3: Getting gllin to run

I was going to make a small trip this weekend but I missed my plane and have to wait until next week. But that means I already have a good excuse for not spending the weekend studying for this week’s exams and I have finally put the time into making gllin behave under Schwartz.

Gllin is a closed-source driver for the Global Locate (now Broadcomm?) GPS known as Hammerhead and it’s been said it didn’t work when the folks compiled it for ARM EABI (i.e. what is used on most ARMs currently) so they only released the OABI binary (the ad-hoc ABI that was used on Linux until ARM came up with a standard ABI and hired people to implement it). So the downloadable gllin package comes with an OABI rootfs which will run under chroot if you have OABI support in your kernel. It seemed wrong to me to have a second rootfs on my phone to run a single program, and it has several other drawbacks.

With the Schwartz loader/linker you can run OABI-compiled programs natively on Linux systems that use different ABIs. This is achieved through translation of library calls that I mentioned previously. Schwartz is by no means complete, and more than anything it’s a proof-of-concept, but it seems to be usable and today my Neo1973 had an actual 3d fix and gave me real coordinates as well as satellite time/date and other info. I took my Neo for an excursion to the shopping mall (not so much to show off, but) to make my first GPS trace for OpenStreetMap. It ran quite stably for the whole 2h and I uploaded the trace here. So here’s how to use it.

Download the schwartz binary from here or here (minimal version). The sources are in this git tree, but building them is not exactly straight-forward. Upload the file to your Neo1973 (or qemu-neo1973). Upload also the gllin binary if you don’t have it there already. In the openmoko package the binary is named gllin.real because gllin is a wrapper script that runs the whole chroot thing. You only need the “.real” binary. You can also safely leave out OABI support from your kernel. Next, make the named pipe for your NMEA data, same way the openmoko package does. After that we’re ready to run gllin and then your favourite gps software.

 $ mknod /tmp/nmeaNP p
 $ cat /tmp/nmeaNP | gzip >> /home/root/gps.gz &
 $ ./ld4 --depnofail --weakdummies --settargetname --noinit gllin -low 5
 $ ./ld4 --depnofail --weakdummies --settargetname --noinit gllin -periodic 2

You can modify the scripts from the package to do all that. ld4 is quite verbose and will print lots of stuff tot he console, which just shows how far it is from completeness. The minimal ld4 differs from the full binary in that the “strace” code is not compiled in. With the full binary, if you append –trick-strace to the cmdline options you will get a strace-like (but more pretty!) log of all functions being called and their parameters. This may potentially be useful for the folks reverse engineering the Hammerhead protocol but I’m not really sure. In the ld4 output you can see a lot of debugging messages and other, that gllin doesn’t normally print out. I have not noticed any anomalies when running gllin under Schwartz but it’s totally possible that the floating-point precision is reduced or something else is broken. gllin is a pretty tough test case for the ABI translation thing for various reasons: all the floating-point arithmetics, heavy usage of memory/files/sockets, C++ libraries, C++ exceptions, real-time constraints and more.

Among other things schwartz enables you to do is running gllin without root privileges (chroot normally requires those). Also an interesting thing to do is compare the strace (the real traditional strace) output of gllin running under a chroot with OABI compiled libc, and the strace output of the same gllin running under schwartz and using EABI libc. You’ll see two different sequences of syscalls being made, but having pretty much the same end effect.

I probably won’t have time to hack schwartz further but improvements from others are welcome. I just wish I had the thing running earlier – ironically I already have a GTA02 on my desk, and GTA02 has a different GPS chip in it which needs no driver on the OS side. There’s very little time left till the mass-production and selling of GTA02 starts and gllin slides into oblivion. (It seems that the TomTom Go’s using the same or a similar driver though).

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3 Responses to “3: Getting gllin to run”

  1. Shoragan Says:

    The current gllin package does not use a chroot, it simply uses the OABI ld-linux.so with different –library-path. So we don’t need to setup any mounts for gllin.

  2. balrog Says:

    Oops, yes, I should have been more precise. However –library-path pointing to a different directory is very much like what chroot does, there’s no need to mount anything just a directory with /lib files present.

  3. Jussi Kukkonen Says:

    Obsolete or not, this is really cool. I’m definitely going to try this out (already cloned your repo but I’m pretty sure this is so far outside my skills that I won’t be of any help).

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