Re: Offset2lib: bypassing full ASLR on 64bit Linux

[Steve Grubb <sgrubb () redhat com>]

On Tuesday, December 09, 2014 03:24:39 PM Daniel Micay wrote:
> > I studied this area 2 years ago for a gray hat talk and in preparation to
> > help  set the policy going forward for Fedora and RHEL. The general
> > reason I've heard mentioned about why its not used as fully as possible
> > is that it adds memory pages that can't be coalesced or consolidated
> > because they are not the same.
>
> AFAIK, it doesn't cause a significant increase in memory usage. The
> whole point of position independent code is that it can be reused across
> processes. Dynamic libraries are already fully position independent.

This was the issue as I remember it, when you use PIE, it introduces a 
writable memory segment for the indirection that is filled in lazily. Because 
this is writeable it is per process. It is also per DSO. With desktop apps 
linking against a 60 or so libraries and an average of 150 apps running at any 
time, the memory used starts to really add up. Server workloads tend to have 
apps with less libraries linked in but many more processes active.

Of course today systems are starting to ship with 16GB standard, but now we 
add virtualization work loads and you have dozens more of the above. I think 
this institutional memory of the old days kind of hinders any movement. We had 
an internal debate of this on bz 786915 (which is not 100% open). The start up 
performance never really a factor in the discussions. It was purely about 
"wasted" memory.

-Steve

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