On Fri, Jan 26, 2018 at 10:37:30AM +0000, David Woodhouse wrote:
> On Tue, 2018-01-23 at 16:25 +0100, Peter Zijlstra wrote:
> > This is boot code, we run this _way_ before userspace comes along to
> > poison our branch predictor.
>
> Hm, objtool knows about sections, doesn't it? Why it is whining about
> indirect jumps in inittext anyway?
>
> In fact, why are we even *doing* retpolines in inittext? Not that we
> are; since we flipped the ALTERNATIVE logic around, at that point we
> still have the 'oldinstr' which is a bare jmp anyway. We might as well
> do this:
>
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -37,10 +37,15 @@
> * as gcc otherwise puts the data into the bss section and not into the init
> * section.
> */
> +#if defined(RETPOLINE) && !defined(MODULE)
> +#define __noretpoline __attribute__((indirect_branch("keep")))
> +#else
> +#define __noretpoline
> +#endif
>
> /* These are for everybody (although not all archs will actually
> discard it in modules) */
> -#define __init __section(.init.text) __cold __inittrace
> __latent_entropy
> +#define __init __section(.init.text) __cold __inittrace
> __latent_entropy __noretpoline
We run module __init text concurrently with userspace.
