Segfault after restart with increased canvas size

[GKnirps]

Hi, I tried out wellenbrecher, and I get a segfault under certain circumstances.

Steps to reproduce:

1. Start wellenbrecher with default parameters
2. stop wellenbrecher
3. start wellenbrecher with a larger screen size (e.g. 1920×1080)
4. let a client write all over the canvas

Expected result: The pixels get printed in the expected area. Actual result: wellenbrecher gets terminated because of a segmentation fault.

Additionally: The canvas is persisted between restarts. I tried to figure out where and how to reset it, but I was unsuccessful. Maybe that can go into the README.md as well?

[GKnirps]

I fiddled around with the code a bit. In wellenbrecher-canvas/srs/lib.rs, in Canvas::open, the variable size is larger than the actual size of shared_memory after the initialization of the shared memory. Later, the shared memory is accessed as if it had the size of size, which leads to memory corruption.

[bits0rcerer]

Hi, thanks for giving wellenbrecher a try :)

---

Yep, you are right.

Any component (wellenbrecher, seebruecke, …) will create a shared memory if there is no present ( :thinking: maybe only wellenbrecher should be able to do that?).

There are mechanics in wellenbrecher to remove that shared memory on exit but they did not receive much attention yet.

wellenbrecher --help
# …
# -l, --canvas-file-link <CANVAS_FILE_LINK>
#          Canvas shared memory file link [default: /tmp/wellenbrecher-canvas]
#     --keep-canvas-file-link
#          Keep canvas shared memory file link
# …

/tmp/wellenbrecher-canvas contains the shared memory that you can find under /dev/shm/.

cat /tmp/wellenbrecher-canvas
# /shmem_<some id>

Remove the canvas manually:

sudo rm "/dev/shm$(cat /tmp/wellenbrecher-canvas)" /tmp/wellenbrecher-canvas

The canvas is persisted between restarts

Yes that is actually a feature :D

[bits0rcerer]

TODOs:

• [x] improve shared canvas ownership
• [x] implement shared canvas size checks