[FFmpeg-devel] [PATCH] Make av_get_random_seed not block when waiting for more entropy

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> On Wed, Jun 30, 2010 at 10:11:27AM +0100, M?ns Rullg?rd wrote:
>> Martin Storsj? <martin at martin.st> writes:
>> 
>> > On Tue, 29 Jun 2010, Martin Storsj? wrote:
>> >
>> >> On Tue, 29 Jun 2010, M?ns Rullg?rd wrote:
>> >> 
>> >> > Martin Storsj? <martin at martin.st> writes:
>> >> > 
>> >> > > On Tue, 29 Jun 2010, M?ns Rullg?rd wrote:
>> >> > >
>> >> > >> This will still block if urandom isn't available.  How about this?
>> >> > >
>> >> > >> From cb60e983c550c2bdffc5fc07075336fb7c465fe9 Mon Sep 17 00:00:00 2001
>> >> > >> From: Mans Rullgard <mans at mansr.com>
>> >> > >> Date: Tue, 29 Jun 2010 14:22:51 +0100
>> >> > >> Subject: [PATCH] Make av_get_random_seed() non-blocking
>> >> > >> 
>> >> > >> Attempt to read from /dev/urandom and /dev/random with O_NONBLOCK set.
>> >> > >> If neither succeeds, proceed with fallbacks.
>> >> > >
>> >> > > In general, this looks like an even better alternative. Neither fcntl, 
>> >> > > F_SETFL, F_GETFL nor O_NONBLOCK are available on windows, though, so they 
>> >> > > need to be hidden within some kind of ifdef.
>> >> > 
>> >> > Cygwin should have them, mingw might not.
>> >> > 
>> >> > Do /dev/[u]random exist on cygwin/mingw?
>> >> 
>> >> Don't know about cygwin, but mingw won't have them at least, since it's 
>> >> almost plain win32.
>> >
>> > Would the cleanest fix be to check for fcntl and/or O_NONBLOCK in 
>> > configure, and just keep the nonblock setting line within ifdef? That 
>> > ought to work correctly on both cygwin and mingw.
>> 
>> Yes, that's what I was planning to do.
>
> before you spend more time on this.
> There is a possible security issue with using non block mode
> namely if we have /dev/*random and not use it we can end up
> using a uninitialized variable. Thats an information leak
> it could leak from pointers (kills ASLR) to OS/platform or
> compiler version or or or ...

That's a valid point.  Ideas what to do instead?  Waiting indefinitely
for something random to show up isn't exactly good either.

-- 
M?ns Rullg?rd
mans at mansr.com