FFmpeg cannot write file to /dev/shm: Permission Denied

Question

Issue: I have an FFmpeg command that I've been running for months now to stream video into the /dev/shm directory. It had been working fine until relatively recently (e.g. within a week), now it throws a permission issue.

The command:

ffmpeg -threads 2 -video_size 640x480 -i /dev/video2 -c:v libx264 -f dash -streaming 1 /dev/shm/manifest.mpd

This is not the exact command (paired down for brevity), however the outcome is the same:

libGL error: No matching fbConfigs or visuals found
libGL error: failed to load driver: swrast
X Error:  GLXBadContext
  Request Major code 151 (GLX)
  Request Minor code 6 ()
  Error Serial #57
  Current Serial #56
ffmpeg version n4.3.1 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 7 (Ubuntu 7.5.0-3ubuntu1~18.04)
  configuration: --prefix= --prefix=/usr --disable-debug --disable-doc --disable-static --enable-cuda --enable-cuda-sdk --enable-cuvid --enable-libdrm --enable-ffplay --enable-gnutls --enable-gpl --enable-libass --enable-libfdk-aac --enable-libfontconfig --enable-libfreetype --enable-libmp3lame --enable-libnpp --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopus --enable-libpulse --enable-sdl2 --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libv4l2 --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxvid --enable-nonfree --enable-nvenc --enable-omx --enable-openal --enable-opencl --enable-runtime-cpudetect --enable-shared --enable-vaapi --enable-vdpau --enable-version3 --enable-xlib
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Input #0, video4linux2,v4l2, from '/dev/video2':
  Duration: N/A, start: 1900.558740, bitrate: 147456 kb/s
    Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, 147456 kb/s, 30 fps, 30 tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 0x55b15d8912c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0x55b15d8912c0] profile High 4:2:2, level 3.0, 4:2:2 8-bit
[libx264 @ 0x55b15d8912c0] 264 - core 152 r2854 e9a5903 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[dash @ 0x55b15d88f600] No bit rate set for stream 0
[dash @ 0x55b15d88f600] Opening '/dev/shm/init-stream0.m4s' for writing
Could not write header for output file #0 (incorrect codec parameters ?): Permission denied
Error initializing output stream 0:0 -- 
Conversion failed!

(tl;dr: Could not write header for output file #0 (incorrect codec parameters ?): Permission denied)

For contrast, this version of the command (writing to the home directory) works fine (/tmp/ also works):

ffmpeg -threads 2 -video_size 640x480 -i /dev/video2 -c:v libx264 -f dash -streaming 1 ~/manifest.mpd

As mentioned above, the strange thing is that I have not (knowingly) changed permissions on anything or altered the application; it seemingly just stopped working (although, not ruling out that I caused it). The last time I remember it working was probably a week ago (~March 20th, 2021).

What I tried:

• Running ffmpeg as sudo (sudo ffmpeg...)
  • Result: sudo: ffmpeg: command not found. This hasn't been necessary in the past, and it had the same output as before.
• sudo sysctl fs.protected_regular=0
  • Result: No change.
• Ran the ffmpeg ... command as su
  • Result: No change
• chmod +777 /dev/shm
  • Result: No change (ls -tls reveals that the directory is indeed rwxrwxrwt)
• chown'd both root:root and my username on /dev/shm
  • Result: No change.
• touch /dev/shm/test.txt and sudo touch /dev/shm/test.txt
  • Result: The file is created without issue.

I've exhausted everything I could think of relating to permissions to get it to work.

The Question What do I need to do to get FFmpeg write files to /dev/shm? Ideally, figuring out why this happened in the first place.

If anyone has any ideas for commands I should run to help diagnose this issue, feel free to add a comment.

System Info:

• Kernel: 4.19.0-14-amd64
• Distro: Debian
• FFmpeg: version n4.3.1 (Was installed using Snapd, if it matters.)

== Solution ==

jsbilling's solution of using snap.<snapname>.* unfortunately did not work, however in the linked forum thread there was a post which basically got around the issue of writing to /dev/shm by mounting a directory in home ~/stmp and writing the ffmpeg output there:

$ mkdir ~/stmp/
$ sudo mount --bind /dev/shm/streaming_front/ ~/stmp/
...
$ ffmpeg -threads 2 -video_size 640x480 -i /dev/video2 -c:v libx264 -f dash -streaming 1  ./stmp/manifest.mpd

Not an ideal solution, but a working one.

Answer 1

If you are using snaps, this forum post indicates there are specific patterns that are allowed for files in /dev/shm:

/dev/shm/snap.<snapname>.*

Another forum member suggested this hack, although it is basically a security bypass:

$ mkdir /dev/shm/shared
$ mkdir ~/shmdir
$ sudo mount --bind /dev/shm/shared ~/shmdir
$ touch ~/shmdir/foo
$ ls /dev/shm/shared/
foo