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Date: Tue, 24 May 2016 22:29:01 +0200
From: Emese Revfy <re.emese@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: "kernel-hardening@lists.openwall.com" 
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Subject: Re: [PATCH v1 3/3] Add the extra_latent_entropy kernel parameter
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On Tue, 24 May 2016 10:09:16 -0700
Kees Cook <keescook@chromium.org> wrote:

> On Mon, May 23, 2016 at 3:17 PM, Emese Revfy <re.emese@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