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Date: Tue, 26 Mar 2019 18:45:41 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Cc: Michael Jeanson <mjeanson@...icios.com>,
Richard Purdie <richard.purdie@...uxfoundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Jonathan Rajotte-Julien <jonathan.rajotte-julien@...icios.com>
Subject: Re: sysconf(_SC_NPROCESSORS_CONF) returns the wrong value
* Jonathan Rajotte-Julien <jonathan.rajotte-julien@...icios.com> [2019-03-26 12:23:34 -0400]:
> > i think we need to know why does a process care if musl returns
> > the wrong number? or what are the valid uses of such a number?
> > (there are heterogeous systems like arm big-little, numa systems
> > with many sockets, containers, virtualization,.. how deep may a
> > user process need to go down in this rabbit hole?)
>
> Does the answers from Mathieu Desnoyers [1] and Florian Weimer [2] fit the bill?
yes
>
> [1] https://www.openwall.com/lists/musl/2019/03/16/3
> [2] https://www.openwall.com/lists/musl/2019/03/19/1
> >
> > note that most of /sys/devices/system/cpu/* is documented under
> > Documentation/ABI/testing in linux, not in Documentation/ABI/stable
> > and the format is not detailed, and some apis (e.g. /proc/cpuinfo)
> > are known to be different on android (and grsec?) kernels it may
> > be unmounted during early boot or in chroots, so sysfs parsing is
> > only done when really necessary.
>
> For what it's worth, uclibc and uclibc-ng seem to iterate over
> /sys/devices/system/cpu/* and fallback on online calculation if necessary.
>
> https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/tree/libc/unistd/sysconf.c#n102
>
> In the mean time, we implemented a fallback similar to this when we do not "know"
> the libc used (since musl does not come with __musl__, I read the reasons why,
> no need to discuss this).
>
> Not sure of the direction musl should take but I strongly believe that the
> behaviour regarding _SC_NPROCESSORS_CONF is not the appropriate one.
i agree that the current behaviour is not ideal, but
iterating over /sys/devices/system/cpu/cpu* may not
be correct either.. based on current linux api docs.
i don't understand why is that number different from the
cpu set in /sys/devices/system/cpu/possible
it seems any upper bound on the number of cpus would be
valid but it's not clear how to provide that guarantee.
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