SANE USB permissions

Today I had a permission problem with SANE on Linux. SANE stands for “Scanner Access Now Easy”, it provides standardized access to scanner hardware (http://www.sane-project.org) and this is the most commonly used scanning tool on UNIX/Linux.

In my case the USB scanner was not recognized when issuing scanimage -L from my user account although it worked correctly under root and my user is in the scanner group. What more is sane-find-scanner reported permissions errors while running the command as user. The owner and group for the device (in my case it was /dev/bus/usb/002/004) were root:root. At this point we already know that something weird is happening and I expected something like root:scanner instead.

Looking into /lib/udev/rules.d/60-libsane.rules, the line in charge of changing the permissions for each scanner device matched by SANE:

ENV{libsane_matched}=="yes", RUN+="/bin/setfacl -m g:scanner:rw $env{DEVNAME}"

This is nice but I do not use ACL and they are disabled in kernel,  so this command is useless. So I replaced this line with:

ENV{libsane_matched}=="yes", RUN+="/bin/setfacl -m g:scanner:rw $env{DEVNAME}", MODE="0664", GROUP="scanner"

Now the owner and group are correctly set to root:scanner and I can use my scanner as a regular user.

Note that on my system the libsane, sane-utils and xsane are the only packages depending on the acl package. According to what I’ve seen in the ChangeLog they do so in order to cope with MFP which I presume should be accessible as a scanner and printer device at the same time. What I would have done instead would be to create special group for MFP devices and use this instead. IMO still less of a mess than enabling ACL on the whole system for a single package.

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