linux-kernel - Re: fs/overlayfs/copy_up.o: warning: objtool: ovl_copy_up_tmpfile() falls through to next function ovl_create

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Date: Tue, 12 Mar 2024 15:49:23 +0800
From: Yujie Liu <yujie.liu@...el.com>
To: Amir Goldstein <amir73il@...il.com>
CC: kernel test robot <lkp@...el.com>, <oe-kbuild-all@...ts.linux.dev>,
	<linux-kernel@...r.kernel.org>, overlayfs <linux-unionfs@...r.kernel.org>,
	Miklos Szeredi <miklos@...redi.hu>
Subject: Re: fs/overlayfs/copy_up.o: warning: objtool: ovl_copy_up_tmpfile()
 falls through to next function ovl_create_index.cold()

Hi Amir,

On Tue, Feb 27, 2024 at 03:56:53PM +0200, Amir Goldstein wrote:
> On Sun, Feb 25, 2024 at 11:25 PM kernel test robot <lkp@...el.com> wrote:
> >
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   70ff1fe626a166dcaadb5a81bfe75e22c91f5dbf
> > commit: 5b02bfc1e7e3811c5bf7f0fa626a0694d0dbbd77 ovl: do not encode lower fh with upper sb_writers held
> > date:   4 months ago
> > config: x86_64-buildonly-randconfig-r001-20230616 (https://download.01.org/0day-ci/archive/20240226/202402260556.0rujs05e-lkp@intel.com/config)
> > compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240226/202402260556.0rujs05e-lkp@intel.com/reproduce)
> >
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Reported-by: kernel test robot <lkp@...el.com>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202402260556.0rujs05e-lkp@intel.com/
> >
> > All warnings (new ones prefixed by >>):
> >
> > >> fs/overlayfs/copy_up.o: warning: objtool: ovl_copy_up_tmpfile() falls through to next function ovl_create_index.cold()
> >
> 
> I have no idea what this is about.
> Does not seem related to the regressing commit.
> This:
> https://lore.kernel.org/lkml/caa4ec6c687931db805e692d4e4bf06cd87d33e6.1460729697.git.jpoimboe@redhat.com/
> suggest that it might be a compiler bug?

Sorry for our late reply. We rechecked this case and found that this can
only be reproduced by a specific version of gcc. gcc-12.2.0 can
reproduce it while gcc-11.3.0 and gcc-13.2.0 cannot. This may indeed be
a flaw in gcc-12. Attached part of the disassembly of the object file
built by gcc-12. We will configure the bot to ignore this warning to
avoid false positives.

Disassembly of section .text.unlikely:
..

00000000000000e6 <ovl_copy_up_metadata.cold>:
..
 155:   e8 00 00 00 00          call   15a <ovl_copy_up_metadata.cold+0x74>
 15a:   80 fb 01                cmp    $0x1,%bl
 15d:   0f 86 00 00 00 00       jbe    163 <ovl_copy_up_metadata.cold+0x7d>
 163:   e8 00 00 00 00          call   168 <ovl_copy_up_metadata.cold+0x82>
 168:   0f b6 f3                movzbl %bl,%esi
 16b:   48 c7 c7 00 00 00 00    mov    $0x0,%rdi
 172:   e8 00 00 00 00          call   177 <ovl_copy_up_metadata.cold+0x91>
 177:   e9 00 00 00 00          jmp    17c <ovl_copy_up_tmpfile.cold>

000000000000017c <ovl_copy_up_tmpfile.cold>:
 17c:   e8 00 00 00 00          call   181 <ovl_create_index.cold>  <--

0000000000000181 <ovl_create_index.cold>:
 181:   e8 00 00 00 00          call   186 <ovl_create_index.cold+0x5>
 186:   0f b6 f3                movzbl %bl,%esi
 189:   48 c7 c7 00 00 00 00    mov    $0x0,%rdi
 190:   e8 00 00 00 00          call   195 <ovl_create_index.cold+0x14>
 195:   e9 00 00 00 00          jmp    19a <ovl_copy_up_workdir.cold>


Best Regards,
Yujie