Lair Of The Multimedia Guru

May 23, 2006


You upgrade your linux distro and suddenly things are failing with error messages like "error while loading shared libraries: cannot enable executable stack as shared object requires: Permission denied" what happened?

Well, you just have become yet another witness of the decay of the GNU toolchain. Besides that gcc and glibc becomes more and more bloated every day, gcc bugreports being randomly closed without fixing them and so on, we since some time have the beautifull PT_GNU_STACK feature in every GNU tool which tries very hard to make the stack of application executable, without grsec your system will be more vulnerable, with grsec stuff will fail hard as grsec will not give apps an executable stack if they dont need one and the redhead ehm i mean gnu libc/loader/… will then simply refuse to continue if the unneccesary executablity isnt provided

what exactly is PT_GNU_STACK?

PT_GNU_STACK is a entry in the elf file format which contains the access rights (read, write, execute) of the stack, so far so good, nothing bad here, the problem is how these access rights are choosen by the tools …

The idea behind PT_GNU_STACK / choosing the access rights

The stack is always read and writeable obviously thats needed, so we are left with the question about it being executable, when linking objects together into an library or libraries into an application, a single object requireing an executable stack will cause the final application and all libraries which contain that object to need an executable stack, if no object needs an executable stack the final application wont need one either

The design and implementation of PT_GNU_STACK

Every tool in the gnu toolchain will from source over assembly, objects and libraries to final application merge the PT_GNU_STACK access rights so that a single object needing a executable stack will cause the final application to need one, in case of doubt (no PT_GNU_STACK entry) the default is that the specific object needs an executable stack

The problem with this / the total failure of the PT_GNU_STACK idea

Undecideablility at the compilation and assembly level

Deciding if a piece of code needs an executable stack is proofably not possible, its indeed equivalent to solving the halting problem
so gcc, gas and so on guess if a pice of code needs an executeable stack and if they are unsure, they claim that it does

Undecideablility at the global level

For example calling a function through a global pointer, the compiler compiling that file cant decide about the stack access rights as it doesnt know all code which changes the pointer, the linker later would have to dissassemble the object files again just to have the info needed to decide it …

several definitions of executability

If you execute code on the stack, does it need to be executable? if you use redheads execshield then AFAIK yes, if you use grsec it depends upon the code, as grsec can detect and emulate some common and harmless sequences of instrucions on the stack (trampolines)

Even if an application needs an executable stack for some feature

Theres no need to have an executable stack if that feature isnt used, actually applications might explicily try to enable / disable executability of a specific part of memory depening upon needs, gcc will in that case almost certainly claim that the app always needs an executable stack …

All these things cause applications to claim that they require an executable stack while they simply do not, which makes your system much more vulnerable to attacks

The alternative

Mark final applications which need an executable stack as such, make the default to not requireing an executable stack

whats the difference

Well, in practice more then 99% of all applications dont need an executable stack and if a application needs an executable stack but doesnt have one then that causes a clear failure which will be noticed immedeatly and can be fixed by marking that application as needing a executable stack
OTOH the current PT_GNU_STACK system with its pass the flags from source to final app ideally needs corretures to be done for each source file which the gnu tools cant deal with automatically, and there are many more source files then applications not to mention that the error rate at the application level seems to be far in excess of 1%
furthermore marking an app as requireing an executable stack while it actually doesnt, wont produce any warning, error or otherwise any hint that something is wrong while security is greatly impaired

Filed under: Off Topic — Michael @ 14:29

May 19, 2006

Coding Style / Code Conventions

There are many coding style guides, IMO actually significantly too many … their existence often only justified by the single hypothesis that having everything in the same style is more readable then having one style per file and letting the original author / maintainer decide it, now maybe my memory sucks but i dont remember a single case where the difference in coding style in different parts of a project caused me any difficulty in understanding. It was rather the lack of any indention, exclusive use of single letter variables or simply the complexity of the code, lack of comments, and my stupidity of course which caused difficulty understanding something …

hmm, mphq cvs is still dead :( so lets try to write the ultimate coding style guide and hope mphq works when iam done so i can finally commit a few minor bugfixes

The ultimate coding (style) guide

Rule 1

Every rule must be justifyable and the justification must not apply to a contradicting rule too
yes, iam trying to avoid the “for consistancy” justification which can be applied to litterally anything

Rule 2a

Write consistant code / Changes should be approximately in the same style as the surrounding code
Justifiction: having the coding and indention style change every few lines makes code somewhat harder to read, while a overly strict style restriction will cause some volunteers (or employees) to leave

Rule 2b

Write indented code (code within a block should be at the same indention level, code in parent and child blocks should be at different levels), placement and indention of stuff like {} is not restricted here, just the actual code
Justification: see mplayer.c for what happens without that rule :)

Rule 2c

Avoid constructs which IDEs/editors have difficulty with, common examples are non-ASCII letters, tab / space mixes, differing tab length in different files, dos line endings, …
Justification: obvious

Rule 2d

avoid non english (in identiers and comments)
Justification: not everyone speaks every language, english is understood my most people, of course if every programmer you care about understands hungarian then its a perfect choice too

Rule 3

maximize usefull information at minimum size and complexity (simple code preferred)

so a variable name like szMyCompleteName is normaly worse then simply name as the type is probably obvious, “complete” also adds no new info … of course a function which converts zero terminated strings to length + chars style would benefit from having the type in the names, it all depends upon how and where a variable is used
comments should not say whats already obvious from the code like int i=sqrt(v); //i is the squre root of v but instead provide usefull info like //i is the largest factor v can contain, so we only need to test integers less then i but then in that case maybe renaming i to largest_factor would have provided more info at less size …
justificaion: the idea is to provide the reader with the info she is interrested in, not to duplicate and spread out the important stuff between noise
also less code means fewer places where a bug could be
Note: this rule also forbids code duplication, and other bloatedness

Rule 4

Write efficient code
Note: efficient here means little memory, disk, cpu, …
Justification: well this one should be obvious

Rule 5

Write portable code
Justification: well this one should be obvious too

Rule 6

Write localized / internationalized applications (user vissible stuff in her native language)
Note, maybe error messages should be dealt with differently as the developers need to understand them in bugreports …
Justification: well this one should be obvious too

Rule 7

Write compatible code (dont break compatibility of file formats, your last library release, …)
Justification: well this one should be obvious too

Rule 8

Write secure code (dont write over the end of an array, checking untrusted input carefully, be carefull with malloc(w*h) due to overflows of the *, …)
Justification: well this one should be obvious too

Rule 9

Write modular code / split code into independant parts
Justification: many dependancies makes code very hard to understand and maintain

Rule 10

If some rules contradict, use common sense (which goal is more important in a specific case)

Filed under: Off Topic — Michael @ 19:49

May 15, 2006

TV sets

Its been a month since my last blog entry, iam not sure if thats bad or not, anyway heres some more halfway offtopic crap, i want(ed) to buy a new TV set as the one i have here is _old_ and doesnt really work very well …

First question CRT or LCD?

CRTs, are old, many shops dont have any CRT based TV sets anymore and everyone seems to know they are old and inferrior
LCDs, shiny new technology, flatter and better, well wait, why exactly does every LCD TV below 1000€ have a _vastly_ inferrior picture quality then my mothers el-cheapo noname CRT TV which probably costed < 200€ … and iam not saying the ones above 1000€ where better, just out of my acceptable price range and many showing high resolution HDTV which made fair comparission hard …


for CRTs size seems strongly correlated with quality and features, no 55cm 100hz TV sets for example, it also seemed like larger isnt realy more expensive, for LCDs of coarse larger is much more expensive …

100hz CRT vs 50hz

50hz flickers, and actually i somehow cant get rid of the feeling that its stronger flickering then 50hz TV sets had a few years ago
100hz dosnt flicker, wait, actually i found just one 100hz TV set <300€ which didnt had at least part of the image flickering, all others had parts of the TV channels logo flickering also with 100hz you can choose between some residue interlacing artifacts or “staircase” artifacts on moving objects, ive not found any 100hz tv set which didnt show some artifacts allthough in the end i prefer 100hz over 50hz the artifcats are less annoying then the flickering IMHO

Comparing TV sets

Well you walk into a shop with TV sets, many TV sets … all showing different stuff, and if you are lucky and they show the same then its a crappy dark low quality music video from MTV completely unsuitable for comparing, but wait one shop had remote controls “attached” to each tv set so customers could change settings and channels as they liked, sadly less then half the remotes where working …

Summary, conclusion and personal oppinion (yeah as if the whole blog was anything but subjective personal oppinion)

LCDs have imporved, more in the written numbers then in reallity, CRTs seems to have become worse but still LCDs are behind CRTs both in quality and quality per price
many TV sets be it CRT or LCD show terrible artifacts
50hz CRTs: flickering
100hz CRTs: staircase or interlacing artifacts on moving objects
CRTs in general: some cases of brightness-size dependance, most have oversharpened and too strongly saturated images (yeah last 2 are purely an adjustment thing, probably done as most customers are idiots and prefer oversharpened and too strongly saturated images)
LCDs: washed out blurry images, colors more or less wrong, aritifacts caused by slow reaction times not to mention 2-4 times as expensive as CRTs

Filed under: Off Topic — Michael @ 11:56

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