I was tinkering today with a project used Texture2D.readPixels to get pixel values from the screen and then called getPixels to access the values therein, but all the pixel values in the entire returned image appeared to be the same, and when accessing the data, it was like a solid white rectangle instead of a copy of the screen data.
I tested the particular project on Unity for OSX and it worked fine, so I think that the bug is in the Linux version only and not the project.
I also tried encoding the texture from the readpixels call as a PNG to see if the problem was there or in getpixels, and the png was a white rectangle, just as getPixels returned, so the problem is definitely in readPixels.