anyway.

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