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• undefined reference to `x264_encoder_open_125'

  18 juin 2013, par Vishal

  While installing ffmpeg on Ubuntu 12.04

  I am getting following error

  libavcodec/libavcodec.a(libx264.o): In function `X264_init':
  
  /root/ffmpeg/libavcodec/libx264.c:492: undefined reference to `x264_encoder_open_125'
  
  collect2: ld returned 1 exit status
  
  make: *** [ffmpeg_g] Error 1

  I am following the instructions given at
  http://ffmpeg.org/trac/ffmpeg/wiki/UbuntuCompilationGuide

  Do anyone have idea about this error ?

• I Really Like My New EeePC

  29 août 2010, par Multimedia Mike — General

  Fair warning : I’m just going to use this post to blather disconnectedly about a new-ish toy.

  I really like my new EeePC. I was rather enamored with the original EeePC 701 from late 2007, a little box with a tiny 7″ screen that is credited with kicking off the netbook revolution. Since then, Asus has created about a hundred new EeePC models.

  Since I’m spending so much time on a train these days, I finally took the plunge to get a better netbook. I decided to stay loyal to Asus and their Eee lineage and got the highest end EeePC they presently offer (which was still under US$500)– the EeePC 1201PN. The ’12′ in the model number represents a 12″ screen size and the rest of the specs are commensurately as large. Indeed, it sort of blurs the line between netbook and full-blown laptop.

  
  
  

  Incidentally, after I placed the order for the 1201PN nearly 2 months ago, and I mean the very literal next moment, this Engadget headline came across announcing the EeePC 1215N. My new high-end (such as it is) computer purchase was immediately obsoleted ; I thought that only happened in parody. (As of this writing, the 1215N still doesn’t appear to be shipping, though.)

  It’s a sore point among Linux aficionados that Linux was used to help kickstart the netbook trend but that now it’s pretty much impossible to find Linux pre-installed on a netbook. So it is in this case. This 1201PN comes with Windows 7 Home Premium installed. This is a notable differentiator from most netbooks which only have Windows 7 Home Starter, a.k.a., the Windows 7 version so crippled that it doesn’t even allow the user to change the background image.

  I wished to preserve the Windows 7 installation (you never know when it will come in handy) and dual boot Linux. I thought I would have to use the Windows partition tool to divide work some magic. Fortunately, the default installation already carved the 250 GB HD in half ; I was able to reformat the second partition and install Linux. The details are a little blurry, but I’m pretty sure one of those external USB optical drives shown in my last post actually performed successfully for this task. Lucky break.

  
  
  The EeePC 1201PN, EeePC 701, Belco Alpha-400, and even a comparatively gargantuan Sony Vaio full laptop– all of the portable computers in the household
  

  So I got Ubuntu 10.04 Linux installed in short order. This feels like something of a homecoming for me. You see, I used Linux full-time at home from 1999-2006. In 2007, I switched to using Windows XP full-time, mostly because my home use-case switched to playing a lot of old, bad computer games. By the end of 2008, I had transitioned to using the Mac Mini that I had originally purchased earlier that year for running FATE cycles. That Mac served as my main home computer until I purchased the 1201PN 2 months ago.

  Mostly, I have this overriding desire for computers to just work, at least in their basic functions. And that’s why I’m so roundly impressed with the way Linux handles right out of the box. Nearly everything on the 1201PN works in Linux. The video, the audio, the wireless networking, the webcam, it all works out of the box. I had to do the extra installation step to get the binary nVidia drivers installed but even that’s relatively seamless, especially compared to “the way things used to be” (drop to a prompt, run some binary installer from the prompt as root, watch it fail in arcane ways because the thing is only certified to run on one version of one Linux distribution). The 1201PN, with its nVidia Ion2 graphics, is able to drive both its own 1366×768 screen simultaneously with an external monitor running at up on 2560×1600.

  The only weird hiccup in the whole process was that I had a little trouble with the special volume keys on the keyboard (specifically, the volume up/down/mute keys didn’t do anything). But I quickly learned that I had to install some package related to ACPI and they magically started to do the right thing. Now I get to encounter the Linux Flash Player bug where modifying volume via those special keys forces fullscreen mode to exit. Adobe really should fix that.

  Also, trackpad multitouch gestures don’t work right away. Based on my reading, it is possible to set those up in Linux. But it’s largely a preference thing– I don’t care much for multitouch. This creates a disparity when I use Windows 7 on the 1201PN which is configured per default to use multitouch.

  
  
  The same 4 laptops stacked up
  

  So, in short, I’m really happy with this little machine. Traditionally, I have had absolutely no affinity for laptops/notebooks/portable computers at all even if everyone around was always completely enamored with the devices. What changed for me ? Well for starters, as a long-time Linux user, I was used to having to invest in very specific, carefully-researched hardware lest I not be able to use it under the Linux OS. This was always a major problem in the laptop field which typically reign supreme in custom, proprietary hardware components. These days, not so much, and these netbooks seem to contain well-supported hardware. Then there’s the fact that laptops always cost so much more than similarly capable desktop systems and that I had no real reason for taking a computer with me when I left home. So my use case changed, as did the price point for relatively low-power laptops/netbooks.

  Data I/O geek note : The 1201PN is capable of wireless-N networking — as many netbooks seem to have — but only 100 Mbit ethernet. I wondered why it didn’t have gigabit ethernet. Then I remembered that 100 Mbit ethernet provides 11-11.5 Mbytes/sec of transfer speed which, in my empirical experience, is approximately the maximum write speed of a 5400 RPM hard drive– which is what the 1201PN possesses.

• Heroic Defender of the Stack

  27 janvier 2011, par Multimedia Mike — Programming

  Problem Statement

  I have been investigating stack smashing and countermeasures (stack smashing prevention, or SSP). Briefly, stack smashing occurs when a function allocates a static array on the stack and writes past the end of it, onto other local variables and eventually onto other function stack frames. When it comes time to return from the function, the return address has been corrupted and the program ends up some place it really shouldn’t. In the best case, the program just crashes ; in the worst case, a malicious party crafts code to exploit this malfunction.

  Further, debugging such a problem is especially obnoxious because by the time the program has crashed, it has already trashed any record (on the stack) of how it got into the errant state.

  Preventative Countermeasure

  GCC has had SSP since version 4.1. The computer inserts SSP as additional code when the -fstack-protector command line switch is specified. Implementation-wise, SSP basically inserts a special value (the literature refers to this as the ’canary’ as in "canary in the coalmine") at the top of the stack frame when entering the function, and code before leaving the function to make sure the canary didn’t get stepped on. If something happens to the canary, the program is immediately aborted with a message to stderr about what happened. Further, gcc’s man page on my Ubuntu machine proudly trumpets that this functionality is enabled per default ever since Ubuntu 6.10.

  And that’s really all there is to it. Your code is safe from stack smashing by default. Or so the hand-wavy documentation would have you believe.

  Not exactly

  Exercising the SSP

  I wanted to see the SSP in action to make sure it was a real thing. So I wrote some code that smashes the stack in pretty brazen ways so that I could reasonably expect to trigger the SSP (see later in this post for the code). Here’s what I learned that wasn’t in any documentation :

  SSP is only emitted for functions that have static arrays of 8-bit data (i.e., [unsigned] chars). If you have static arrays of other data types (like, say, 32-bit ints), those are still fair game for stack smashing.

  Evaluating the security vs. speed/code size trade-offs, it makes sense that the compiler wouldn’t apply this protection everywhere (I can only muse about how my optimization-obsessive multimedia hacking colleagues would absolute freak out if this code were unilaterally added to all functions). So why are only static char arrays deemed to be "vulnerable objects" (the wording that the gcc man page uses) ? A security hacking colleague suggested that this is probably due to the fact that the kind of data which poses the highest risk is arrays of 8-bit input data from, e.g., network sources.

  The gcc man page also lists an option -fstack-protector-all that is supposed to protect all functions. The man page’s definition of "all functions" perhaps differs from my own since invoking the option does not have differ in result from plain, vanilla -fstack-protector.

  The Valgrind Connection

  "Memory trouble ? Run Valgrind !" That may as well be Valgrind’s marketing slogan. Indeed, it’s the go-to utility for finding troublesome memory-related problems and has saved me on a number of occasions. However, it must be noted that it is useless for debugging this type of problem. If you understand how Valgrind works, this makes perfect sense. Valgrind operates by watching all memory accesses and ensuring that the program is only accessing memory to which it has privileges. In the stack smashing scenario, the program is fully allowed to write to that stack space ; after all, the program recently, legitimately pushed that return value onto the stack when calling the errant, stack smashing function.

  Valgrind embodies a suite of tools. My idea for an addition to this suite would be a mechanism which tracks return values every time a call instruction is encountered. The tool could track the return values in a separate stack data structure, though this might have some thorny consequences for some more unusual program flows. Instead, it might track them in some kind of hash/dictionary data structure and warn the programmer whenever a ’ret’ instruction is returning to an address that isn’t in the dictionary.

  Simple Stack Smashing Code

  Here’s the code I wrote to test exactly how SSP gets invoked in gcc. Compile with ’gcc -g -O0 -Wall -fstack-protector-all -Wstack-protector stack-fun.c -o stack-fun’.

  stack-fun.c :

  PLAIN TEXT

  C :

  1. /* keep outside of the stack frame */
  2. static int i ;
  3.  
  4. void stack_smasher32(void)
  5. {
  6.  int buffer32[8] ;
  7.  // uncomment this array and compile without optimizations
  8.  // in order to force this function to compile with SSP
  9. // char buffer_to_trigger_ssp[8] ;
  10.  
  11.  for (i = 0 ; i <50 ; i++)
  12.   buffer32[i] = 0xA5 ;
  13. }
  14.  
  15. void stack_smasher8(void)
  16. {
  17.  char buffer8[8] ;
  18.  for (i = 0 ; i <50 ; i++)
  19.   buffer8[i] = 0xA5 ;
  20. }
  21.  
  22. int main()
  23. {
  24. // stack_smasher8() ;
  25.  stack_smasher32() ;
  26.  return 0 ;
  27. }

  The above incarnation should just produce the traditional "Segmentation fault". However, uncommenting and executing stack_smasher8() in favor of stack_smasher32() should result in "*** stack smashing detected *** : ./stack-fun terminated", followed by the venerable "Segmentation fault".

  As indicated in the comments for stack_smasher32(), it’s possible to trick the compiler into emitting SSP for a function by inserting an array of at least 8 bytes (any less and SSP won’t emit, as documented, unless gcc’s ssp-buffer-size parameter is tweaked). This has to be compiled with no optimization at all (-O0) or else the compiler will (quite justifiably) optimize away the unused buffer and omit SSP.

  For reference, I ran my tests on Ubuntu 10.04.1 with gcc 4.4.3 compiling the code for both x86_32 and x86_64.