Why does a grep reboot the machine?

Question

On a RHEL 6.5 Desktop, if I use:

sudo grep -r foo /*

my desktop reboots.

Q: Why?? How to debug this? It's the third time my Desktop reboots because of this :D

Because you shouldn't do this. By greping the root, you may read a file in /proc or /sys which initiates the system reboot. — Spack, Apr 24 '14 at 14:12
Absolutly no idea. You can try sudo grep -r foo /* > /root/grep.log and may find some additional info... — Tobias, Apr 24 '14 at 14:13
@Spack I can't imagine there's a proc file that reboots on read. — goldilocks, Apr 24 '14 at 14:15
@Tobias : Why not? It is pretty weird and worth investigating. Contra Spack, I think you should be able to grep -r the entire filesystem safely. — goldilocks, Apr 24 '14 at 14:17
I just wundered if there is a good reason for it. I must confess, I also like to try such things. And I also think, this shoud work without reboot. — Tobias, Apr 24 '14 at 14:23
How does it reboot? Is it a clean init 6 or does the screen just go black and the machine reboots? — Bananguin, Apr 24 '14 at 18:39
strace helped, please post it as answer, the bad device was the /dev/cdrom — evachristine, Apr 28 '14 at 15:06

score 7 · Answer 1 · answered Apr 24 '14 at 16:00

There are things you're not supposed to blindly read from.

/dev/mem and /dev/kmem come to mind, but /dev/port sounds even more dangeous to read - it maps to I/O ports directly. Reading blindly from that sounds like a recipe for disaster (hard-locked my machine a few minutes ago... for science I guess). I don't think reading from that file will result in the same symptoms for all machines, but a reboot doesn't sound unreasonable.

At the very least, you should exclude /dev entirely from that, and /sys and /proc shouldn't be included either - it makes no sense to search those filesystems even if it was entirely safe.

Consider the performance as well. Do you really want to grep all of /dev/sda, then all of /dev/sda1, then whatever filesystem contents it has (assuming it's mounted)?

Since version 2.12, GNU grep only searches in regular files (or special files passed on the command line) when invoked with -r. But RHEL6 has version 2.6.3 which indeed searches in all files including devices. — Gilles 'SO- stop being evil', Apr 24 '14 at 22:38
/proc/kcore would be another. And that one appears as a completely normal file. It even has a size on it. — phemmer, Apr 25 '14 at 00:43

score 1 · Accepted Answer · answered Jun 26 '14 at 18:59

1

strace helped, the bad device was the /dev/cdrom

answered Jun 26 '14 at 18:59

evachristine

2,613

wow!! my machine is rebooting, and I need to find the source of that problem too!!! http://unix.stackexchange.com/questions/172396/how-does-auto-reboot-happens – Aquarius Power Dec 09 '14 at 21:16

Why does a grep reboot the machine?

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