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Date: Wed, 01 Jun 2011 08:33:55 -0700
From: Richard Henderson <rth@twiddle.net>
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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: cedric.vincent@st.com, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Laurent ALFONSI <Laurent.ALFONSI@st.com>, Riku Voipio <riku.voipio@iki.fi>

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);


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.


r~