How to destroy your OS with tar

by vorakl • 2 years ago

98 61 2 years ago

Interesting read. Thanks for sharing. Maybe it is by lack of experience, but I always treat tarballs like a loaded gun. I extract then inside an empty subdirectory in my home first just to be sure and then move the data as required. It is no fun having to cleanup the mess left by an incorrect extraction.

vorakl8• 2 years ago

This is a good and safe practice. And I think most people do it this way after cleaning up the mess of a badly built tarball at least once :)

nox1013• 2 years ago

I'd argue this is a design bug. Extracting into the current directory should either not be possible or be the exception (tar xvfz --current-directory) and not singling out tar here, unzip, pkunzip, etc all have this issue and have all cause people data loss and worse because if this default unsafe behavior

kennethrc• 2 years ago

I extract then inside an empty subdirectory in my home first

... AFTER a "tvf[jzx]", I hope

marcosdumay• 2 years ago

What's the gain of -t if the extraction target is disposable?

Does -x have some side effect that -t would list for you?

chatmasta• 2 years ago

Yeah, then I just hope I get the `mv` or `cp` right when I'm done, and don't end up with a directory full of files from the top part of the tar...

zzzeek• 2 years ago

I have 30 years of professional experience and with one off tarballs that I'm not deeply familiar with, this is usually what I do as well (certainly with a tarball that has a /usr like structure inside of it). You're good

PhilipRoman• 2 years ago

Most distros should have a useful Linux package "atool", containing the command "aunpack" which does the least surprising thing for all archive types (without creating duplicate root when it already exists).

BearOso• 2 years ago

I believe the unar tool creates a containing directory by default.

Gormo3• 2 years ago

Extracting archives directly into your system root as a superuser is in the same class of activity as piping curl output into your shell interpreter as a superuser: things that no one should ever do.

cbsks• 2 years ago

Any time you see a command starting with “sudo” and with a path of “/“ in the arguments, alarm bells should be going off. Here be dragons.

vorakl8• 2 years ago

Yes, but someone has to do the dirty work ;)

millzlane• 2 years ago

Appreciate the sacrifice

bigstrat2003• 2 years ago

Right, I would be more surprised if this didn't break your system. I never would've tried unpacking a tarball to my FS root in the first place.

klyrs5• 2 years ago

What, you mean the increasingly popular install pattern

   curl ... | sudo bash

might have pitfalls? Gasp.

mike256• 2 years ago

You shouldn't do that. At least download the installer first, have a short look at it and decide if you like to run it.

I personally avoid software whose manual suggests this type of install because I think of the developers as not that intelligent ones.

plasticeagle• 2 years ago

Well... Yes you're not wrong. But this isn't really any different from downloading an installer and entering your password to permit the install to proceed.

So I assume that you also never do that - and the downloaded installer is even worse, because you can't easily look inside the executable to determine what it does.

Shocking news : Installing software installs the software.

pests• 2 years ago

Yet we all npm install or run random binaries without a second thought.

rob742• 2 years ago

Yup... tar alone can't destroy your system, you need sudo (and the appropriate parameters) for that.

nurb• 2 years ago

https://gtfobins.github.io/gtfobins/tar/

PresidentWolf• 2 years ago

Been there. That was an interesting day.

silisili2• 2 years ago

If I'm reading this right, using --no-overwrite-dir should have prevented this, I think.

I'd still extract to a local folder as nonroot, though, so still the right conclusion.

vorakl8• 2 years ago

--no-overwrite-dir (Preserve metadata of existing directories)

This is a really good point! It solves the main problem. I'll add it to the article.

Although that's what, unfortunately, is set by default (at least in GNU tar):

--overwrite-dir (Overwrite metadata of existing directories when extracting (default).)

skynetv2• 2 years ago

Never untar anything to /. In this case, inexpensive, but a very valuable lesson.

m463• 2 years ago

This reminds me once of using gparted on a disk.

I got in and out of it a few times to make sure what I was going to do, then I launched it again (but I didn't add any arguments).

Not only didn't it complain, it defaulted to my root disk, and I ended up destroying my partition table.

Luckily the system was still running and I was able to backup everything before I shut the system down.

hcfman2• 2 years ago

It’s funny how tar came out first. Then cpio came out which was a lot better but tar had the momentum and it never lost it. I still find cpio more controllable and use that as a preference.

But cpio existed when I first used Unix in 1982/3

eichin2• 2 years ago

and now there's pax (since POSIX looked at both of them and shuddered :-)

hcfman2• 2 years ago

My WOW with cpio is pretty elementary. I've never found anything in all my usage to shudder about. Possibly there are smarty pants usages in there that could make you shudder I don't know. But why choose smarty pants if elementary is perfectly satisfactory.

orliesaurus• 2 years ago

ah the joys of tar...the one time I got the input and output args confused and I lost a lot of data woops

prmoustache4• 2 years ago

I never understood why so few people use `pax` instead of `tar`. It has always seemed more user friendly to begin with.

With pax, the `-k` option will not overwrite existing files. Also not using `-p o` or `-p e` will not preserve rights. I don't know why you would extract and preserve rights to extract binaries in a chroot, usually you would want to control the permissions instead.

rob742• 2 years ago

The main reason is probably that tar is installed by default on 99% of Linux systems you might come across, while pax isn't...

prmoustache4• 2 years ago

Now that is funny, a few days ago I was downvoted to oblivion for stating the obvious: that nobody cares about POSIX anymore. And now you are telling me that all those downvoters are probably not following POSIX systems[1], either on their own desktop or their servers.

[1] pax is the recommended archive utility in the POSIX shell&utilities section while tar and cpio aren't mentionned.

silisili2• 2 years ago

Because tar is super versatile, old, and as the name implies, creates an archive. Useful for creating an archive to image something, and many times you want reproducible ownership and permissions.

Tar can do all the same things, it just requires options either on the archiving or extraction end.

lm411• 2 years ago

Alternative title: How to destroy your OS by not understanding your OS.

Don't take this the wrong way. Nice write up. :)

......

Sudo ... why would you sudo a command that you don't understand?

RTFM ... all of the behaviour you experienced is well documented. There are no surprises.

POSIX and UNIX utils are flexible and powerful. This is why we love them. Don't blame the hammer.

Sandbox anything you extract and review it. Extract in a test environment. Extract without sudo and copy the files and permissions you need.

tar -tv is simply tests the archive and outputs the paths. You ignored what it told you - that it would overwrite ./ :)

#### Never ever do this! $ sudo tar -C / -xvfp xbps-static-latest.x86_64-musl.tar.xz

Insanity.

Cheers :)

anthk• 2 years ago

Bubblewrap will allow you to create a chroot for xbps. No root needed. If $HOME/void_chroot is a void rootfs:

https://docs.voidlinux.org/installation/guides/chroot.html#t...

       bwrap --bind $HOME/void_chroot/  /  --ro-bind /etc/resolv.conf /etc/resolv.conf  --rw-bind /home/ /home/  --proc /proc --dev /dev /bin/sh

Instead of xchroot, use bwrap.

doublerabbit2• 2 years ago

I'm kind of annoyed that tar doesn't back up directories starting with a period unless you throw in a command flag.

I wish I knew this before taking a backup and reformatting.

Always test your backups I guess..

vorakl8• 2 years ago

This is an interesting one. What was the version of tar? Was it gnu tar? I wonder, how did you create that archive?

I'm asking this because I was trying to reproduce the same situation and everything seems to be working fine:

  $ tar -C /tmp/root3 -cvf test.tar .
  ./
  ./.config/
  ./.config/test
  ./.local/
  ./var/
  ./var/db/
  ./var/db/xbps/

or even like this

  $ tar -cvf test2.tar root3/
  root3/
  root3/.config/
  root3/.config/test
  root3/.local/
  root3/var/
  root3/var/db/
  root3/var/db/xbps/

No special options were needed. But, if I do it this way, then, there are definitely missing all dot directories:

  $ tar -cvf test2.tar root3/*
  root3/usr/
  root3/usr/bin/
  root3/usr/bin/xbps-uunshare

But this problem is not a tar's problem. That's only because "*" mask doesn't match dot files:

  $ echo root3/*
  root3/usr root3/var

For that purpose, you need to clearly add a dot:

  $ echo root3/.*
  root3/.config root3/.local

Thus, the solution might be

  $ tar -cvf test2.tar root3/.* root3/*
  root3/.config/
  root3/.config/test
  root3/.local/
  root3/usr/
  root3/usr/bin/

But, I'd rather stick to "-C dir/" option instead of relying on "*" mask in this case.

doublerabbit2• 2 years ago

   tar -cvf mytar.tar /home/myuser/*

Was definitely the syntax I ran. I'm on FreeBSD so just tar.

How to destroy your OS with tar

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