Patches or kernel modules were installed on a system but the initrd file wasn't updated to reflect that properly. The /boot or root filesystem changed locations and the initrd file can't find them. The /boot or root file system can't be mounted due to a file system issue or a corrupted initrd file issue. To boil it down a little more, a kernel panic can occur because: When the initrd can't mount the boot or root partitions, or the Linux kernel can't load properly, this is when a kernel panic occurs. The TLDR Version of Why Kernel Panics Happen A kernel panic occurs because something goes wrong with this part of the boot process. Once those partitions are mounted, the initrd file starts loading the Linux kernel and the rest of the OS. The boot partition contains the Linux kernel. When the initrd file is mounted, that file then mounts key system partitions like the root filesystem and the boot partition. That file is like a tiny virtual hard drive that contains enough files and information to start the rest of the boot loader process. When the MBR calls the Grub2 bootloader, it mounts a file called initrd or initramfs (depending on the Linux OS). So, when the MBR calls the Grub2 bootloader, it is instead calling another bit of information that holds the instructions to start Grub2 and not Grub2 itself. It is too large to fit entirely into the MBR partitions because of that. Grub2 is a Linux bootloader capable of working with various types of hardware (including non-x86 based architectures) and filesystems. Most major Linux distributions have migrated to using Grub2 as their default bootloader. Taking a quick detour, the most common bootloader for Linux today is Grub2. This is especially true with multi-OS environments. In the world of modern OSes, it is not large enough to hold the entire bootloader needed to start the OS. That MBR partition is incredibly small, though. Traditionally the MBR would contain the information and instructions for calling the files needed to start the OS. The MBR is a small partition at the beginning of the primary storage drive in a computer. When the BIOS/UEFI passes priority to the bootloader, it calls the MBR, or master boot record. The job of the BIOS/UEFI is to enumerate the hardware, make sure everything on the computer is in working order, and then pass priority to the bootloader to startup the OS installed on the storage drives of that computer. When a computer first turns on, the BIOS or UEFI takes control. The following is the Makefile.The boot process for Linux occurs throughout multiple steps. *killer = 1 // it puts the value 1 in the memory area pointed to, which is NULL, i.e., nonexistent MODULE_LICENSE("GPL") // required by the compilerĬhar *killer = NULL // this is the pointer with NULL value The null-pointer-dereference.c module will generate the NULL pointer dereference error: #include Let’s create a kernelnpd folder in our home and put the following null-pointer-dereference.c and Makefile files in it. To tell the kernel to panic immediately, let’s set kernel.panic_on_oops to 1: # echo "1" > /proc/sys/kernel/panic_on_oops In this scenario, system reliability is compromised. In this case, 0 means that the kernel will attempt to continue operation when it encounters an oops, which is a serious but not fatal error. Let’s inspect kernel.panic_on_oops: $ sysctl kernel.panic_on_oops In kernel modules, it’s a common programming error and will cause a kernel panic if the value of /proc/sys/kernel/ panic_on_oops is 1. A pointer with a NULL (zero) value doesn’t point to a valid memory location, so dereferencing it results in an invalid memory access. Dereferencing a pointer means accessing the value to which the pointer points. In C, a pointer is a variable that stores the address of another variable.
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