Re: [dpdk-dev] [PATCH 2/2] eal: resolve getentropy at run time for random seed

[Dan Gora <dg@adax.com>]

On Thu, Apr 23, 2020 at 9:36 AM Mattias Rönnblom
<mattias.ronnblom@ericsson.com> wrote:
> >>
> >> /dev/urandom is basically only a different interface to the same
> >> underlying mechanism.
> >>
> >> Such an alternative would look something like:
> >>
> >> static int
> >> getentropy(void *buffer, size_t length)
> >> {
> >>           int rc = -1;
> >>           int old_errno = errno;
> >>           int fd;
> >>
> >>           fd = open("/dev/urandom", O_RDONLY);
> >>
> >>           if (fd < 0)
> >>                   goto out;
> >>
> >>           if (read(fd, buffer, length) != length)
> >>                   goto out_close;
> >>
> >>           rc = 0;
> >>
> >> out_close:
> >>           close(fd);
> >> out:
> >>           errno = old_errno;
> >>
> >>           return rc;
> >> }
> > That's fine with me, but like I said I wasn't trying to change how any
> > of this worked, just work around glibc dependencies.  There seems to
> > be some subtle difference between /dev/urandom and /dev/random, but...
> >
> > https://protect2.fireeye.com/v1/url?k=1705be57-4b8f6b41-1705fecc-862f14a9365e-bb983def357fdfad&q=1&e=10fec9c1-51b3-4bc3-b77d-7eb39787d007&u=https%3A%2F%2Fpatches-gcc.linaro.org%2Fcomment%2F14484%2F
> >
> >>>> Failure to run on old libc seems like a non-issue to me.
> >>> Well, again, it's a new dependency that didn't exist before.. We sell
> >>> to telco customers, so we have to support 10s of different target
> >>> platforms of various ages.  If they update their system, we'd have to
> >>> recompile our code to be able to use getentropy().  Similarly, if we
> >>> compiled on a system which has getentropy(), but the target system
> >>> doesn't, then they cannot run our binary because of the glibc 2.25
> >>> dependency.  That means that we have to have separate versions with
> >>> and without getentropy().  It's a maintenance headache for no real
> >>> benefit.
> >>
> >> I'm not sure I follow. Why would you need to recompile DPDK in case they
> >> upgrade their system? It sounds like you care about initial seeding,
> >> since you want getentropy() if it exists, but then in the next paragraph
> >> you want to throw it out, so I'm a little confused.
> > Well  _I_ wouldn't but maybe someone wants getentropy() for the
> > initial seed.. I assume that's why it was added in the first place..
> > For my application we don't care at all.  I just want to get rid of
> > this dependency on glibc 2.25 and have the behavior be the same on
> > meson and Makefile builds on the same complication system.
>
>
> The reason for trying to avoid a wall time-based seed as the default is
> that application instances started at the roughly the same time might
> end up having a the same seed, which in turn might impact their behavior
> in an adverse way. For example, random back-off timers may be the same.
> On x86_64, TSC has a high resolution, but on other platforms its
> equivalent the clock rate is much lower.
>
>
> >> Why doesn't the standard practice of compiling against the oldest
> >> supported libc work for you?
> > I guess I didn't realize that was "standard practice" but even so it
> > still adds an unnecessary restriction on the complication platform.
>
>
> If DPDK has the policy of attempting to allow DPDK applications compiled
> against one glibc version to run against another, older, version, we can
> go ahead and discuss the details further. That would be up to the tech
> board to decide. I would vote against it.

I don't know why anyone would vote against removing an unnecessary
dependency, which was only introduced in v19.08 anyways.

> If the fix was simple, that's one thing. dlopen()/dlsym() doesn't
> qualify as such, nor does a syscall wrapper, as you pointed out.

The dlopen/dlsym() method is used in at least 4 other places in DPDK.
It's not that complicated.  There is plenty of precedence for it being
done this way.

I sent a v4 of the patch which emulated getentropy() using
/dev/urandom as you suggested. Did you see that?

thanks
dan