Re: [linux-stable-rc:linux-5.10.y 6996/7120] arch/x86/kernel/ftrace_64.o: warning: objtool: ftrace_epilogue()+0x10: unreachable instruction

[Ben Hutchings]

On Wed, 2022-07-13 at 03:57 +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:   ba398a0e54049bb6a1c102245ae84103f103ff2c
> commit: 8ca9dceaa17983afec1cc8e1a6e76d6bd142447a [6996/7120] objtool: Assume only ELF functions do sibling calls
> config: x86_64-rhel-8.3-syz (https://download.01.org/0day-ci/archive/20220713/202207130344.dwXyDmTk-lkp@xxxxxxxxx/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=8ca9dceaa17983afec1cc8e1a6e76d6bd142447a
>         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 8ca9dceaa17983afec1cc8e1a6e76d6bd142447a
>         # 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@xxxxxxxxx>
> 
> All warnings (new ones prefixed by >>):
> 
> > > arch/x86/kernel/ftrace_64.o: warning: objtool: ftrace_epilogue()+0x10: unreachable instruction
> 
> 
> objdump-func vmlinux.o ftrace_epilogue:
> 0000 000000000008ca70 <ftrace_epilogue>:
> 0000    8ca70:	e9 00 00 00 00       	jmp    8ca75 <ftrace_epilogue+0x5>	8ca71: R_X86_64_PLT32	ftrace_stub-0x4
> 0005    8ca75:	66 66 2e 0f 1f 84 00 00 00 00 00 	data16 cs nopw 0x0(%rax,%rax,1)
> 

I don't know what's going on here.  There's a jump to a ret (now
potentialy a rethunk), and nothing obviously unreachable. 
ftrace_epilogue got simplified by commit 0c0593b45c9b "x86/ftrace: Make
function graph use ftrace directly" which maybe avoids the warning in
the mainline.

Ben.

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