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Autres articles (58)

  • Use, discuss, criticize

    13 avril 2011, par

    Talk to people directly involved in MediaSPIP’s development, or to people around you who could use MediaSPIP to share, enhance or develop their creative projects.
    The bigger the community, the more MediaSPIP’s potential will be explored and the faster the software will evolve.
    A discussion list is available for all exchanges between users.

  • L’espace de configuration de MediaSPIP

    29 novembre 2010, par

    L’espace de configuration de MediaSPIP est réservé aux administrateurs. Un lien de menu "administrer" est généralement affiché en haut de la page [1].
    Il permet de configurer finement votre site.
    La navigation de cet espace de configuration est divisé en trois parties : la configuration générale du site qui permet notamment de modifier : les informations principales concernant le site (...)

  • Contribute to a better visual interface

    13 avril 2011

    MediaSPIP is based on a system of themes and templates. Templates define the placement of information on the page, and can be adapted to a wide range of uses. Themes define the overall graphic appearance of the site.
    Anyone can submit a new graphic theme or template and make it available to the MediaSPIP community.

Sur d’autres sites (9191)

  • 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.

  • Remove black frames and sync audio with ffmpeg

    24 janvier 2020, par rahim123

    I’m trying to figure out a way to automatically process .mp4 or .mkv videos that have been manually edited to remove a bunch of segments, leaving many blank spaces (black frames with no audio). I want to remove all but one black frame (although I would settle for removing all of them if it’s much simpler) and then also sync the audio after the video timings change.

    I found this question and tried the three main suggestions :

    1. ffmpeg -i in.mkv -vf mpdecimate,setpts=N/FRAME_RATE/TB out.mkv
    2. ffmpeg -i in.mkv -vf [mpdecimate][] out.mkv
    3. ffmpeg -i in.mkv -vf [decimate][]=cycle=6,[setpts][]=N/25/TB out.mkv

    The first one does eliminate the black segments, but it doesn’t sync the audio. The other two commands don’t work :

    > ffmpeg -i in.mp4 -vf [decimate][]=cycle=6,[setpts][]=N/25/TB out.mp4
    
ffmpeg version 4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
    
  built with gcc 9 (SUSE Linux)
    
  configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64 --incdir=/usr/include/ffmpeg --extra-cflags='-O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g' --optflags='-O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g' --disable-htmlpages --enable-pic --disable-stripping --enable-shared --disable-static --enable-gpl --disable-openssl --enable-avresample --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcelt --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libdc1394 --enable-libdrm --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libssh --enable-libsrt --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libv4l2 --enable-libvpx --enable-libwebp --enable-libxml2 --enable-libzimg --enable-libzmq --enable-libzvbi --enable-lto --enable-lv2 --enable-libmfx --enable-vaapi --enable-vdpau --enable-libfdk-aac-dlopen --enable-nonfree --enable-libvo-amrwbenc --enable-version3 --enable-libx264 --enable-libx265 --enable-librtmp --enable-libxvid
    
  libavutil      56. 31.100 / 56. 31.100
    
  libavcodec     58. 54.100 / 58. 54.100
    
  libavformat    58. 29.100 / 58. 29.100
    
  libavdevice    58.  8.100 / 58.  8.100
    
  libavfilter     7. 57.100 /  7. 57.100
    
  libavresample   4.  0.  0 /  4.  0.  0
    
  libswscale      5.  5.100 /  5.  5.100
    
  libswresample   3.  5.100 /  3.  5.100
    
  libpostproc    55.  5.100 / 55.  5.100
    
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'in.mp4':
    
  Metadata:
    
    major_brand     : isom
    
    minor_version   : 512
    
    compatible_brands: isomiso2avc1mp41
    
    encoder         : Lavf58.20.100
    
  Duration: 00:34:23.62, start: 0.000000, bitrate: 506 kb/s
    
    Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/unknown/smpte170m), 640x480 [SAR 1:1 DAR 4:3], 412 kb/s, 29.97 fps, 29.97 tbr, 1867500000.00 tbn, 59.94 tbc (default)
    
    Metadata:
    
      handler_name    : VideoHandler
    
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 86 kb/s (default)
    
    Metadata:
    
      handler_name    : SoundHandler
    
Stream mapping:
    
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
    
  Stream #0:1 -> #0:1 (aac (native) -> aac (native))
    
Press [q] to stop, [?] for help
    
[AVFilterGraph @ 0x55ece8def080] Bad (empty?) label found in the following: "[]=cycle=6,[setpts][]=N/25/TB".
    
Error reinitializing filters!
    
Failed to inject frame into filter network: Invalid argument
    
Error while processing the decoded data for stream #0:0
    
[aac @ 0x55ece8c5be80] Qavg: 222.643
    
[aac @ 0x55ece8c5be80] 2 frames left in the queue on closing
    
Conversion failed!
    > ffmpeg -i in.mp4 -vf [decimate][]=cycle=6,[setpts][]=N/25/TB out.mp4
    
ffmpeg version 4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
    
  built with gcc 9 (SUSE Linux)
    
  configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64 --incdir=/usr/include/ffmpeg --extra-cflags='-O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g' --optflags='-O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -Werror=return-type -flto=auto -g' --disable-htmlpages --enable-pic --disable-stripping --enable-shared --disable-static --enable-gpl --disable-openssl --enable-avresample --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcelt --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libdc1394 --enable-libdrm --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libssh --enable-libsrt --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libv4l2 --enable-libvpx --enable-libwebp --enable-libxml2 --enable-libzimg --enable-libzmq --enable-libzvbi --enable-lto --enable-lv2 --enable-libmfx --enable-vaapi --enable-vdpau --enable-libfdk-aac-dlopen --enable-nonfree --enable-libvo-amrwbenc --enable-version3 --enable-libx264 --enable-libx265 --enable-librtmp --enable-libxvid
    
  libavutil      56. 31.100 / 56. 31.100
    
  libavcodec     58. 54.100 / 58. 54.100
    
  libavformat    58. 29.100 / 58. 29.100
    
  libavdevice    58.  8.100 / 58.  8.100
    
  libavfilter     7. 57.100 /  7. 57.100
    
  libavresample   4.  0.  0 /  4.  0.  0
    
  libswscale      5.  5.100 /  5.  5.100
    
  libswresample   3.  5.100 /  3.  5.100
    
  libpostproc    55.  5.100 / 55.  5.100
    
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'in.mp4':
    
  Metadata:
    
    major_brand     : isom
    
    minor_version   : 512
    
    compatible_brands: isomiso2avc1mp41
    
    encoder         : Lavf58.20.100
    
  Duration: 00:34:23.62, start: 0.000000, bitrate: 506 kb/s
    
    Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/unknown/smpte170m), 640x480 [SAR 1:1 DAR 4:3], 412 kb/s, 29.97 fps, 29.97 tbr, 1867500000.00 tbn, 59.94 tbc (default)
    
    Metadata:
    
      handler_name    : VideoHandler
    
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 86 kb/s (default)
    
    Metadata:
    
      handler_name    : SoundHandler
    
Stream mapping:
    
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
    
  Stream #0:1 -> #0:1 (aac (native) -> aac (native))
    
Press [q] to stop, [?] for help
    
[AVFilterGraph @ 0x558b0d48af80] Bad (empty?) label found in the following: "[]=cycle=6,[setpts][]=N/25/TB".
    
Error reinitializing filters!
    
Failed to inject frame into filter network: Invalid argument
    
Error while processing the decoded data for stream #0:0
    
[aac @ 0x558b0d2f7e80] Qavg: 222.643
    
[aac @ 0x558b0d2f7e80] 2 frames left in the queue on closing
    
Conversion failed!

    I also tried this script, but it also fails to sync the audio. I tried with both .mp4 and .mkv source files. Thanks for the help.

  • avcodec/encode : Always use intermediate buffer in ff_alloc_packet2()

    11 mai 2021, par Andreas Rheinhardt
    avcodec/encode : Always use intermediate buffer in ff_alloc_packet2()

    Up until now, ff_alloc_packet2() has a min_size parameter :
    
It is supposed to be a lower bound on the final size of the packet
    
to allocate. If it is not too far from the upper bound (namely,
    
if it is at least half the upper bound), then ff_alloc_packet2()
    
already allocates the final, already refcounted packet ; if it is
    
not, then the packet is not refcounted and its data only points to
    
a buffer owned by the AVCodecContext (in this case, the packet will
    
be made refcounted in encode_simple_internal() in libavcodec/encode.c).
    
The goal of this was to avoid data copies and intermediate buffers
    
if one has a precise lower bound.
    

    Yet those encoders for which precise lower bounds exist have recently
    
been switched to ff_get_encode_buffer() (which automatically allocates
    
final buffers), leaving only two encoders to actually set the min_size
    
to something else than zero (namely aliaspixenc and hapenc). Both of
    
these encoders use a very low lower bound that is not helpful in any
    
nontrivial case.
    

    This commit therefore removes the min_size parameter as well as the
    
codepath in ff_alloc_packet2() for the allocation of final buffers.
    
Furthermore, the function has been renamed to ff_alloc_packet() and
    
moved to encode.h alongside ff_get_encode_buffer().
    

    Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
    • [DH] libavcodec/aacenc.c
    • [DH] libavcodec/alacenc.c
    • [DH] libavcodec/aliaspixenc.c
    • [DH] libavcodec/asvenc.c
    • [DH] libavcodec/audiotoolboxenc.c
    • [DH] libavcodec/cfhdenc.c
    • [DH] libavcodec/cinepakenc.c
    • [DH] libavcodec/encode.c
    • [DH] libavcodec/encode.h
    • [DH] libavcodec/ffv1enc.c
    • [DH] libavcodec/flashsv2enc.c
    • [DH] libavcodec/flashsvenc.c
    • [DH] libavcodec/gif.c
    • [DH] libavcodec/hapenc.c
    • [DH] libavcodec/huffyuvenc.c
    • [DH] libavcodec/internal.h
    • [DH] libavcodec/j2kenc.c
    • [DH] libavcodec/lclenc.c
    • [DH] libavcodec/libfdk-aacenc.c
    • [DH] libavcodec/libilbc.c
    • [DH] libavcodec/libopencore-amr.c
    • [DH] libavcodec/libopenjpegenc.c
    • [DH] libavcodec/libopusenc.c
    • [DH] libavcodec/libspeexenc.c
    • [DH] libavcodec/libtwolame.c
    • [DH] libavcodec/libvo-amrwbenc.c
    • [DH] libavcodec/libxvid.c
    • [DH] libavcodec/ljpegenc.c
    • [DH] libavcodec/magicyuvenc.c
    • [DH] libavcodec/mlpenc.c
    • [DH] libavcodec/mpegaudioenc_template.c
    • [DH] libavcodec/mpegvideo_enc.c
    • [DH] libavcodec/msvideo1enc.c
    • [DH] libavcodec/opusenc.c
    • [DH] libavcodec/pcxenc.c
    • [DH] libavcodec/pngenc.c
    • [DH] libavcodec/proresenc_anatoliy.c
    • [DH] libavcodec/proresenc_kostya.c
    • [DH] libavcodec/qtrleenc.c
    • [DH] libavcodec/roqvideoenc.c
    • [DH] libavcodec/rpzaenc.c
    • [DH] libavcodec/sgienc.c
    • [DH] libavcodec/snowenc.c
    • [DH] libavcodec/sonic.c
    • [DH] libavcodec/sunrastenc.c
    • [DH] libavcodec/svq1enc.c
    • [DH] libavcodec/targaenc.c
    • [DH] libavcodec/tiffenc.c
    • [DH] libavcodec/ttaenc.c
    • [DH] libavcodec/utvideoenc.c
    • [DH] libavcodec/vorbisenc.c
    • [DH] libavcodec/wavpackenc.c
    • [DH] libavcodec/wmaenc.c
    • [DH] libavcodec/xbmenc.c

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