Using trickle with Flatpak applications

README.md

Trickle is a command-line tool to throttle network bandwidth of other applications. You use it on the command-line to start the application. It uses LD_PRELOAD to override network-related calls. As a result, using it with containerized apps, like Flatpak, does not work out of the box.

To get trickle to function with a Flatpak application, the preloaded library needs to be available in the container. And it needs to be compatible with the software in the container (notably, a glibc version that is compatible).

First try

1. Install trickle on your (host) system, on Debian/Ubuntu that would be apt install trickle.
2. Make sure you have the flatpak application installed. I'm using com.slack.Slack as an example here.
3. Run the flatpak application with trickle

   trickle -s -d 250 -u 250 flatpak run com.slack.Slack
   

4. Find out it doesn't work. Error message:

   ERROR: ld.so: object '/usr/lib/x86_64-linux-gnu/trickle/trickle-overload.so' from LD_PRELOAD cannot be preloaded (cannot open shared object file): ignored.
   

Second try

1. (as above)
2. (as above)
3. Find out what runtime the flatpak application uses. flatpak info com.slack.Slack, e.g. org.freedesktop.Platform/x86_64/21.08
4. Copy the trickle library to the runtime

   RUNTIME=`flatpak info com.slack.Slack | sed 's/^\s*Runtime:\s*//p;d'`
   cd "/var/lib/flatpak/runtime/$RUNTIME/active/files/lib/x86_64-linux-gnu"
   sudo mkdir trickle
   sudo ln /usr/lib/x86_64-linux-gnu/trickle/trickle-overload.so trickle # use cp on different filesystem
   

5. Run the flatpak with trickle (as above)
6. Find out it still doesn't work

   /bin/sh: /usr/lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /usr/lib/x86_64-linux-gnu/trickle/trickle-overload.so)
   

You can find out which GLIBC version a library needs using objdump -T. Running it on the system's trickle library, we find

$ objdump -T /usr/lib/x86_64-linux-gnu/trickle/trickle-overload.so | grep GLIBC
0000000000000000      DF *UND*	0000000000000000 (GLIBC_2.2.5) getenv
0000000000000000      DF *UND*	0000000000000000 (GLIBC_2.3.4) __snprintf_chk
...
0000000000000000      DF *UND*	0000000000000000 (GLIBC_2.4)  __stack_chk_fail
0000000000000000      DF *UND*	0000000000000000 (GLIBC_2.15) __fdelt_chk
...
0000000000000000      DF *UND*	0000000000000000 (GLIBC_2.34) dlsym
0000000000000000      DF *UND*	0000000000000000 (GLIBC_2.2.5) exit

You see that there is one external symbol that needs glibc 2.34, hence the requirement or this minimum version.

Third try

1. (as above)
2. (as above)
3. Download a binary package of trickle for an older operating system. Debian stretch will do here (package). Extract the library.

   wget http://ftp.de.debian.org/debian/pool/main/t/trickle/trickle_1.07-10+b1_amd64.deb
   mkdir trickle-stretch
   dpkg-deb -x trickle_1.07-10+b1_amd64.deb trickle-stretch
   sudo cp trickle-stretch/usr/lib/trickle/trickle-overload.so /tmp
   

4. Copy the older library to the flatpak runtime

   RUNTIME=`flatpak info com.slack.Slack | sed 's/^\s*Runtime:\s*//p;d'`
   cd "/var/lib/flatpak/runtime/$RUNTIME/active/files/lib/x86_64-linux-gnu"
   sudo mkdir -p trickle
   sudo cp /tmp/trickle-overload.so trickle/
   

5. Run the flatpak application with trickle

   trickle -s -d 250 -u 250 flatpak run com.slack.Slack
   

6. Finally it runs.