Re: Support getrandom() for pg_strong_random() source

On Tue, Jul 22, 2025 at 4:12 AM Dagfinn Ilmari Mannsåker
<ilmari@ilmari.org> wrote:
>
> Masahiko Sawada <sawada.mshk@gmail.com> writes:
>
> > On Tue, Jul 22, 2025 at 12:13 AM Michael Paquier <michael@paquier.xyz> wrote:
> >>
> >> On Mon, Jul 21, 2025 at 11:43:35PM -0700, Masahiko Sawada wrote:
> >> > The patch supports the getrandom() function as a new source of
> >> > pg_strong_random(). The getrandom() function uses the same source as
> >> > the /dev/urandom device but it seems much faster than opening,
> >> > reading, and closing /dev/urandom. Here is the execution time of
> >> > generating 1 million UUIDv4 data measured on my environment:
> >> >
> >> > HEAD(/dev/urandom): 1863.064 ms
> >> > Patched(getrandom()): 516.627 ms
> >>
> >> Interesting.  Are there platforms where this is not available?  I'd be
> >> pretty sure that some animals in the buildfarm would not like this
> >> suggestion but I'm saying it anyway.  Perhaps we could even drop
> >> /dev/urandom?
> >
> > As far as I know macOS doesn't support getrandom() but supports
> > getentropy() instead. And an older glibc version might not support it.
> > It's supported since Linux 3.17 and glibc 2.25.
>
> getrandom() is Linux-specific, while getentropy() is specified by POSIX
> (since 2024).  It was originally introduced by OpenBSD 5.6 in 2014, and
> was added to macOS 10.12 in 2016, glibc 2.25 (same as getrandom()) in
> 2017, musl 1.1.20 and FreeBSD 12.0 in 2018, and NetBSD 10.0 in 2024
>
> Sources:
>
> https://pubs.opengroup.org/onlinepubs/9799919799/functions/getentropy.html
> https://dotat.at/@/2024-10-01-getentropy.html
>
> So I think it's more worthwhile to add support for getentropy() than
> getrandom().

While getentropy() has better portability, according to the
getentropy() manual, the maximum length is limited to 256 bytes. It
works in some cases such as generating UUID data but seems not
appropriate for our general pg_strong_random() use cases.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com