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Date: Tue, 14 Jul 2009 17:38:23 +1000
From: Anton Blanchard <anton@samba.org>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [patch 1/3] powerpc: Move 64bit VDSO to improve context switch
	performance
Message-ID: <20090714073823.GB9805@kryten>
References: <20090714065350.659537380@samba.org>
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	<1247555233.29462.86.camel@pasglop>
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Hi Ben,

> Don't we lose randomization ? Or do we randomize the whole mem map
> nowadays ?

The start of the top down mmap region is randomized, so the VDSO will be in a
different position each time. A quick example:

run 1:
fffb01f6000-fffb01f9000 r-xp 00000000 00:00 0         [vdso]
fffb01f9000-fffb01fa000 r--p 00027000 08:06 4333852   /lib64/ld-2.9.so
fffb01fa000-fffb01fd000 rw-p 00028000 08:06 4333852   /lib64/ld-2.9.so
fffb01fd000-fffb01fe000 rw-p 00000000 00:00 0 
ffff7c6f000-ffff7c84000 rw-p 00000000 00:00 0         [stack]

run 2:
fff9a094000-fff9a097000 r-xp 00000000 00:00 0         [vdso]  
fff9a097000-fff9a098000 r--p 00027000 08:06 4333852   /lib64/ld-2.9.so
fff9a098000-fff9a09b000 rw-p 00028000 08:06 4333852   /lib64/ld-2.9.so
fff9a09b000-fff9a09c000 rw-p 00000000 00:00 0 
fffea0a6000-fffea0bb000 rw-p 00000000 00:00 0         [stack]

You will notice we aren't randomising each mmap, so the relative offset
between ld.so and the vdso will be consistent. I just checked and it 
looks like x86 does the same.

It might make sense to add a small amount of randomness between mmaps
on both x86 and PowerPC, at least for 64bit applications where we have
enough address space.

Anton