linux-kernel - Re: [PATCH 3/4] x86/jump_label: Implement arch_static

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Date:   Mon, 15 Jan 2018 12:28:15 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 3/4] x86/jump_label: Implement arch_static_assert()

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@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Borislav Petkov <bp@...en8.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  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"

The "aw" flags aren't needed, the section is neither allocatable nor
writable.

-- 
Josh