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Date: Fri, 3 Jun 2011 08:46:19 +0200
From: <cedric.vincent@st.com>
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Subject: Re: [Qemu-devel] [PATCH] linux-user: Define AT_RANDOM to support
 target	dynamic linkers that do ASLR
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To: Richard Henderson <rth@twiddle.net>
Cc: Laurent ALFONSI <Laurent.ALFONSI@st.com>, Riku Voipio <riku.voipio@iki.fi>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

On Wed, Jun 01, 2011 at 05:33:55PM +0200, Richard Henderson wrote:
> On 06/01/2011 06:47 AM, cedric.vincent@st.com wrote:
> > On Wed, Jun 01, 2011 at 03:26:09PM +0200, Richard Henderson wrote:
> >>
> >> 16 bytes, not 16 bits.
> > 
> > You're right it's not 16 bits, it's "sizeof(uintptr_t)" actually:
> 
> No, it's not.
> 
>         unsigned char k_rand_bytes[16];
>         elf_addr_t __user *u_rand_bytes;
> ...
>         /*
>          * Generate 16 random bytes for userspace PRNG seeding.
>          */
>         get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes));
>         u_rand_bytes = (elf_addr_t __user *)
>                        STACK_ALLOC(p, sizeof(k_rand_bytes));
>         if (__copy_to_user(u_rand_bytes, k_rand_bytes, sizeof(k_rand_bytes)))
>                 return -EFAULT;
> ...
>         NEW_AUX_ENT(AT_RANDOM, (elf_addr_t)(unsigned long)u_rand_bytes);

It's clearer to me now, thanks.


> Frankly, it's trivial to do this right in create_elf_tables.
> Grab 16 bytes at SP right at the beginning of the function,
> fill it with whatever random values seem good.
> 
> I suggest at minimum a command-line argument to force a 
> particular AT_RANDOM value, for repeatability.

OK, I will submit you such a patch soon.

Thanks,
Cédric.