switch arc4random to chacha

[Konstantin Belousov]

On Fri, May 30, 2014 at 10:49:53AM -0700, Xin Li wrote:
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> On 05/30/14 08:41, Konstantin Belousov wrote:
> > On Thu, May 29, 2014 at 09:04:11PM -0400, Ted Unangst wrote:
> >> This syncs libc arc4random.c with OpenBSD, mostly to change the 
> >> implementation to ChaCha20.
> >> 
> >> I removed the more complicated seed fetching code and changed it 
> >> to just sysctl(). A quick check revealed that the FreeBSD kernel 
> >> supports this for at least five years now. It's much simpler to 
> >> use code that always works instead of a series of untested 
> >> fallbacks that are even less likely to work.
> >> 
> >> Also removes the addrandom interface as a useless complication. 
> >> If the kernel is incapable of properly seeding arc4random, 
> >> application code can't do any better.
> >> 
> >> Unfortunately, I don't have any FreeBSD systems running at the 
> >> moment, so I can't make any promises that this will even
> >> compile, but it passed the eyeball test.
> > 
> > Am I right that the patch removes arc4random_stir and 
> > arc4random_addrandom symbols ?  If yes, this is done incorrect,
> > and it in fact is disallowed, since it breaks ABI.
> > 
> > The compat shims must be provided, possibly issuing a warning, and 
> > default version for the symbols must be removed to prevent linking 
> > new consumers.
> 
> Actually I have a WIP patchset for this at:
> 
> 	https://github.com/delphij/freebsd/compare/featurefork;chacha20
> 
> It provided compatibility shims for arc4random_stir and
> arc4random_addrandom that logs the event for each process once.
What you do WRT ABI is almost fine.  You should remove the symbols from
the gen/Symbol.map for the change to be complete.  Did you verified
readelf output on the patched libc to ensure that there is no default
versions for the compat symbols ?

> 
> Another difference (which I haven't seek for review and would like to
> see criticizes) from OpenBSD is that my version have added threading
> support.  What it does is that the system will create a maximum of CPU
> number's random states and use the states in a LIFO manner, new state
> is created on demand when a contention happens and the CPU number
> limit haven't been reached.
> 
> (I made a further tweak which basically do #define arc4random_stir()
> and #define arc4random_addrandom(a,b) in stdlib.h.  This allows
> existing applications that insists arc4random_stir() on FreeBSD to
> compile -- is there a way to give a compile time warning?)
There is a GNU linker feature which issues a warning when symbol
is referenced, see sys/cdefs.h:__warn_referenced().

> 
> One thing I haven't done yet is to make the kernel portion of
> arc4random() (i.e. kern.arand) to use Chacha20.
> 
> Cheers,
> - -- 
> Xin LI <delphij at delphij.net>    https://www.delphij.net/
> FreeBSD - The Power to Serve!           Live free or die
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