scary man pages

while doing a little reading and research,  i ended up reading a bit of the hdparm man page. never in my life have i read such a scary man page. i guess it is appropriate for halloween, but it came 2 days late in my case. as a result of my being so impressed with the sheer amount of warnings in the manual, i have decided to compile them here.

–dco-restore
Reset all drive settings, features, and accessible capacities back to factory defaults and full capabilities. This command will fail if DCO is frozen/locked, or if a -Np maximum size restriction has also been set. This is EXTREMELY DANGEROUS and will very likely cause massive loss of data. DO NOT USE THIS COMMAND.

–drq-hsm-error
VERY DANGEROUS, DON’T EVEN THINK ABOUT USING IT. This flag causes hdparm to issue an IDENTIFY command to the kernel, but incorrectly marked as a “non-data” command. This results in the drive being left with its DataReQust(DRQ) line “stuck” high. This confuses the kernel drivers, and may crash the system immediately with massive data loss. The option exists to help in testing and fortifying the kernel against similar real-world drive malfunctions. VERY DANGEROUS, DO NOT USE!!

–fwdownload
When used, this should be the only flag given. It requires a file path immediately after the flag, indicating where the new drive firmware should be read from. The contents of this file will be sent to the drive using the (S)ATA DOWNLOAD MICROCODE command, using either transfer protocol 7 (entire file at once), or, if the drive supports it, transfer protocol 3 (segmented download). This command is EXTREMELY DANGEROUS and HAS NEVER BEEN PROVEN TO WORK and will probably destroy both the drive and all data on it. DO NOT USE THIS COMMAND.

-m
Get/set sector count for multiple sector I/O on the drive. A setting of 0 disables this feature. Multiple sector mode (aka IDE Block Mode), is a feature of most modern IDE hard drives, permitting the transfer of multiple sectors per I/O interrupt, rather than the usual one sector per interrupt. When this feature is enabled, it typically reduces operating system overhead for disk I/O by 30-50%. On many systems, it also provides increased data throughput of anywhere from 5% to 50%. Some drives, however (most notably the WD Caviar series), seem to run slower with multiple mode enabled. Your mileage may vary. Most drives support the minimum settings of 2, 4, 8, or 16 (sectors). Larger settings may also be possible, depending on the drive. A setting of 16 or 32 seems optimal on many systems. Western Digital recommends lower settings of 4 to 8 on many of their drives, due tiny (32kB) drive buffers and non-optimized buffering algorithms. The -i flag can be used to find the maximum setting supported by an installed drive (look for MaxMultSect in the output). Some drives claim to support multiple mode, but lose data at some settings. Under rare circumstances, such failures can result in massive filesystem corruption.

-R
Register an IDE interface (DANGEROUS). See the -U option for more information.

-u
Get/set interrupt-unmask flag for the drive. A setting of 1 permits the driver to unmask other interrupts during processing of a disk interrupt, which greatly improves Linux´s responsiveness and eliminates “serial port overrun” errors. Use this feature with caution: some drive/controller combinations do not tolerate the increased I/O latencies possible when this feature is enabled, resulting in massive filesystem corruption. In particular, CMD-640B and RZ1000 (E)IDE interfaces can be unreliable (due to a hardware flaw) when this option is used with kernel versions earlier than 2.0.13. Disabling the IDE prefetch feature of these interfaces (usually a BIOS/CMOS setting) provides a safe fix for the problem for use with earlier kernels.

-U
Un-register an IDE interface (DANGEROUS). The companion for the -R option. Intended for use with hardware made specifically for hot-swapping (very rare!). Use with knowledge and extreme caution as this can easily hang or damage your system. The hdparm source distribution includes a ´contrib´ directory with some user-donated scripts for hot-swapping on the UltraBay of a ThinkPad 600E. Use at your own risk.

-w
Perform a device reset (DANGEROUS). Do NOT use this option. It exists for unlikely situations where a reboot might otherwise be required to get a confused drive back into a useable state.

-x
Tristate device for hotswap (DANGEROUS).

–security-unlock PWD
–security-set-pass PWD
–security-disable PWD
–security-erase PWD
–security-erase-enhanced PWD
–user-master USER
–security-mode MODE
[…]
THIS FEATURE IS EXPERIMENTAL AND NOT WELL TESTED. USE AT YOUR OWN RISK.

i think it is now safe to say that:

# rm -rf /

has now officially been replaced with the much more dangerous:

# hdparm -mRuUwx --dco-restore --drq-hsm-error --fwdownload --security-unlock PWD --security-set-pass PWD --security-disable PWD --security-erase PWD --security-erase-enhanced PWD --user-master USER --security-mode MODE [device]

have fun kiddies!

why linux is powerful or: how to erase half your system and then fix it

after a brief bout of stupidity i quickly realized that my makefile had gone awry and was quickly eating through my filesystem.

after ^C killing it, it seems i only took out most of /lib/* and /usr/sbin/* — who needs those anyways… apparently almost everyone.

what happened next. well it turns out i was lucky and had a few shells and a webbrowser open– attempts to launch new programs will fail, but existing programs are already loaded in memory so i was able to work.

since almost everything was broken, i first had to get dpkg and apt/apt-utils going again. after much anguish, i manually installed the missing library files and binaries from http://packages.ubuntu.com/ and i was on my way with apt.

if you’re manually installing files from .deb’s use:
dpkg -x <package_file.deb> outputfolder/

which will let you get in and use mv and cp to put back the missing .so files.
once some basic tools were working, you can try and fix up your tool chain doing things like:
sudo apt-get --reinstall install <package_name>

it’s good to do this to apt-utils, dpkg, and whatever other utilities are throwing library errors. the packages themselves need various utilities installed, and as you get a missing abc.so file, find out what package it needs with:
dpkg -S <filename>

my apt seems to be back, but many utilities still aren’t. finding out what should go in /lib was a little harder but i was able to get a list of packages like this:

(find out what *should* be installed)
dpkg --get-selections > installed-software

(list which files come from these packages, sort and uniq it)
dpkg -L `cat installed-software` | sort | uniq > uniq-software

(find what we deleted)
cat uniq-software | grep ^/lib/ > missing-software

(which packages does this come from, sort and uniq it. (might get too long argument list))
dpkg -S `cat missing-software` > package-list

(find the package name by itself, get rid of the colons, sort, uniq)
cat package-list | awk '{print $1}' | sed -e 's/://' | sed -e 's/,//' | sort | uniq > reinstall-me

(do the reinstall)
sudo apt-get --reinstall install `cat reinstall-me`

at the moment this is running, and i luckily had a cd lying around to help speed up the process. use:
apt-cdrom add

(with the cd in the drive) to add it to your /etc/apt/sources.list

turns out this eventually failed with some obscure errors… it ultimately might be a more concise list if you know what got killed, but i think the sheer number of packages i messed up and that needed to get updated somehow confused apt with some cyclical cycles, and i had to go about it slightly more manually and generally.

first of all, when every i had an annoying library error, i ran this script:
#!/bin/bash
in=`echo $1 | sed -e 's/://'`
fix=`dpkg -S $in | awk '{print $1}' | sed -e 's/://'`
sudo apt-get -y --reinstall install $fix

which would reinstall the missing library. since i ended up doing this rather repetitively, it helped a lot.

secondly, i was forced to run something like this:
for i in `cat get-selections`; do
sudo apt-get -y --reinstall install $i;
done


which reinstalls each package. slowly after pruning the list for problem packages, and deleting the successful ones, my system came back. at one point you won’t have to run them individually, and you can run:
sudo apt-get --reinstall install `cat get-selections`

and be done! i finished with a:
do-release-upgrade

and now things are looking great. luckily no important user files were squished, and hopefully this is a good reference for you. i hopefully won’t have to reference it again for all the dpkg commands i’ll forget.