# Linux Kernel Module Cheat

### 38.17. Bisection

When updating the Linux kernel, QEMU and gem5, things sometimes break.

However, for many types of crashes, it is trivial to bisect down to the offending commit, in particular because we can make QEMU and gem5 exit with status 1 on kernel panic as mentioned at: Section 17.6.1.3, “Exit emulator on panic”.

For example, when updating from QEMU `v2.12.0` to `v3.0.0-rc3`, the Linux kernel boot started to panic for `arm`.

We then bisected it as explained at: https://stackoverflow.com/questions/4713088/how-to-use-git-bisect/22592593#22592593 with the bisect-qemu-linux-boot script:

    root_dir="$(pwd)"
    cd "$(./getvar qemu_source_dir)"
    git bisect start

    # Check that our test script fails on v3.0.0-rc3 as expected, and mark it as bad.
    "${root_dir}/bisect-qemu-linux-boot"
    # Should output 1.
    echo #?
    git bisect bad

    # Same for the good end.
    git checkout v2.12.0
    "${root_dir}/bisect-qemu-linux-boot"
    # Should output 0.
    echo #?
    git bisect good

    # This leaves us at the offending commit.
    git bisect run "${root_dir}/bisect-qemu-linux-boot"

    # Clean up after the bisection.
    git bisect reset
    git submodule update
    "${root_dir}/build-qemu" --clean --qemu-build-id bisect

Other bisection helpers include:

- bisect-linux-boot-gem5

- bisect-gem5-linux-boot