Re: [PATCH 3/4] x86/jump_label: Implement arch_static_assert()

[Josh Poimboeuf]

On Mon, Jan 15, 2018 at 05:44:31PM +0100, Peter Zijlstra wrote:
> Implement the static (branch) assertion. It simply emits the address
> of the next instruction into a special section which objtool will read
> and validate against either __jump_table or .altinstructions.
> 
> Use like:
> 
> 	if (static_branch_likely(_key)) {
> 		arch_static_assert();
> 		/* do stuff */
> 	}
> 
> Or
> 
> 	if (static_cpu_has(_feat)) {
> 		arch_static_assert();
> 		/* do stuff */
> 	}
> 
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/jump_label.h |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -62,6 +62,15 @@ static __always_inline bool arch_static_
>  	return true;
>  }
>  
> +static __always_inline void arch_static_assert(void)
> +{
> +	asm volatile ("1:\n\t"
> +		      ".pushsection .discard.jump_assert, \"aw\" \n\t"
> +		      _ASM_ALIGN  "\n\t"
> +		      _ASM_PTR "1b \n\t"
> +		      ".popsection \n\t");
> +}
> +

This needs a nice comment about what exactly it asserts.

And also, people without objtool enabled (i.e., no ORC or livepatch)
won't see the assertion.  Do we care about those people? :-)

-- 
Josh