[FFmpeg-devel] [PATCH 2/2] configure: libbsd support for arc4random()

[Ganesh Ajjanagadde]

On Wed, Dec 9, 2015 at 9:44 AM, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Wed, Dec 09, 2015 at 09:39:47AM -0500, Ganesh Ajjanagadde wrote:
>> On Wed, Dec 9, 2015 at 8:28 AM, wm4 <nfxjfg at googlemail.com> wrote:
>> > On Tue, 8 Dec 2015 09:01:47 -0500
>> > Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>> >
>> >> On Tue, Dec 8, 2015 at 8:16 AM, Clément Bœsch <u at pkh.me> wrote:
>> >> > On Tue, Dec 08, 2015 at 07:34:51AM -0500, Ganesh Ajjanagadde wrote:
>> >> >> On Tue, Dec 8, 2015 at 7:27 AM, wm4 <nfxjfg at googlemail.com> wrote:
>> >> >> > On Sun,  6 Dec 2015 22:56:33 -0500
>> >> >> > Ganesh Ajjanagadde <gajjanagadde at gmail.com> wrote:
>> >> >> >
>> >> >> >> On non-BSD machines, there exists a package libbsd for providing BSD
>> >> >> >> functionality. This can be used to get support for arc4random.
>> >> >> >>
>> >> >> >> Thus, an opt-in --enable-libbsd is added to configure for this
>> >> >> >> functionality.
>> >> >> >>
>> >> >> >> Tested on GNU/Linux.
>> >> >> >>
>> >> >> >
>> >> >> > This doesn't seem worth the trouble at all. Who is even going to use
>> >> >> > it, and why, and what additional hidden bugs will it cause?
>> >> >>
>> >> >> arc4random is a far superior interface, in that it can never fail. See
>> >> >> http://www.linuxfromscratch.org/hints/downloads/files/entropy.txt for
>> >> >> details.
>> >> >> As for hidden bugs, apart from configuration/detection issues, I see none.
>> >> >> If anything, it is easier to use /dev/urandom incorrectly.
>> >> >> Ultimately any code change is a tradeoff, with different people
>> >> >> feeling differently about various things.
>> >> >> I still feel that it is worth inclusion due to its technical merits.
>> >> >>
>> >> >
>> >> > Note that the behaviour of arc4random differs between implementations.
>> >> >
>> >> > http://insanecoding.blogspot.gr/2014/05/libressl-porting-update.html
>> >>
>> >> That is for libressl, not libbsd, though the remark may still apply.
>> >> And there is no real fundamental issue: FFmpeg's seeding code anyway
>> >> varies between platforms, in the worst case using time.
>> >>
>> >> Second, a getrandom system call has been added to the kernel, so
>> >> libbsd/libressl should upgrade to the new interface over time.
>> >>
>> >> Whatever, if people don't want this, I will drop 2/2 but keep within
>> >> my own tree for a future date potentially.
>> >>
>> >> 1/2 is still very much worthwhile: platforms supporting natively
>> >> arc4random should use it (e.g the BSD's).
>> >
>> > You should wait until glibc supports the getrandom syscall, instead of
>> > using a wrapper lib that is merely meant to make porting BSD programs
>> > simpler. (Looking at libbsd, it does try to use the getrandom syscall,
>> > but also tries potentially dangerous fallbacks like using the system
>> > time as random seed? Isn't that exactly the wrong thing if you want
>> > 100% correctness?)
>>
>> I looked at the libbsd code right now. Strictly speaking, you are
>> right, but it falls back to time of day only on very rare platforms.
>> In particular, it uses /dev/urandom when available. Thus, it turns out
>> that using it over /dev/urandom should actually be a Pareto
>> improvement (ref: getentropy in _rs_stir). Or more generally, using it
>
>> when available via --enable-libbsd will never lower FFmpeg's random
>> seeding quality.
>
> how does it use time of day to create a seed and on what platform ?

Seems super rare, /dev/urandom is available essentially everywhere
apparently: https://en.wikipedia.org/wiki//dev/random. The code
http://cgit.freedesktop.org/libbsd/tree/src/arc4random.c calls
getentropy while stirring, which uses /dev/urandom generally:
http://cgit.freedesktop.org/libbsd/tree/src.

The fallback uses timeofday in some weird way, that is hard to
understand at a glance (just grep for gettimeofday).

>
>
>>
>> I am now 90-10 for 2/2.
>>
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>
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> I know you won't believe me, but the highest form of Human Excellence is
> to question oneself and others. -- Socrates
>
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