linux-kernel - TFD_CANCEL_ON_SET race when making a wall clock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 02 Sep 2011 10:37:12 -0400
From:	Colin Walters <walters@...bum.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: TFD_CANCEL_ON_SET race when making a wall clock

Hi,

So I was recently making GNOME use the new timerfd TFD_CANCEL_ON_SET so
we get woken up when the system clock changes.  It works generally well,
except Ryan Lortie pointed out a race condition in my use of
timerfd_settime() that I think anyone using it to make a wall clock
display might not realize at first:

https://bugzilla.gnome.org/show_bug.cgi?id=655129#c36

For the link-averse, basically the system clock can move backwards
between when the process gets the current time, and computes the wakeup
(typically for the next minute).

I was able to work around it in userspace with this patch:
http://bugzilla-attachments.gnome.org/attachment.cgi?id=195252
But it's clearly not what I'd call beautiful.

I don't see a nice way to handle this in the kernel given the current
API, but maybe someone else does?

TFD_CANCEL_ON_SET isn't documented in man-pages at all right now...maybe
this is just a useful note for a future patch to
man-pages/man2/timerfd_create.2.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/