Kernel compilation modes

The CheriBSD kernel can be compiled as hybrid, pure-capability, or benchmark code:

• The hybrid kernel enables capability use in userspace while making relatively little use of capabilities in the kernel's own implementation.
• The pure-capability kernel implements strong referential and spatial memory protection internally in the kernel, protecting against memory-safety vulnerabilities in components such as the network stack and system-call layer.
• The benchmark kernel implements the Benchmark ABI which improves performance on the Arm Morello board at the cost of reduced security. The benchmark kernel is considered appropriate for a performance evaluation of the Arm Morello architecture and third-party software executed on it. It should not be used for a security evaluation and the pure-capability kernel should be used instead.

A number of precompiled kernels are provided in the CheriBSD release, enabling or disabling debugging in combination with hybrid, pure-capability, and benchmark compilations. In general, we recommend that performance benchmarking be done only with benchmark NODEBUG kernels; see Benchmarking guidance for further details.

Precompiled kernels in CheriBSD

CheriBSD ships with a number of precompiled kernels. Kernel configuration files themselves may be found in sys/arm64/conf in the CheriBSD source tree, and will have names such as GENERIC-MORELLO-PURECAP. Kernel configuration file names start with one of the following prefixes identifying the compilation mode of the kernel:

In addition to the base kernels for each compilation mode, CheriBSD includes precompiled kernels which disable debugging. These kernel configurations append a -NODEBUG suffix to the kernel configuration file name.

The default kernel uses the GENERIC-MORELLO-PURECAP kernel configuration, and is a pure-capability kernel with debugging enabled. It is unsuitable for benchmarking; instead, use the GENERIC-MORELLO-PURECAP-BENCHMARK-NODEBUG kernel configuration.

Kernels and their associated modules are stored in subdirectories of /boot matching the pattern kernel*; the default kernel is named /boot/kernel.

# cd /boot
# ls -d kernel*
kernel
kernel.GENERIC-MORELLO
kernel.GENERIC-MORELLO-NODEBUG
kernel.GENERIC-MORELLO-PURECAP-BENCHMARK
kernel.GENERIC-MORELLO-PURECAP-BENCHMARK-NODEBUG
kernel.GENERIC-MORELLO-PURECAP-NODEBUG

Identifying the currently booted kernel

The uname(1) command prints information on the kernel configuration. The following command-line options may be of use:

For example:

# uname -i
GENERIC-MORELLO-PURECAP

Additionally, the current kernel file path is stored in the kern.bootfile sysctl variable that you can read with the sysctl(8) utility. For example:

# sysctl kern.bootfile
kern.bootfile: /boot/kernel/kernel

Selecting another kernel using nextboot(8)

You can request an alternate kernel for the next boot of the system via the -k flag of the nextboot(8) command. The value of the flag should be set to the name of the kernel directory under /boot; for example, to use a pure-capability kernel without debugging on the next boot, use:

# nextboot -k kernel.GENERIC-MORELLO-PURECAP-NODEBUG

prior to rebooting. The system will revert back to the kernel configured in /boot/loader.conf on subsequent boots.

Selecting another kernel using loader.conf

You can set the kernel to boot via /boot/loader.conf using the variable kernel. This is set to the name of the kernel directory under /boot; for example, the default is:

kernel="kernel"

You can persistently set the system to boot with a Benchmark ABI kernel by setting kernel as follows in /boot/loader.conf:

kernel="kernel.GENERIC-MORELLO-PURECAP-BENCHMARK"

After a reboot, uname(1) and sysctl(8) should then report that this is the kernel in use:

# uname -i
GENERIC-MORELLO-PURECAP-BENCHMARK
# sysctl kern.bootfile
kern.bootfile: /boot/kernel.GENERIC-MORELLO-PURECAP-BENCHMARK/kernel

Selecting another kernel from the boot loader

It is also possible to interactively select a different kernel from the boot loader on the video or serial console:

    _____ _               _ ____   _____ _____
   / ____| |             (_)  _ \ / ____|  __ \
  | |    | |__   ___ _ __ _| |_) | (___ | |  | |
  | |    | '_ \ / _ \ '__| |  _ < \___ \| |  | |
  | |____| | | |  __/ |  | | |_) |____) | |__| |
   \_____|_| |_|\___|_|  |_|____/|_____/|_____/
                                                 ```                        `
                                                s` `.....---.......--.```   -/
 /---------- Welcome to CheriBSD ----------\    +o   .--`         /y:`      +.
 |                                         |     yo`:.            :o      `+-
 |  1. Boot Multi user [Enter]             |      y/               -/`   -o/
 |  2. Boot Single user                    |     .-                  ::/sy+:.
 |  3. Escape to loader prompt             |     /                     `--  /
 |  4. Reboot                              |    `:                          :`
 |  5. Cons: Serial                        |    `:                          :`
 |                                         |     /                          /
 |  Options:                               |     .-                        -.
 |  6. Kernel: kernel (1 of 6)             |      --                      -.
 |  7. Boot Options                        |       `:`                  `:`
 |                                         |         .--             `--.
 |                                         |            .---.....----.
 \-----------------------------------------/

Use the 6 key to rotate through kernel selections. Once the desired kernel is selected, press the 1 key to proceed with the boot.