Soooooo, I felt like I needed to score a new pair of pants, so I went to the closeout-store and got some cheap-o black corduroys. I need to get rid of my other pants that I do not wear too often, to keep things simple. I'm not a fan of acquiring new possessions. Anyways, so after I scored some pants, I decided to go to the little market store deal. As I made my way to the store, I heard some people cheering. Then.... yes.... then.... I saw two dudes pants-less riding their bicycles. Well, ya know, this is Melbourne, so I didn't think too much of it, other than, right on dude! All this craziness while I'm carrying my new pants, because I do not see the need for the store to waste a plastic bag on me. Then... more... a whole parade, women, men, some with pants, some without, frikkin' naked just riding their bikes down the road. I kinda cheered and waved my pants at them. Welcome to Melbourne. Turns out, today is Melbourne's World Naked Bike Ride. Way-cool. I guess it takes some balls to ride a bike in public.
-Matt
Sunday, March 6, 2011
Saturday, March 5, 2011
I went to church yesterday, SLAYERRRRRRRrrrrrrrrrrrrRRRR (Soundwave was my church)
Being a music fan, and a lover of attending concerts, I often check the music schedules of places when I travel. When I got to Melbourne I soon learned of the most-awesomest musical orgy in this area, the Soundwave music festival. Well, that was my holy-land, and I proclaimed my pilgrimage would start on March the 4th. Well, yesterday, I was successful. For I have witnessed/experienced once more the love of Primus, Slayer, Iron Maiden, and some Rob Zombie. I didn't see all the shows I wanted to see, heck, there were 6 stages, but I had a blast. Note: Soundwave was one way of forcing myself to stay here in Melbs, even if I didn't like it (I scored my ticket pre-sale). So, my bud Mark picked me up at my house, we then picked up our other friend Ian, and made the pilgrimage. While things were already kickin' (the shows started at noon, we arrived at 2PM), we basically saw: DevilDriver, All that Remains, Primus, High on Fire, Murderdolls, Ill Nino, Slayer, Rob Zombie (very little of it), Queens of the Stone Age, and Kylesa.... ohhh yeah almost forgot... Iron Maiden!!!!!!!!!!! Some of the shows I have listed we did not see in entirety. But it was a blast! When Slayer hit the stage, awe man, it was crushing! The likes of data compression would have been impressed on how much space was compressed in so little time. People cover big area, slayer, people now in tightly packed area. Like one giant pulsating organism. Slayer is magic.
Much love
-Matt
Much love
-Matt
Saturday, February 26, 2011
Goin' back to...
Just got back from the Mother Land. That's right peoples, I just traveled to the USA. I'm back here in Melbourne now. So, the trip was fast (left Oz last Monday the 21st around noon). The cool piece of this deal was that I left at noon that Monday, arrived in Cali-for-ni-a (the home of the Governator) the same day at 2PM. Like a super-crazy worm-hole. In-fact, many abductees claim the extraterrestrial abduction was responsible for their time shift. But, I am pretty sure, science can explain the loss in hours, I think it was probably do to flying against the international dateline. None-the-less, just a mere 2 hours after I left, I arrived in LA, but on a ~14 hour plane flight. Anyways, flying home that following Thursday, and just got back in today... Saturday... freaky. Anyways, California is mega-sweetness. I crossed over the Golden Gate Bridge.... but no jumpers.
-Matt
-Matt
Sunday, February 13, 2011
Acca Dacca Lane and More
SoooooooOOoOoOoOoOOooOoOo.... posted pictures, I have. Enjoy.
http://www.flickr.com/matt-davis
http://www.flickr.com/matt-davis
Saturday, February 12, 2011
Linux syscall, vsyscall, and vDSO... Oh My!
The following article is a brief investigation I wrote up, which looks into the system call functionality of the Linux kernel. Mainly, I was trying to clarify the difference between vsyscall and vdso. This was conducted, more or less, on 2.6.32 and 2.6.37 branches, if I recall properly. Anyways here goes:
Often these calls are not even accessed directly by an application, rather they are executed via a wrapper. The GNU C library, glibc, has wrapper functions that do all the handy-dandy wrapping. For instance, a call to gettimeofday() in an application is really just a call to glibc's wrapper for the system call for gettimeofday(). There is no overhead because the wrapper, at link-time, just associates gettimeofday() to the system's appropriate version of the routine. If one were to desire to make a true system call, avoiding the wrapper, Linux provides a syscall() routine allowing such. Both wrapper and the explicit syscall versions of gettimeofday() are demonstrated below:
System calls are loaded at kernel boot-time. All of these calls are accessed by a number. In the example above SYS_gettimeofday is just a constant integer. And this integer is the index into an array (called an interrupt descriptor table) of system calls that are constructed at boot-time. This table/vector can be thought of as pointers to the actual routines [3]. So at index SYS_gettimeofday is a pointer to a piece of code which is where the actual sys_gettimeofday() routine exists.
Ok, so interrupts are relatively cheap in execution time. However, the way the kernel is designed does add some overhead when a syscall is executed. Linux is divided into two primary memory segments. User-space and kernel-space (userland, kernelland). User-space memory is that which user applications run in. Kernel-space is where all the kernel services run. This segmentation acts as a security barrier, so that crafty and/or malicious user apps cannot directly access the kernel's memory. Making a syscall, as mentioned previously, is just a bridge, a mere wormhole, between these two segments of memory. It is this hop between address spaces that causes quite some overhead, as the kernel must switch between user and kernel-land memory segments, and then back again as the syscall completes [5, 6]. Changing the memory addressing requires some register shuffling, and just imparts more overhead on the whole syscall process.
userland, that should not cause a security hole for the kernel. The page of memory where these calls lies is mapped into each running process' user-space. Thus, when a call to one of these syscalls is made, no context switch between the memory regions of user and kernel-space is conducted, thus less overhead.
Another interesting means of reducing syscall overhead comes specific to the underlying CPU architecture. Both the more recent AMD and Intel chips have implemented a fast syscall functionality. Instead of issuing an interrupt, programs can issue instructions (SYSCALL/SYSENTER and SYSRET/SYSEXIT) that act faster than a traditional interrupt. The usage of these are based on object code in the OS. Therefore, a programmer does not have to consider how to implement/request the use of a SYSCALL/SYSENTER over a traditional "int 0x80" [1]. When applications are built, based on the architecture and the system, vsyscall and vdso linkage is done automagically.
As demonstrated above, by using the syscall() routine, a traditional syscall will be conducted, even if there is vDSO support (virtual Dynamically Shared Object). However, despite this fact, that call might still be using the newer SYSCALL/SYSENTER CPU instructions. The glibc wrapped gettimeofday() call is what most programs would use. Since the kernel has been designed to use the most efficient mechanism of syscall, that version has the potential to be a virtualized syscall that is mapped into userspace.
To determine if a specific call is using a virtualized (user-space) syscall or a traditional, memory segment-shuffling, syscall, the strace utility can be used. If a true traditional syscall is being conducted, the routine will be output by strace, will look similar to the following:
According to comments in glibc-2.12.1: "The vsyscall page is a virtual DSO (Dynamic Shared Object) pre-mapped by the kernel" [7].
vsyscall and vDSO are similar in how they work, however there are some slight differences. vsyscall is limited to 4 entries, and is static in memory. Therefore, any statically linked applications can guarantee where vsyscalls are loaded. On-the-other-hand, vDSO is dynamically loaded into the user process, therefore it is not predictable due to Linux's randomized address space layout. If more than 4 vsyscalls are needed, then a vdso should be used instead [8].
For example, run `cat /proc/self/maps` and look at both the '[vdso]' and '[vsyscall]' entries. If your system supports these, the memory range for vDSO is different for each process issued, and vsyscall is totally predictable. If you dont believe me, run 'cat /proc/self/maps' a few times and note the addresses of vdso and vsyscall. Since this example is looking at '/proc/self/maps' the memory mappings displayed are for that 'cat' process [9, 10].
[1] http://en.wikipedia.org/wiki/System_call
[2] Linux User's Manual: intro(2)
[3] http://www.tldp.org/LDP/khg/HyperNews/get/syscall/syscall86.html
[4] http://en.wikipedia.org/wiki/Interrupt
[5] http://en.wikipedia.org/wiki/Kernel_(computing)
[6] http://www.linux.it/~rubini/docs/ksys/ksys.html
[7] glibc-2.12.1
[8] Linux Kernel 2.6.37 arch/x86/kernel/vsyscall_64.c
[9] http://anomit.com/2010/04/18/examining-the-linux-vdso/
[10] http://www.trilithium.com/johan/2005/08/linux-gate/
-Matt
-= What are System Calls =-
System calls are routines that communicate directly with the operating system in hopes of attaining some specific piece of information, or to make a specific request that the OS needs to fulfill. For example, gettimeofday() is a request often from user-land for an application to obtain the current timing information from the operating system's kernel. In Linux this call is implemented as a system call. Often this interface exists as a means of fulfilling a request by a user for kernel information. This interface allows a lesser privileged application to access higher-privileged kernel information [1, 2].Often these calls are not even accessed directly by an application, rather they are executed via a wrapper. The GNU C library, glibc, has wrapper functions that do all the handy-dandy wrapping. For instance, a call to gettimeofday() in an application is really just a call to glibc's wrapper for the system call for gettimeofday(). There is no overhead because the wrapper, at link-time, just associates gettimeofday() to the system's appropriate version of the routine. If one were to desire to make a true system call, avoiding the wrapper, Linux provides a syscall() routine allowing such. Both wrapper and the explicit syscall versions of gettimeofday() are demonstrated below:
#include <stdlib.h>
#include <sys/syscall.h>
#include <sys/time.h>
void main(void)
{
struct timeval tv;
/* glibc wrapped */
gettimeofday(&tv, NULL);
/* Explicit syscall */
syscall(SYS_gettimeofday, &tv, NULL);
return 0;
}
System calls are loaded at kernel boot-time. All of these calls are accessed by a number. In the example above SYS_gettimeofday is just a constant integer. And this integer is the index into an array (called an interrupt descriptor table) of system calls that are constructed at boot-time. This table/vector can be thought of as pointers to the actual routines [3]. So at index SYS_gettimeofday is a pointer to a piece of code which is where the actual sys_gettimeofday() routine exists.
-= Overhead =-
Originally, invoking a syscall in Linux was actually a pretty expensive process, as it was implemented as a system interrupt (int 0x80). When the syscall interface was called, via the "int 0x80" instruction, the CPU would pass control to the OS. The OS, would look at what value was passed in, such as SYS_gettimeofday, and then the interrupt vector would be indexed and gettimeofday() would get called. Interrupts force the CPU to save the state of execution just before the system call was executed and the system interrupted. This state is restored after the interrupt (in this case a syscall) completes.Ok, so interrupts are relatively cheap in execution time. However, the way the kernel is designed does add some overhead when a syscall is executed. Linux is divided into two primary memory segments. User-space and kernel-space (userland, kernelland). User-space memory is that which user applications run in. Kernel-space is where all the kernel services run. This segmentation acts as a security barrier, so that crafty and/or malicious user apps cannot directly access the kernel's memory. Making a syscall, as mentioned previously, is just a bridge, a mere wormhole, between these two segments of memory. It is this hop between address spaces that causes quite some overhead, as the kernel must switch between user and kernel-land memory segments, and then back again as the syscall completes [5, 6]. Changing the memory addressing requires some register shuffling, and just imparts more overhead on the whole syscall process.
-= vsyscall and vDSO =-
To reduce the overhead of hopping between user and kernel spaces, a newer mechanism in Linux allows certain syscalls to be accessed directly from userspace, without the need to cross the user/kernel space barrier. This is just what the vsyscall and vdso (Virtual Dynamic Share Object) interfaces do. At boot-time a page of memory is dedicated to containing a subset of syscalls, deemed safe to execute fromuserland, that should not cause a security hole for the kernel. The page of memory where these calls lies is mapped into each running process' user-space. Thus, when a call to one of these syscalls is made, no context switch between the memory regions of user and kernel-space is conducted, thus less overhead.
Another interesting means of reducing syscall overhead comes specific to the underlying CPU architecture. Both the more recent AMD and Intel chips have implemented a fast syscall functionality. Instead of issuing an interrupt, programs can issue instructions (SYSCALL/SYSENTER and SYSRET/SYSEXIT) that act faster than a traditional interrupt. The usage of these are based on object code in the OS. Therefore, a programmer does not have to consider how to implement/request the use of a SYSCALL/SYSENTER over a traditional "int 0x80" [1]. When applications are built, based on the architecture and the system, vsyscall and vdso linkage is done automagically.
As demonstrated above, by using the syscall() routine, a traditional syscall will be conducted, even if there is vDSO support (virtual Dynamically Shared Object). However, despite this fact, that call might still be using the newer SYSCALL/SYSENTER CPU instructions. The glibc wrapped gettimeofday() call is what most programs would use. Since the kernel has been designed to use the most efficient mechanism of syscall, that version has the potential to be a virtualized syscall that is mapped into userspace.
To determine if a specific call is using a virtualized (user-space) syscall or a traditional, memory segment-shuffling, syscall, the strace utility can be used. If a true traditional syscall is being conducted, the routine will be output by strace, will look similar to the following:
gettimeofday({1297472587, 581519}, NULL) = 0
According to comments in glibc-2.12.1: "The vsyscall page is a virtual DSO (Dynamic Shared Object) pre-mapped by the kernel" [7].
vsyscall and vDSO are similar in how they work, however there are some slight differences. vsyscall is limited to 4 entries, and is static in memory. Therefore, any statically linked applications can guarantee where vsyscalls are loaded. On-the-other-hand, vDSO is dynamically loaded into the user process, therefore it is not predictable due to Linux's randomized address space layout. If more than 4 vsyscalls are needed, then a vdso should be used instead [8].
For example, run `cat /proc/self/maps` and look at both the '[vdso]' and '[vsyscall]' entries. If your system supports these, the memory range for vDSO is different for each process issued, and vsyscall is totally predictable. If you dont believe me, run 'cat /proc/self/maps' a few times and note the addresses of vdso and vsyscall. Since this example is looking at '/proc/self/maps' the memory mappings displayed are for that 'cat' process [9, 10].
[1] http://en.wikipedia.org/wiki/System_call
[2] Linux User's Manual: intro(2)
[3] http://www.tldp.org/LDP/khg/HyperNews/get/syscall/syscall86.html
[4] http://en.wikipedia.org/wiki/Interrupt
[5] http://en.wikipedia.org/wiki/Kernel_(computing)
[6] http://www.linux.it/~rubini/docs/ksys/ksys.html
[7] glibc-2.12.1
[8] Linux Kernel 2.6.37 arch/x86/kernel/vsyscall_64.c
[9] http://anomit.com/2010/04/18/examining-the-linux-vdso/
[10] http://www.trilithium.com/johan/2005/08/linux-gate/
-Matt
Saturday, February 5, 2011
matt += food;
Soooo. Well, I never posted about me getting out of the hospital, but I was there for three weeks, and was released about two weeks ago. I have been going back to work and research, but I have also been eating more. I have weekly doctor appointments to run blood tests. I have also been enjoying cooking more, and ya know, it's not that hard and it's not boring. It's not hard cause I'm not trying to be the next Master Chef. I need to keep eating, as my ultimate goal is to regain more health. So, for all of those that cared or offered to come out to see me, or sent me messages, or were legitimately concerned... much props.
BTW, I had no idea that I would acquire so many books while being in the crazy-house. Much props to my advisor, friend, and professor for bringing me a copy of Douglas Hofstadter's Godel, Escher, Bach. I have not completed it, but got just under halfway through while being re-fed. I must say, as much as it sucked to be held against my will, I needed it. Technically, I was only against my will for one night, but I have a feeling if I were to have tried to discharge myself, I would have gotten another Section 12 (involuntary) status; thereby preventing me from leaving. I must say I did meet some really cool people. Cool people are interesting. That could have been said in reverse, interesting people are cool. It is a commutative relation.
I did learn two things when in the hospital:
-Matt
BTW, I had no idea that I would acquire so many books while being in the crazy-house. Much props to my advisor, friend, and professor for bringing me a copy of Douglas Hofstadter's Godel, Escher, Bach. I have not completed it, but got just under halfway through while being re-fed. I must say, as much as it sucked to be held against my will, I needed it. Technically, I was only against my will for one night, but I have a feeling if I were to have tried to discharge myself, I would have gotten another Section 12 (involuntary) status; thereby preventing me from leaving. I must say I did meet some really cool people. Cool people are interesting. That could have been said in reverse, interesting people are cool. It is a commutative relation.
I did learn two things when in the hospital:
- If you carry around a large book about science, you might be perceived a genius.
- If you can access facebook via a proxy, you might be perceived as a hacker.
- Sultanas are seemingly the equivalent to raisins in America.
-Matt
Friday, January 14, 2011
137 Trimethylxanthine!!!!
So one beauty of coming to Melbourne is that it has the most cafes per square meter. For a coffee nerd like myself (I take pride in the fact that I used to roast my own), this is a pretty cool place. Well, I had no idea I would be able to sample the delicacies of the beautiful bean from the confines of a mental ward (still here re-feeding trying to get healthier, I love how they do this in the mental ward).
Anyways, the only coffee that is available, typically, here in this ward is instant decrap. Well the decrap, instant, turns to rocks in the high-humidity of this region. Likewise, since it is a mental ward, and also people with eating problems, less caffeine to stimulate the body and also metabolism is the choice. Well, I have had a bit of caffeine from the tea (they also have soda here but I don't drink that). Anyways, they had real coffee, with a press pot here today. I just sampled it...... mmmmmmm I FEEL LIKE I CAN LIFT A SMALL CAR. AHHHH HAHAHAHHAHAHA. Ok, neurons stimulated. One of the other patients here did bring in some instant (caffeinated) coffee. I did sample some of that a while back. Anyways, hopefully I am getting healthier and my bloods are stabilizing. I want out soon!
Much Love
-Matt
Anyways, the only coffee that is available, typically, here in this ward is instant decrap. Well the decrap, instant, turns to rocks in the high-humidity of this region. Likewise, since it is a mental ward, and also people with eating problems, less caffeine to stimulate the body and also metabolism is the choice. Well, I have had a bit of caffeine from the tea (they also have soda here but I don't drink that). Anyways, they had real coffee, with a press pot here today. I just sampled it...... mmmmmmm I FEEL LIKE I CAN LIFT A SMALL CAR. AHHHH HAHAHAHHAHAHA. Ok, neurons stimulated. One of the other patients here did bring in some instant (caffeinated) coffee. I did sample some of that a while back. Anyways, hopefully I am getting healthier and my bloods are stabilizing. I want out soon!
Much Love
-Matt
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