File permission denied after matrix=RGBMatrix() call

[BNNorman]

I thought I'd modify the python/samples/image-viewer.py script to cycle through a list of images and display them in a round robin fashion. I've called it slideshow.py, of course. However, I get permission denied when I run the script using sudo python slideshow.py at the point where it attempts to open the image files but ONLY within the code that follows the matrix=RGBMatrix(options=options). If I comment out that statement and the matrix.SetImage() statement the loop runs just fine with no errors. I'm running it on a Pi 3. All the images have permissions 777.

Your original code loads the image before the matrix = RGBMatrix(options = options) statement. I did try moving the display code within the for loop into a method (def showImage) which was defined before the matrix instantiation but I still got a permission denied error.

I then placed os.getegid() calls before and after the matrix= statement and lo-and-behold it changed from 0 to 1. So it looks like you RGBMatrix call is clobbering the effective user.

I will try a workaround of loading the images into a list before the loop but that's going to be memory hungry with a lot of images.

Here's my code if you want to try it.

!/usr/bin/env python

''' modified image-viewer.py cycle through a list of images found in config.txt and display on rgb matrix ''' import time import sys from rgbmatrix import RGBMatrix, RGBMatrixOptions from PIL import Image

open the list of images to display

try: images=open("config.txt") except: print "Unable to open config.txt" sys.exit(0)

Configuration for the RGB LED matrix

options = RGBMatrixOptions() options.rows = 32 options.chain_length = 2 options.parallel = 2 options.hardware_mapping = 'regular' options.gpio_slowdown=2

print "egid before=",os.getegid()

after this line I get permission denied trying to open the images

matrix = RGBMatrix(options = options)

print "egid after=",os.getegid()

for image_file in images: image_file=image_file.strip() # remove trailing cr/lf

# this code fails with permission denied 
# if the matrix = RGBMatrix(options = options) is commented out
# the code opens the image ok.

try:
    image=Image.open(image_file)
except Exception as e:
    print "Unable to open ["+image_file+"]",e.args
    continue

# Make image fit our screen.
image.thumbnail((matrix.width, matrix.height), Image.ANTIALIAS)
matrix.SetImage(image.convert('RGB'))

try:
        print("Press CTRL-C to stop.")
        time.sleep(5)
except KeyboardInterrupt:
        sys.exit(0)

print "Show finished" sys.exit(0)

[hzeller]

The reason is, that the library drops the privileges from root to daemon, once it has initialized the hardware. This adds some safety in case something goes wrong. But it also means, that other interactions (e.g. reading images) are then done as a different user.

You can switch that off with --led-no-drop-privs or the equivalent option when starting the matrix. You also could just simply make your images readable to everyone chmod a+r <images>.

[BNNorman]

Thank you again for your quick replies.

On 7 Aug 2017 4:39 pm, "Henner Zeller" notifications@github.com wrote:

The reason is, that the library drops the privileges from root to daemon, once it has initialized the hardware. This adds some safety in case something goes wrong. But it also means, that other interactions (e.g. reading images) are then done as a different user.

You can switch that off with --led-no-drop-privs or the equivalent option when starting the matrix. You also could just simply make your images readable to everyone chmod a+r .

— You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub https://github.com/hzeller/rpi-rgb-led-matrix/issues/371#issuecomment-320699180, or mute the thread https://github.com/notifications/unsubscribe-auth/APHW3bAe_nr1QOqKMsbQanu7MzNRPn1sks5sVy_JgaJpZM4OvO_h .

[digital-cinema-arts]

I'm having the same problem but with calling from the python bindings. Is there a switch for 'led-no-drop-privs' in the python interface? I haven't found it.

[digital-cinema-arts]

I've posted the issue on Adafruit's forum:

https://forums.adafruit.com/viewtopic.php?f=47&t=155192

[digital-cinema-arts]

Found it after grepping core.pyx:

RGBMatrixOptions.drop_privileges = False