[Xymon] Detecting read-only file system in Linux

[Thomas Eckert]

> On 11 Mar 2015, at 00:58, Jeremy Laidman <jlaidman at rebel-it.com.au> wrote:
> 
> On 11 March 2015 at 00:44, Thomas Eckert <thomas.eckert at it-eckert.de <mailto:thomas.eckert at it-eckert.de>> wrote:
> Just for future reference as you already found your solution:
> 
> From looking at the util-linux package I understand that _if_ the /proc-filesystem is mounted the information is used from there.
> 
> I don't think this is the case.  Running "strace" on the "mount" command shows that it first looks at /etc/mtab, and if that exists, it doesn't look at /proc/mounts.  Only if /etc/mtab doesn't exist, does mount look at /proc/mounts.

Strange, I did also check with strace and did not see `/etc/mtab` used. This is on Debian 7.x, Debian kernel 3.2.0-4-amd64 and util-linux 2.20.1-5.3

	root at bb:~# strace -e trace=open mount 2>&1 | egrep 'mtab|proc'
	open("/proc/filesystems", O_RDONLY)     = 3
	open("/proc/self/mountinfo", O_RDONLY)  = 3
	proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)

> So in a situation where the filesystem containing /etc is having problems and goes read-only, /etc/mtab won't get updated because it's now read-only.


Agreed.
So re-using the `mount`-info for the r/o-check is at least highly dependent on the individual environment and versions.

Cheers
Thomas


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