Re: [tip:x86/core 16/17] vmlinux.o: warning: objtool: do_jit+0x276: relocation to !ENDBR: .noinstr.text+0x6a60

[Kees Cook]

On Mon, Mar 03, 2025 at 10:24:59AM +0100, Peter Zijlstra wrote:
> On Mon, Mar 03, 2025 at 07:47:57AM +0800, kernel test robot wrote:
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86/core
> > head:   dfebe7362f6f461d771cdb9ac2c5172a4721f064
> > commit: 0c92385dc05ee9637c04372ea95a11bbf6e010ff [16/17] x86/ibt: Implement FineIBT-BHI mitigation
> > config: x86_64-randconfig-071-20250303 (https://download.01.org/0day-ci/archive/20250303/202503030704.H9KFysNS-lkp@xxxxxxxxx/config)
> > compiler: clang version 19.1.7 (https://github.com/llvm/llvm-project cd708029e0b2869e80abe31ddb175f7c35361f90)
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250303/202503030704.H9KFysNS-lkp@xxxxxxxxx/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@xxxxxxxxx>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@xxxxxxxxx/
> > 
> > All warnings (new ones prefixed by >>):
> > 
> > >> vmlinux.o: warning: objtool: do_jit+0x276: relocation to !ENDBR: .noinstr.text+0x6a60
> 
> Thanks, below seems to cure it for me.
> 
> ---
> Subject: x86/ibt: Make cfi_bhi a constant for FINEIBT_BHI=n
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Mon Mar 3 10:21:47 CET 2025
> 
> Robot yielded a .config that tripped:
> 
>   vmlinux.o: warning: objtool: do_jit+0x276: relocation to !ENDBR: .noinstr.text+0x6a60
> 
> This is the result of using __bhi_args[1] in unreachable code; make
> sure the compiler is able to determine this is unreachable and trigger
> DCE.
> 
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202503030704.H9KFysNS-lkp@xxxxxxxxx/
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/cfi.h    |    5 +++++
>  arch/x86/kernel/alternative.c |    3 +++
>  2 files changed, 8 insertions(+)
> 
> --- a/arch/x86/include/asm/cfi.h
> +++ b/arch/x86/include/asm/cfi.h
> @@ -100,7 +100,12 @@ enum cfi_mode {
>  };
>  
>  extern enum cfi_mode cfi_mode;
> +
> +#ifdef CONFIG_FINEIBT_BHI
>  extern bool cfi_bhi;
> +#else
> +#define cfi_bhi (0)
> +#endif

Just for my own curiosity, how did you track this down? (The
relationship between cfi_bhi and do_jit is not immediately obvious to
me. Or rather, what was needing DCE?)

-Kees

-- 
Kees Cook