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

[Ganesh Ajjanagadde]

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.

I am now 90-10 for 2/2.

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