Re: vmlinux.o: warning: objtool: mptscsih_abort+0x7f4: unreachable instruction

[Peter Zijlstra]

On Mon, Feb 27, 2023 at 12:38:13PM +0000, Mark Rutland wrote:
> [adding Peter and Josh, given objtool]
> 
> On Mon, Feb 27, 2023 at 05:28:53PM +0800, kernel test robot wrote:
> > Hi Mark,
> > 
> > FYI, the error/warning still remains.
> > 
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   f3a2439f20d918930cc4ae8f76fe1c1afd26958f
> > commit: c27cd083cfb9d392f304657ed00fcde1136704e7 Compiler attributes: GCC cold function alignment workarounds
> > date:   5 weeks ago
> > config: x86_64-randconfig-r026-20230227 (https://download.01.org/0day-ci/archive/20230227/202302271743.78emit6I-lkp@xxxxxxxxx/config)
> > compiler: gcc-11 (Debian 11.3.0-8) 11.3.0
> > reproduce (this is a W=1 build):
> >         # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c27cd083cfb9d392f304657ed00fcde1136704e7
> >         git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> >         git fetch --no-tags linus master
> >         git checkout c27cd083cfb9d392f304657ed00fcde1136704e7
> >         # save the config file
> >         mkdir build_dir && cp config build_dir/.config
> >         make W=1 O=build_dir ARCH=x86_64 olddefconfig
> >         make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash
> > 
> > If you fix the issue, kindly add following tag where applicable
> > | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > | Link: https://lore.kernel.org/oe-kbuild-all/202302271743.78emit6I-lkp@xxxxxxxxx/
> > 
> > All warnings (new ones prefixed by >>):
> > 
> > >> vmlinux.o: warning: objtool: mptscsih_abort+0x7f4: unreachable instruction

> > 07ef  2fb7bff:	e8 00 00 00 00       	call   2fb7c04 <mptscsih_abort+0x7f4>	2fb7c00: R_X86_64_PLT32	mpt_halt_firmware-0x4
> > 07f4  2fb7c04:	e9 75 fb ff ff       	jmp    2fb777e <mptscsih_abort+0x36e>

this is one of those noreturn things again... objtool was clever enough
to figure out that mpt_halt_firmware() does not return (all it's control
flow ends in panic()) and hence it 'knows' that +7f4 will not be
executed. But GCC didn't figure out the same.