linux-kernel - Re: random: /dev/random often returns short reads

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Message-ID: <CAK1hOcPrkLM4H0FZNpgFE66n2Eb4vxD6iST+OrzOvkmPoydV7w@mail.gmail.com>
Date:   Thu, 19 Jan 2017 22:45:49 +0100
From:   Denys Vlasenko <vda.linux@...glemail.com>
To:     "Theodore Ts'o" <tytso@....edu>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Denys Vlasenko <vda.linux@...glemail.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: random: /dev/random often returns short reads

On Wed, Jan 18, 2017 at 7:07 PM, Theodore Ts'o <tytso@....edu> wrote:
> In the ideal world, yes.  I've acknowledged this is a bug, in the "be
> conservative in what you send, liberal in what you receive" sense..
> But no one complained for three year, and userspace needs to be able
> to retry short reads instead of immediately erroring out.
>
> The problem is changing that code to figure out exactly how many bytes
> you need to get in order to have N random bytes is non-trivial.  So
> our choices are:
>
> 1) Transfer more bytes than might be needed to the secondary pool,
> which results in resource stranding --- since entropy in the secondary
> pool isn't available for reseeding the CRNG.  OTOH, given that we're
> now using the CRNG solution, and we're only reseeding every five
> minutes, I'm not actually all that worried about stranding some extra
> entropy bits in the blocking pool, since that's only going to happen
> if we have people *using* the /dev/random pool, and so that entropy
> will likely be used eventually anyway
...
...
> I'm leaning a bit towards 1 if we have to do something (which is my
> proposed, untested patch).

Thanks, this solution is okay for me.