Re: [dpdk-dev] [PATCH v4 0/2] eal: choose initial PRNG seed source at runtime

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

On 2020-06-29 20:01, Dan Gora wrote:
> On Mon, Jun 29, 2020 at 6:32 AM Mattias Rönnblom
> <mattias.ronnblom@ericsson.com> wrote:
>> On 2020-04-23 01:42, Dan Gora wrote:
>>> Hi All,
>>>
>>> The following patches updates the rte_random subsystem to dynamically find
>>> the best source of the initial seed to the PRNG at run time.
>>>
>>> The first patch enables dynamic checking for the rdseed instruction and
>>> removes the requirement for it on the execution system.  It also ensures
>>> that the code to use the rdseed instruction is generated, even if the host
>>> compilation system does not support it (on x86 systems).
>>>
>>> The second patch emulates the getentropy() glibc function by reading bytes
>>> from /dev/urandom.  This removes an unnecessary dependency on glibc 2.25.
>>>
>>> v4:  Note that emulating getentropy by reading from /dev/urandom should
>>> never fail, so now the question is if we should just remove the rdseed
>>> method entirely since it's x86 only?  Should we remove the fallback to
>>> rte_get_tsc_cycles()?
>>>
>>> Thanks
>>> Dan
>>>
>>> -----
>>> v2:
>>> * Fix patch apply issue.
>>> * dlclose() handle if dlsym() fails in __rte_getentropy().
>>>
>>> v3:
>>> * Fix error checking of dlsym() in __rte_getentropy().
>>> * Style changes recommended by Mattias.
>>>
>>> v4:
>>> * Replace dlopen/dlsym method with reading from /dev/urandom.
>>> * Try rdseed method before getentropy() method since the
>>>     latter should never fail.
>>
>> I see no reason to prefer rdseed over the kernel.
> Let me give you a good reason.  The reason that this change was done...
>
> Since rte_getentropy() is now available on all systems (with these
> patches) and cannot fail. rdseed can either 1) go before
> rte_getentropy() or 2) be completely removed.
>

It's unlikely to fail, and if it does something is probably seriously 
wrong with your system. You also seem to think it might fail, since you 
take great care of setting errno and having non-zero return code in 
__rte_getentropy().


 From what I recall, it was Bruce that suggested rdseed should be 
included as one of the sources. I have no opinion on that particular 
subject, other than I think kernel-originated randomness should have 
priority.