linux-kernel - Polling(2) /proc/mounts is broken?

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Message-ID: <d8aa5094-8042-4172-8e1d-7d2c7f2299a7@us-col-exch1.green.sophos>
Date:	Fri, 10 Dec 2010 15:32:48 +0000
From:	Tvrtko Ursulin <tvrtko.ursulin@...hos.com>
To:	Eric Paris <eparis@...hat.com>, Al Viro <viro@...iv.linux.org.uk>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Polling(2) /proc/mounts is broken?

Hi all,

So since we cannot get fanotify global mode we were told to poll(2) on
/proc/mounts and set up new mount marks dynamically.

Is the attached C program roughly how it should work?

Because on 2.6.37-rc5 it busy loops for me (POLLIN always reported), even
though if I remove the seek it gets EOF immediately after POLLIN.

Under 2.6.32 (Ubuntu flavour) I see the same behaviour.

Going further back to 2.6.24 (again Ubuntu flavour) and there it works, but it
set POLLERR instead of POLLIN in revents.

So I guess it regressed somewhere in between. I'll look to provide a fix
unless someone more familiar with this area beats me to it.

Tvrtko

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View attachment "pollmounts.c" of type "text/x-csrc" (760 bytes)