On Tue, 24 May 2016 10:09:16 -0700
Kees Cook <keesc...@chromium.org> wrote:
> On Mon, May 23, 2016 at 3:17 PM, Emese Revfy <re.em...@gmail.com> wrote:
> > @@ -1235,6 +1236,15 @@ static void __free_pages_ok(struct page *page,
> > unsigned int order)
> > }
> >
> > #ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
> > +bool __meminitdata extra_latent_entropy;
> > +
> > +static int __init setup_extra_latent_entropy(char *str)
> > +{
> > + extra_latent_entropy = true;
> > + return 0;
> > +}
> > +early_param("extra_latent_entropy", setup_extra_latent_entropy);
> > +
> > volatile u64 latent_entropy __latent_entropy;
> > EXPORT_SYMBOL(latent_entropy);
> > #endif
> > @@ -1254,6 +1264,19 @@ static void __init __free_pages_boot_core(struct
> > page *page, unsigned int order)
> > __ClearPageReserved(p);
> > set_page_count(p, 0);
> >
> > +#ifdef CONFIG_GCC_PLUGIN_LATENT_ENTROPY
> > + if (extra_latent_entropy && !PageHighMem(page) && page_to_pfn(page)
> > < 0x100000) {
> > + u64 hash = 0;
> > + size_t index, end = PAGE_SIZE * nr_pages / sizeof hash;
> > + const u64 *data = lowmem_page_address(page);
> > +
> > + for (index = 0; index < end; index++)
> > + hash ^= hash + data[index];
> > + latent_entropy ^= hash;
> > + add_device_randomness((const void *)&latent_entropy,
> > sizeof(latent_entropy));
> > + }
> > +#endif
> > +
>
> We try to minimize #ifdefs in the .c code, so in this case, I think I
> would define "extra_latent_entropy" during an #else above so this "if"
> can be culled by the compiler automatically:
>
> #else
> # define extra_latent_entropy false
> #endif
>
> Others may have better suggestions to avoid the second #ifdef, but
> this seems the cleanest way to me to tie this to the earlier #ifdef.
Hi,
I think the best way would be if I removed all #ifdefs because
this is useful without the latent_entropy plugin.
I don't know wether the default value of extra_latent_entropy
should be true or false. I'll do some performance measurements.
--
Emese
