Swap Kernel on Rocky 9

• Swap Kernel on Rocky 9
  • Build the Kernel
  • Create RPM Package for It
  • Research
    • What is vmlinuz
      • Where to Find vmlinuz
    • What is the System Map
      • Purpose and Usage
      • Location
      • Security Considerations
      • Updating and Managing

Build the Kernel

Pull the Linux kernel from here

# Pull in Rocky's existing config
cp -f /boot/config-$(uname -r) .config

# This accepts all the defaults for the new kernel options
yes "" | make -j$(nproc --all) oldconfig &&
make -j$(nproc) &&
make -j$(nproc) modules &&
make -j$(nproc) modules_install &&
make -j$(nproc) install &&
grub2-mkconfig -o /boot/efi/EFI/rocky/grub.cfg
# sudo dnf reinstall grub2-efi-x64 shim-x64

CERTIFICATE ERROR If you get a certificate error you need to get rhel.pem from the kernel source. Do this: https://unix.stackexchange.com/a/294116/240147

RANDOM ERRORS I also had some random errors I think were due to race conditions. I just dropped it down to make -j 10 and it worked.

Create RPM Package for It

• vim .config - go to CONFIG_LOCALVERSION AND ADD -grant-nvme

sudo dnf -y install rpm-build redhat-rpm-config make gcc &&
rm -rf ~/rpmbuild 6.6.16-grant-nvme &&
mkdir -p ~/rpmbuild/{BUILD,BUILDROOT,RPMS,SOURCES,SPECS,SRPMS} &&
mkdir 6.6.16-grant-nvme/ &&
cd 6.6.16-grant-nvme/ &&
cp /boot/vmlinuz-6.6.16-grant-nvme vmlinuz-6.6.16-grant-nvme &&
cp /boot/System.map-6.6.16-grant-nvme System.map-6.6.16-grant-nvme &&
cp ~/linux-6.6.16/.config config-6.6.16-grant-nvme &&
cp -R /lib/modules/6.6.16-grant-nvme modules &&
tar --use-compress-program=pigz -cvf ../6.6.16-grant-nvme.tar.gz . &&
cd .. &&
mv 6.6.16-grant-nvme.tar.gz ~/rpmbuild/SOURCES/ &&
vim ~/rpmbuild/SPECS/6.6.16-grant-nvme.spec

Paste the following content and save it.

Summary: Custom Kernel Package with NVMe Debug Driver
Name: kernel-grant-nvme
Version: 6.6.16
Release: 1
License: GPL
Group: System Environment/Kernel
Source: 6.6.16-grant-nvme.tar.gz
BuildRoot: %{_tmppath}/%{name}-%{version}-%{release}-root

# Disable automatic debuginfo and debugsource package generation
%global debug_package %{nil}

%description
This package provides a custom Linux kernel version 6.6.16-grant-nvme, specifically built for debugging and enhancing NVMe support.

%prep
%setup -c -T -n 6.6.16-grant-nvme
pigz -dc %{_sourcedir}/6.6.16-grant-nvme.tar.gz | tar -xf -

%build
# Since we're packaging a precompiled kernel, no build commands are necessary

%install
rm -rf $RPM_BUILD_ROOT
mkdir -p $RPM_BUILD_ROOT/boot
mkdir -p $RPM_BUILD_ROOT/lib/modules/6.6.16-grant-nvme
mkdir -p $RPM_BUILD_ROOT/boot/loader/entries
cp vmlinuz-6.6.16-grant-nvme $RPM_BUILD_ROOT/boot/
cp System.map-6.6.16-grant-nvme $RPM_BUILD_ROOT/boot/
cp config-6.6.16-grant-nvme $RPM_BUILD_ROOT/boot/
cp -a modules/* $RPM_BUILD_ROOT/lib/modules/6.6.16-grant-nvme/
dracut --force $RPM_BUILD_ROOT/boot/initramfs-6.6.16-grant-nvme.img 6.6.16-grant-nvme

%files
/boot/vmlinuz-6.6.16-grant-nvme
/boot/System.map-6.6.16-grant-nvme
/boot/config-6.6.16-grant-nvme
/boot/initramfs-6.6.16-grant-nvme.img
/lib/modules/6.6.16-grant-nvme/*

%clean
rm -rf $RPM_BUILD_ROOT

%post
# Create the BLS snippet for the custom kernel
# Note: No /boot prefix, as BLS expects paths relative to the boot partition
cat > /boot/loader/entries/6.6.16-grant-nvme.conf << EOF
title Custom Kernel 6.6.16 with NVMe Debug
version 6.6.16-grant-nvme
linux /vmlinuz-6.6.16-grant-nvme
initrd /initramfs-6.6.16-grant-nvme.img
options root=/dev/mapper/rl-root ro crashkernel=1G-4G:192M,4G-64G:256M,64G-:512M resume=/dev/mapper/rl-swap rd.lvm.lv=rl/root rd.lvm.lv=rl/swap rhgb quiet
EOF

# Regenerate the GRUB2 configuration to pick up the new BLS entry
grub2-mkconfig -o /boot/efi/EFI/rocky/grub.cfg

%postun
if [ $1 -eq 0 ]; then
    # This is a full uninstall
    rm -f /boot/loader/entries/6.6.16-grant-nvme.conf
    # It is not recommended to automatically regenerate GRUB config in %postun
    # because it can leave the system unbootable if something goes wrong.
    # Instead, we inform the user to do this manually.
    echo "Please run 'grub2-mkconfig -o /boot/efi/EFI/rocky/grub.cfg' to update your GRUB configuration."
fi

Next run these commands:

rpmbuild -ba ~/rpmbuild/SPECS/6.6.16-grant-nvme.spec

Research

What is vmlinuz

vmlinuz is the name of the Linux kernel executable. It’s a compressed, bootable image of the kernel. When the system boots, this is the file loaded by the bootloader (GRUB) into memory to start the Linux operating system. The “vm” in vmlinuz stands for Virtual Memory, highlighting the kernel’s capability to manage hardware virtual memory. The “linuz” part is a play on Linux, with “z” indicating it’s compressed (historically, z stood for zlib compression, but modern kernels might use other compression methods like LZMA).

Where to Find vmlinuz

On Rocky Linux, vmlinuz is located in the /boot directory. The full path is typically /boot/vmlinuz-$(uname -r), where $(uname -r) is replaced with the version of the kernel currently running. For instance, if you’re running kernel version 5.11.0, the path would likely be /boot/vmlinuz-5.11.0.

When you compile a new kernel from source, the vmlinuz file for your new kernel needs to be manually placed in the /boot directory (or wherever your distribution expects kernel images) and then referenced in your bootloader’s configuration so that it can be loaded at boot time.

What is the System Map

The System Map, often referred to as System.map, is a file generated during the compilation of the Linux kernel. This file maps the memory addresses to the symbol names within the kernel. It’s essentially a lookup table that contains the addresses of various functions, variables, and data structures used by the kernel.

Purpose and Usage

• Debugging and Diagnostic Tools: The System Map is crucial for kernel developers and system administrators for debugging purposes. Tools like kdump, kprobe, and SystemTap, as well as many others, use the System Map to translate memory addresses into human-readable symbol names when analyzing kernel crashes or behavior.
• Security: The addresses of kernel symbols can vary between different builds of the kernel due to features like Kernel Address Space Layout Randomization (KASLR), which enhances system security by randomizing the memory address at which kernel code is loaded. However, the System Map can be used to map these addresses to specific functions or variables, making it a sensitive file in terms of system security.
• Kernel Module Development: Developers working on kernel modules may refer to the System Map to understand kernel internals, such as function addresses they wish to hook or modify.

Location

The System Map file is typically located in the /boot directory alongside the kernel it corresponds to. For example, if you’re running a kernel version 5.11.0, you might find it at /boot/System.map-5.11.0. The exact path can vary depending on your distribution’s configuration.

When installing a new kernel, the corresponding System Map file is also installed to ensure that tools and operations requiring symbol resolution can function correctly with the new kernel.

Security Considerations

Given its detailed insight into the kernel’s memory layout, the System Map file is considered sensitive. Unauthorized access to this file could potentially help an attacker understand the kernel’s structure, making it easier to exploit vulnerabilities. Therefore, access to the System Map is usually restricted to root or users with specific permissions.

Updating and Managing

When you compile and install a new kernel, a new System Map file is generated and should be installed in the appropriate location (/boot, for most systems). Managing multiple kernels involves keeping the corresponding System Map files organized and correctly named, ensuring that diagnostic tools can automatically find and use the correct System Map file for the currently running kernel.