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In-Reply-To: <ca31cb51-30b8-e970-c33c-7b848ae5ed45@huawei.com>
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Mon, 19 Sep 2022 11:40:13 +0100
Message-ID: <CAHmME9qWQxkDRY+TG=QL_mSZqd+vNTn186L4kbZfKEbimQcD0Q@mail.gmail.com>
Subject: Re: Inquiry about the removal of flag O_NONBLOCK on /dev/random
To:     "Guozihua (Scott)" <guozihua@huawei.com>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Andrew Lutomirski <luto@kernel.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        zhongguohua <zhongguohua1@huawei.com>
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Hi,

On Mon, Sep 19, 2022 at 11:27 AM Guozihua (Scott) <guozihua@huawei.com> wrote:
>
> On 2022/9/8 17:51, Jason A. Donenfeld wrote:
> > Hi,
> >
> > On Thu, Sep 08, 2022 at 11:31:31AM +0800, Guozihua (Scott) wrote:
> >> For example:
> >>
> >>
> >> --
> >> Best
> >> GUO Zihua
> >>
> >> --
> >> Best
> >> GUO Zihua
> >
> > Looks like you forgot to paste the example...
> >
> >> Thank you for the timely respond and your patient. And sorry for the
> >> confusion.
> >>
> >> First of all, what we think is that this change (removing O_NONBLOCK) is
> >> reasonable. However, this do cause issue during the test on one of our
> >> product which uses O_NONBLOCK flag the way I presented earlier in the
> >> Linux 4.4 era. Thus our colleague suggests that returning -EINVAL when
> >> this flag is received would be a good way to indicate this change.
> >
> > No, I don't think it's wise to introduce yet *new* behavior (your
> > proposed -EINVAL). That would just exacerbate the (mostly) invisible
> > breakage from the 5.6-era change.
> >
> > The question now before us is whether to bring back the behavior that
> > was there pre-5.6, or to keep the behavior that has existed since 5.6.
> > Accidental regressions like this (I assume it was accidental, at least)
> > that are unnoticed for so long tend to ossify and become the new
> > expected behavior. It's been around 2.5 years since 5.6, and this is the
> > first report of breakage. But the fact that it does break things for you
> > *is* still significant.
> >
> > If this was just something you noticed during idle curiosity but doesn't
> > have a real impact on anything, then I'm inclined to think we shouldn't
> > go changing the behavior /again/ after 2.5 years. But it sounds like
> > actually you have a real user space in a product that stopped working
> > when you tried to upgrade the kernel from 4.4 to one >5.6. If this is
> > the case, then this sounds truly like a userspace-breaking regression,
> > which we should fix by restoring the old behavior. Can you confirm this
> > is the case? And in the meantime, I'll prepare a patch for restoring
> > that old behavior.
> >
> > Jason
> > .
>
> Hi Jason
>
> Thank for your patience.
>
> To answer your question, yes, we do have a userspace program reading
> /dev/random during early boot which relies on O_NONBLOCK. And this
> change do breaks it. The userspace program comes from 4.4 era, and as
> 4.4 is going EOL, we are switching to 5.10 and the breakage is reported.
>
> It would be great if the kernel is able to restore this flag for
> backward compatibility.

Alright then. Sounds like a clear case of userspace being broken. I'll
include https://git.zx2c4.com/linux-rng/commit/?id=b931eaf6ef5cef474a1171542a872a5e270e3491
or similar in my pull for 6.1, if that's okay with you. For 6.0, we're
already at rc6, so maybe better to let this one stew for a bit longer,
given the change, unless you feel strongly about having it earlier, I
guess.

Jason