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

[Luca Boccassi <bluca@debian.org>]

On Mon, 2020-04-27 at 13:57 -0300, Dan Gora wrote:
> On Mon, Apr 27, 2020 at 1:19 PM Luca Boccassi <bluca@debian.org> wrote:
> > On Thu, 2020-04-23 at 14:38 -0300, Dan Gora wrote:
> > > On Thu, Apr 23, 2020 at 12:59 PM Luca Boccassi <bluca@debian.org> wrote:
> > > > > > /dev/urandom is basically only a different interface to the same
> > > > > > underlying mechanism.
> > > > 
> > > > This is not the whole story though - while the end result when all
> > > > works is the same, there are important differences in getting there.
> > > > There's a reason a programmatic interface was added - it's just better
> > > > in general.
> > > > Just to name one - opening files has implications for LSMs like
> > > > SELinux. You now need a specific policy to allow it, which means
> > > > applications that upgrade from one version of DPDK to the next will
> > > > break.
> > > 
> > > DPDK opens _tons_ of files. This would not be the first file that DPDK
> > > has to open.  And it's not like /dev/urandom is a new interface.  It's
> > > been around forever.
> > 
> > That might be the case, but it is not reason in itself to make things
> > harder. Especially in light of the new stability promise - this might
> > or might not be considered part of it, but it's worth mentioning at the
> > very least, as it has a real impact on users.
> 
> "make things harder" seems especially subjective.. I would argue that
> I am in fact making things much easier by removing unnecessary
> dependecies

For someone with selinux, things would be harder. It's a consequence
worth highlighting, that's all.

> > > If this is such a major problem, then that would argue for using the
> > > dlsym()/dlopen() method to try to find the getentropy glibc function
> > > that I sent in v3 of these patches.
> > > 
> > > > In general, I do not think we should go backwards. The programmatic
> > > > interface to the random pools are good and we should use them by
> > > > default - of course by all means add fallbacks to urandom if they are
> > > > not available.
> > > 
> > > The original problem was that the "programmatic interface to the
> > > random pools" (that is, getentropy()) can only be determined at
> > > compilation time and if found introduce a new dependency on glibc 2.25
> > > that can easily be avoided by emulating it (as I did here in v4 of the
> > > patches) or by trying to dynamically find the symbol at run time using
> > > dlopen()/dlsym() (as I did in v3 of the patches).
> > > 
> > > > But as Stephen said glibc generally does not support compiling on new +
> > > > running on old - so if it's not this that breaks, it will be something
> > > > else.
> > > 
> > > Well that's not necessarily true.  Most glibc interfaces have been
> > > around forever and you can easily see what versions of glibc are
> > > needed by running ldd on your application.  I don't see the point in
> > > introducing a new dependency on a very recent version of glibc which
> > > is not supported by all supported DPDK platforms when it can easily be
> > > worked around.
> > > 
> > > The issue here is that the original patch to add getentropy():
> > > 1) Added a _new_ dependency on glibc 2.25.
> > > 2) Added a _new_ dependency that the rdseed CPU flag on the execution
> > > machine has to match the complication machine.
> > > 3) Has different behavior if the DPDK is compiled with meson or with
> > > Make on the same complication platform.
> > > 
> > > thanks,
> > > dan
> > 
> > Adding a new dependecy happens only when building with the new version
> > of the library. If it's not available, then there's no new dependency.
> 
> But you also do not get to use the new getentropy() if you happen to
> compile on a system which does not have the latest glibc, or if you
> use the makefile system..

And that's perfectly fine - backward compatibility workarounds are not
a problem to me.

> > It sounds to me like you are trying to add workarounds for issues in
> > your downstream build/deployment model, where your build dependencies
> > are newer than your runtime dependencies. This in general is rarely
> > well supported.
> 
> I am fully aware of that.  I am not adding "workarounds", I am
> eliminating unnecessary dependencies which probably never should have
> been introduced in the first place.

It's not unnecessary. It's a better interface, and worth using if
available.

> > Now I'm fine with adding workarounds as _fallbacks_ - what I am
> > explicitly NACKing is forcibly restricting to the least common
> > denominator because of issues in a third party build/deployment system
> > that doesn't follow the common norm.
> 
> ugh.. this is the exact _opposite_ of what this patch does.  It is not
> restricting anything to a least common denominator.  It is allowing
> the DPDK to use the "best" available function, regardless of the build
> system.
> 
> Restricting to the least common denominator is what the original patch did...

This is restricting to the least common denominator of /dev/urandom,
which is a bad interface, frail with issues that everybody is moving
away from, in favour of the programmatic API that this patch is
removing, in order to fix a corner case with a non-standard, third-
party build system that downgrades dependencies at runtime vs build
time.

> > This is especially true when dealing with RNG APIs, where the tiniest
> > and most innocent-looking mistake could have disastrous consequences.
> 
> This does not change the functionality of the RNG at all.  It just
> makes it work in the way that it was intended.  These changes were
> only introduced into 19.08, so they are not historical artifacts or
> anything.

It's reimplementing a syscall using a different interface which has
different semantics. A small mistake there is going to cost us dearly.

-- 
Kind regards,
Luca Boccassi