riteshharjani / gup-repro

Reproduce ext4 WARN_ON() on file-backed GUP mapping with userland clean + kernel dirty

GUP repro

What it does

Reproduces the issue (somewhat) addressed in the patch series mm/gup: disallow GUP writing to file-backed mappings by default.

The steps being:-

1. Map some file-backed memory.
2. msync it so it's clean.
3. Use a kernel module to GUP-pin it.
4. Write to the file.
5. msync again so it's clean.
6. Trigger the kernel module to write/mark dirty.
7. ???
8. Profit.

Building and using

A make should suffice, to build against a specific kernel (and you should really be doing this in qemu so this is advised), specify the kernel tree path like:-

make KERNEL_TREE_PATH=/path/to/your/kernel

Then, in the vm, simply sudo insmod kgup-repro.ko and sudo ./gup-repro.

The warning should appear in dmesg.

IMPORTANT: The code relies on the test file test.txt. This can be regenerated using the gen-test-txt.sh script. Make sure to copy this to the VM along with the binaries.

Disclaimer

It'd be unwise to run this on a host system you care about, please do prefer a VM, but if things do explode then it's not my fault yada yada ;)

All code is GPLv2.