Subject: Re: Are those hacks still valid on powerpc kernel ?

From: Benjamin Herrenschmidt <[email-masked]>
Date: Thu, 25 Jan 2018 10:49:42 +1100

On Wed, 2018-01-24 at 11:17 +0100, Christophe LEROY wrote:
> Below comments are very old.
> 
> Aren't new glibc and binutils now able to go without this ?
> 
> Note that the code inside the #if 0 is wrong as we have no vma defined 
> in the function.
> 
> Or does it just have no performance impact anyway ?
> 
> 
>  From /arch/powerpc/mm/mem.c:
> 
> void clear_user_page(void *page, unsigned long vaddr, struct page *pg)
> {
> 	clear_page(page);
> 
> 	/*
> 	 * We shouldn't have to do this, but some versions of glibc
> 	 * require it (ld.so assumes zero filled pages are icache clean)
> 	 * - Anton
> 	 */
> 	flush_dcache_page(pg);
> }
> EXPORT_SYMBOL(clear_user_page);

Well, I think it would be a security issue to potentially leave garbage
icache content (possibly instructions from another process) accessible
to userspace. So I don't think we can avoid that one.

> void copy_user_page(void *vto, void *vfrom, unsigned long vaddr,
> 		    struct page *pg)
> {
> 	copy_page(vto, vfrom);
> 
> 	/*
> 	 * We should be able to use the following optimisation, however
> 	 * there are two problems.
> 	 * Firstly a bug in some versions of binutils meant PLT sections
> 	 * were not marked executable.
> 	 * Secondly the first word in the GOT section is blrl, used
> 	 * to establish the GOT address. Until recently the GOT was
> 	 * not marked executable.
> 	 * - Anton
> 	 */
> #if 0
> 	if (!vma->vm_file && ((vma->vm_flags & VM_EXEC) == 0))
> 		return;
> #endif

Well, we try not to break userspace.... This doesn't affect newer CPUs
that much because they have CPU_FTR_COHERENT_ICACHE, so
flush_dcache_page is pretty much a nop on them.

Cheers,
Ben.

> 	flush_dcache_page(pg);
> }
> Christophe