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I want to set world read/write permissions on a raw block device file (yeah, I know). I can set the permission to 666, but it goes back to 660 immediately after I touch it or write to it. Why?

[root@mysystem udev]# ll /dev/sdz6
brw-rw-rw-. 1 root disk 65, 150 Aug 30 12:09 /dev/sdz6
[root@mysystem udev]# touch /dev/sdz6
[root@mysystem udev]# ll /dev/sdz6
brw-rw----. 1 root disk 65, 150 Aug 30 13:58 /dev/sdz6
[root@mysystem udev]# chmod 666 /dev/sdz6
[root@mysystem udev]# ll /dev/sdz6
brw-rw-rw-. 1 root disk 65, 150 Aug 30 13:58 /dev/sdz6
[root@mysystem udev]#

I don't see anything in /var/log/messages, /var/log/dmesg or /var/log/secure. I turned off SELinux (setenforce 0).

edit - here's the script I wrote to fix it:

[root@mysystem rules.d]# cat /etc/udev/rules.d/99-drives.rules
KERNEL=="sd*", SUBSYSTEM=="block", MODE="0666"

[root@mysystem rules.d]# udevadm info -q path -n /dev/sdz

[root@mysystem rules.d]# udevadm test /devices/pci0000:80/0000:80:03.0/0000:8b:00.0/0000:8c:01.0/0000:8e:00.0/host19/rport-19:0-2/target19:0:2/19:0:2:12/block/sdz 2>&1 | grep '99-drives'
parse_file: reading '/etc/udev/rules.d/99-drives.rules' as rules file
udev_rules_apply_to_event: MODE 0666 /etc/udev/rules.d/99-drives.rules:1
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You probably want to look at writing a udevd rules.d set that makes MODE=0666. I assume udev is overwriting this 'for you'. – polynomial Aug 30 '11 at 19:37
Yeah, I just created a simple udev rule that seems to work. Documentation was not helpful, but I was able to google around and stitch together enough examples. I would post my script, but this site is forcing me to wait a day to post my answer because I don't have enough reputation. – twblamer Aug 30 '11 at 19:49
up vote 3 down vote accepted

Setting permissions in udev manually will almost never work. You should really look at adding a rule in /etc/udev that will set "MODE=0666" on the device in question. That will make it so the system creates it that way.

(adding per comment discussion)

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