Re: [PATCH v2] x86: fix early boot crash on gcc-10

[Peter Zijlstra]

On Wed, Apr 22, 2020 at 12:23:09PM +0200, Borislav Petkov wrote:
> Ok,
> 
> let's try the simple and clean fix first. Nick, would that work on LLVM
> too?
> 
> And I hope this will remain working and the compiler won't jump over an
> inline asm and go nuts.
> 
> Thx.
> 
> ---
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 3b9bf8c7e29d..06d2e16bedbb 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -266,6 +266,13 @@ static void notrace start_secondary(void *unused)
>  
>  	wmb();
>  	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
> +
> +	/*
> +	 * Prevent tail call to cpu_startup_entry() because the stack protector
> +	 * guard has been changed in the middle of this function and must not be
> +	 * checked before tail calling another function.
> +	 */
> +        asm ("");
>  }

You haz a whitespace issue there.

Also, can we get this in writing, signed in blood, from the various
compiler teams ;-)