Hello,
I thought maybe some people here may be interested in a tool that I
primarily developed for my studies where I have to use MATLAB and
Simulink to program an Arduino board. Trying to access the Arduino
serial port caused me trouble because of missing write access to /var/lock.
Of course it would have been easier to screw up my system security by
blindly allowing access to a directory where my distributor decided that
users should not write to. Instead of doing this, I decided to take this
opportunity to learn something about LD_PRELOAD to redirect /var/lock
access to a (newly created) directory below $XDG_RUNTIME_DIR and
fine-tuned until MATLAB was happy and finally connected to my Arduino
without actual write access to /var/lock.
What it does is overriding glibc functions that are commonly used for
device locking mechanisms. If the accessed path is in a list of known
lock file paths, then the path is rewritten to point to a directory
below $XDG_RUNTIME_DIR before forwarding the call to the actual glibc
function and the return value back to the caller.
For development, I used "rxtx" (Java library) and "liblockdev" (Debian
device locking library) first. They are also included in my repo for
testing after build. The MATLAB part (that I actually wanted to solve)
was way more difficult as I don't know how device locking is implemented
there. I hoped that making open source libraries work, first, may cover
everything MATLAB uses, but it didn't. This required many hours of trial
and error with adding random additional glibc function overrides until I
finally catched the right ones and then removing unneeded overrides
again as I didn't want to mess with unnecessarily many glibc functions.
This also means that I probably didn't catch them all and there may
still be issues there.
If you are interested in trying out my tool, you can find it here:
https://github.com/M-Reimer/lockdev-redirect
Regards,
Manuel
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