linux-kernel - Re: [linux-stable-rc:linux-5.10.y 7082/7120] arch/x86/kernel/head_64.o: warning: objtool: xen_hypercall_mmu_update(): can't find starting instruction

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Message-ID: <Ys9MKAriCchlEO8S@decadent.org.uk>
Date:   Thu, 14 Jul 2022 00:50:16 +0200
From:   Ben Hutchings <ben@...adent.org.uk>
To:     kernel test robot <lkp@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...e.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>,
        stable@...r.kernel.org
Subject: Re: [linux-stable-rc:linux-5.10.y 7082/7120]
 arch/x86/kernel/head_64.o: warning: objtool: xen_hypercall_mmu_update():
 can't find starting instruction

On Wed, Jul 13, 2022 at 05:38:47AM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git linux-5.10.y
> head:   53b881e19526bcc3e51d9668cab955c80dcf584c
> commit: 7575d3f3bbd1c68d6833b45d1b98ed182832bd44 [7082/7120] x86: Use return-thunk in asm code
> config: x86_64-rhel-8.3-syz (https://download.01.org/0day-ci/archive/20220713/202207130531.SkRjrrn8-lkp@intel.com/config)
> compiler: gcc-11 (Debian 11.3.0-3) 11.3.0
> reproduce (this is a W=1 build):
>         # https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git/commit/?id=7575d3f3bbd1c68d6833b45d1b98ed182832bd44
>         git remote add linux-stable-rc https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git
>         git fetch --no-tags linux-stable-rc linux-5.10.y
>         git checkout 7575d3f3bbd1c68d6833b45d1b98ed182832bd44
>         # save the config file
>         mkdir build_dir && cp config build_dir/.config
>         make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash arch/x86/
> 
> If you fix the issue, kindly add following tag where applicable
> Reported-by: kernel test robot <lkp@...el.com>
> 
> All warnings (new ones prefixed by >>):
> 
> >> arch/x86/kernel/head_64.o: warning: objtool: xen_hypercall_mmu_update(): can't find starting instruction
> 
> -- 
> 0-DAY CI Kernel Test Service
> https://01.org/lkp

Please add the following patch to fix this.  This would also be
needed for 5.15-stable.

Ben.

From: Ben Hutchings <ben@...adent.org.uk>
Date: Thu, 14 Jul 2022 00:39:33 +0200
Subject: [PATCH] x86/xen: Fix initialisation in hypercall_page after rethunk

The hypercall_page is special and the RETs there should not be changed
into rethunk calls (but can have SLS mitigation).  Change the initial
instructions to ret + int3 padding, as was done in upstream commit
5b2fc51576ef "x86/ibt,xen: Sprinkle the ENDBR".

Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/xen/xen-head.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index 38b73e7e54ba..2a3ef5fcba34 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -69,9 +69,9 @@ SYM_CODE_END(asm_cpu_bringup_and_idle)
 SYM_CODE_START(hypercall_page)
 	.rept (PAGE_SIZE / 32)
 		UNWIND_HINT_FUNC
-		.skip 31, 0x90
 		ANNOTATE_UNRET_SAFE
-		RET
+		ret
+		.skip 31, 0xcc
 	.endr
 
 #define HYPERCALL(n) \


-- 
Ben Hutchings
Always try to do things in chronological order;
it's less confusing that way.

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