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 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.

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