Tag: linux

Interrupting a select without a timeout

select is a POSIX syscall which allows you to wait on several different filedescriptors (including sockets) for the event that they won’t block on write; won’t block or read or are in error. This syscall is very convenient when you’re writing a server.

When I want to shutdown an instance of the server, I have to interrupt the select. I have yet to find a satisfying way of doing this. At the moment I create a pair of linked sockets with socketpair. I include one of them to the sockets on which to block until there is data to read in the select call. To interrupt, I simply write some data to the other socket which will cause data to be available on the socket which in turn will interrupt the select.

There must be a more elegant solution.

C&C 3 on Linux

I’ve got Command and Conquer 3: Tiberium Wars running on my Gentoo Linux installation with wine 0.9.34 by following the instructions here. I had to first install it on windows though, and copy the folder for the installer didn’t work, even with Crossover Office.

Except for (very glitchly) video and sometimes a crash everything seems to run. (Didn’t try multiplayer yet though). I had to put all quality settings to lowest, which makes me wonder whether that is my radeon X1400 being not so good as I expected or wine just being slow in emulating Direct3D.

The Filesystem Failed. Part I: introduction

The Filesystem (I’ll consider the Linux VFS as an example) has failed:

  • Database storage is implemented on top of the Filesystem, because the Filesystem is incapable of serving the needs of relational storage.
  • Metadata is stored inside files in many different formats which can only be guessed by clumsy ‘magic’ in the headers. This forces many media player and desktop search application to duplicate tag information in their own databases. Each of them has only limited support for each of the many different formats.
  • More and more device and service abstractions are moving from the Filesystem to seperate namespaces, because the Filesystem’s API is inadequate. Take for instance oss which used /dev/dsp, whereas alsa uses its own. Many new abstractions don’t even go near the filesystem anymore, for instance kevents, futexes, networking, dbus and hal.
  • Small files are stored in (compressed) packs and archives because the Filesystem can’t handle them. This happens with for instance your mailbox.

The problem comes down to fragmentation of data and metadata in too many namespaces because the Filesystem doesn’t seem to be an adequate one.

In a series of posts I’ll look at the possibilities to create one unified filesystem.

ati-drivers-8.33.6 for Gentoo

This is a slightly adjusted 25.3 ebuild that will give you the 8.33.6 ati-drivers for Gentoo. Yes, it’s dirty. They aren’t in the main tree yet because they are considered broken, although it works just fine for me.

Download: ati-drivers-overlay-8.33.6.tar.bz2

Extract them to an overlay.

Update, the 8.33.6 drivers are in the mainline tree now, so you should use those instead of mine.

(auto)mounting removable media as user

I’ve always been bothered by the fact that you need to be root to mount anything (like an usb stick). It can be solved a bit by setting up udev rules and putting a specific device in /etc/fstab, but that only works for that single usb stick. Pretty annoying.

Googling only gives you stupid and silly solution (like allowing users to mount /dev/sd[a-z] — security risk).

Luckily I’ve recently been pointed to ivman, which is an automounter. It automatically mounts removable media for you in /media.

I looked at the internals of ivman, and noticed that it uses pmount, which is a wrapper around mount which allows users to mount removable media on a /media folder. Great!

Btw, you need to be in the plugdev group to use pmount.

Update It seems that gnome-mount also works fine when you’re in the plugdev group. Gnome-mount does about the same as pmount with the advantage that gnome-mount has got the nice gui integration everywhere in gnome.

RTFM, where?

Recently a buddy on msn asked me a linux question, he just started linux so he had some problems getting stuff done.

He downloaded an installer, he said, a .run, but he doesn’t know how to execute it. He tried googling for it and asking on forums, but didn’t get an answer, so he asked me.

I solved his problem, but I still wondered, where you can find that you need to put ‘./’ in front of a file in bash to execute it and where can you find that you probably need to chmod +x the file too if you downloaded it from somewhere, if you are a total newcomer to linux.

The bash tutorial would’ve probably solved it, but do you know that that thing in which you are typing actually is a separate program? Probably not.

I basically learned all this trivial stuff while following the gentoo installation manual, but I guess that’s a bit too much to ask from each new linux user. There should be a good linux introduction that explains this trivial stuff somewhere to which I can redirect new users. Anyone knows one?

bcmap 0.1

bcmap is a simply program to be linked against klibc to map Broadcom RAIDCORE disks to more sensible and stable device nodes.

Currently the bcraid drivers assign the next available sd node to a detected disk, which is very inconvenient when you have a usb stick inserted on boot which pushes your root disk from sda to sdb.

bcmap ensures that your first array will always be /dev/bca, which is a nice thing.

>>> bcmap-0.1.tar.bz2 (source code)

Licensed under the (holy) GNU GPL v.2

bcraid drivers part two

Here’s a small patch that tweak the broadcom raid drivers for the bc4000 series to run on the latest kernels:

http://w-nz.com/~darkshines/projects/bcraid-2.1.0-bw1.patch

Note that it is everything but complete. Some other pointers getting the bcraid to work:

  • Use sparse memory or conitguous memory instead of incontiguous memory in the memory layout configuration in the kernel. (CONFIG_SPARSEMEM=y)
  • Enable the magic sysrq key (CONFIG_MAGIC_SYSRQ=y)

At the moment I’m trying to get the bcraid module working in an initrd to allow the root partition on the raid drivers.

The rm -r / typo

Today I accidently made a (yes, very stupid) typo in a root console:

rm -r /

I noticed the typo almost directly, but rm managed to wipe out my /bin and started removing parts of /boot. This situation wasn’t very helpful for the stability of my system, as you might understand.

For the windows user: it’s a bit like deleting half of all executables in the windows folder.

One key difference: when running linux, you can fix it easily. I booted a livecd, mounted my system, copied the /bin from a stage3 tarball to my root partition and rebooted.

And it’s working again! There were some complaints about a libproc version mismatch with the binaries, but that’ll be easily solved by a emerge -e system.

You just got to love linux. (and other nixes for that matter)

Linux Mount Security

With the linux Set UID Attribute you can let the owner of the file be the one the execute it when another user executes the file. This feature has traditionaly be used for system tools in linux which require root access to run but also must be able to be run my users.

It came to mind that a floppy with the ext2 filesystem could contain files of the root user with this Set UID Attribute set. Which theoraticly would allow anyone who is allowed to mount floppy’s or other media with an filesystem that supports this attribute to gain root access for a program.

On my system I got this entry in my /mnt/fstab, which allows an user to mount the floppy:

/dev/floppy/0 /mnt/floppy auto noauto,user 0 0

I made a simple C program which would show the contents of /etc/shadow, which contains the password hashes of the users, and chmodded it accordingly. (chmod = showshadow; chmod go+x showshadow; chmod u+rs showshadow)

I ran my program, and it seemed to work! The contents of the /etc/shadow file was streaming on my console.

Euforicly I went to another linux computer and tried the same trick.

darkshines@darkshines-one /mnt/floppy $ ./showshadow
bash: ./showshadow: Permission denied

Dissapointed but releived it seemed that linux had already some precaution against a root Set UID-ed executable.

I copied the contents of the folder whilest preserving permissions to another folder outside the /mnt/floppy and it all seemed to work again, although I couldn’t do it with a normal user account for I can’t preserve the owner when copying a file as a normal user.

I wondered how linux would secure it and tried to run the program while it was unchmodded.

darkshines@darkshines-one /mnt/floppy $ ./showshadow.unchmodded
bash: ./showshadow.unchmodded: Permission denied

The warning is from bash which can’t seem to execute the program. (note that it isn’t the program that can’t acces shadow) . After recompiling it on the floppy itself it seems that linux prevents any program to be executed in an user mounted folder.

I recon that that security precaution is a bit too strict. Although copying the file from the medium to a normal folder and then executing is still possible, I find it a bit strange that nothing of the user itself can be executed.

This could result in trouble when dealing with netmounts where one user can has a share on a server where he puts his files and can access only that mount for space on a terminal, when dealing with an user mount which would be required for security.

BSD

BSD is Unix.. said to be the professional cousin of Linux.

This piece of propoganda for BSD against linux gives as reason that linux is bad for it is told to be just hacked together. This because of the people who develop linux are just people from the community who put a little bit of time in it to get a feature (they probably want) added into Linux and don’t really concern about making it perfect, was said.

They took some of the sarcastic ‘todo’ comments in the kernel as example, blaming that if that stuff is in the kernel linux can’t be trusted at all.

But why does BSD hasn’t got the widespread hardware support Linux has? They blame the big company’s like IBM for instance. I just wonder whether their 60 dedicated BSD programmers could code all the hardware drivers that the thousands of contributers of Linux have coded, even if it were as bad as it is now according to them.

I bet that *BSD has got more than enough of those comments in their code too. If they haven’t they are just hiding the truth and hiding points of improvement, for these TODO’s and FIXME’s are fixed in the end. And even if they get rid of all the TODO’s and FIXME’s before they release any of it they waste a lot of time it could have been used already (less efficiently though.. but it usualy doesn’t make such a big difference).

Gentoo Linux

3 days ago I came accross gentoo linux.

Gentoo Linux is a linux redist that provides you with a livecd from which you are able to build a whole new linux installation practicly from scratch.

Yes, that means compiling everything yourself.

Normally compiling means reading through tons of documentation, trying to find dependencies, trying to get stuff working, making tweaks, and reading again through tons of documentation, trying to find dependencies, hoping version won’t collide, and again, and again.

Luckily gentoo provides a very neat tool called portage.

Portage allows you to simply install for instance gnome by just typing:

emerge gnome

Portage will look up which dependencies are required, download them, configure them, compile them, and clean up garbage. The only thing you got to do is hang back and relax.

Portage also automaticly implements your preferenced optimalization settings to create the best possible build for every single application you compile.

Gentoo seems to have resulted in a lot faster linux installation, although there is one little issue, it takes time.. A lot of time…

Compiling gnome (with all dependencies) took about 8 hours…

Maybe gentoo will be the first linux to kick windows from my desktop :-).

The Linux Paradox

Linux wants to get popular. Everyone using linux wants linux to grow bigger and bigger to ultimately have linux be the operating system dominating. err.. correction.. independant distributions and inheritances of linux to avoid the nasty monopoly stuff. Even your mom should start using it.

Sounds good. Even if linux is in some people`s opinion not the best OS it still has the best community backing it and is the most secure OS. It also has got the most support from the community. If something is required or has to be fixed it`ll happen in notime.

So why doesn’t everyone use it already?

The problem is that linux demands too much from the user.

A linux user should be able to at least have a grip of how a console works. And preferably has to know how to compile application and fix bugs when it won’t compile. And a user should know how to get dependencies it misses. An user should read through the fast different documentation of each application to even begin to hope to get it running easily. And an user should start over every time a new version is released.

Sure, they got RPM’s and yumm which makes life a lot easier. But the yum repositries don’t contain everything. If you want some new stuff, or an application with a certain module installed you have to go dig into configuration files, master the console, read through documentation, compile, and offcourse check your version dependencies, etc.

The average windows user like your mom doesn’t even know how drives work and certainly have never heard about the command line. It just wouldn’t work out. They got a lot of trouble even with just one or two options. When confronted with linux with hundreds of possible options and even stranger errors it just won’t work out well between them.

It certainly is possible to stick to the main applications provided by your redistribution and using the yum repositry. But linux will never run as satisfying as when you get it all compiled yourself and configured yourself. This really makes a big difference! I made linux start up a lot faster by compiling the kernel with only the modules I require with computer specific optimalizations. These little things give linux a really big edge over windows.

Linux is and remains only interested for adventerous ‘power’ users with quite some free time on their hands.