Why does Linux on x86 use different segments for user processes and the kernel?

Question

So, I know that Linux uses four default segments for an x86 processor (kernel code, kernel data, user code, user data), but they all have the same base and limit (0x00000000

Answer 1

The x86 memory management architecture uses both segmentation and paging. Very roughly speaking, a segment is a partition of a process's address space that has its own protection policy. So, in the x86 architecture, it is possible to split the range of memory addresses that a process sees into multiple contiguous segments, and assign different protection modes to each. Paging is a technique for mapping small (usually 4KB) regions of a process's address space to chunks of real, physical memory. Paging thus controls how regions inside a segment are mapped onto physical RAM.

All processes have two segments:

1. one segment (addresses 0x00000000 through 0xBFFFFFFF) for user-level, process-specific data such as the program's code, static data, heap, and stack. Every process has its own, independent user segment.

2. one segment (addresses 0xC0000000 through 0xFFFFFFFF), which contains kernel-specific data such as the kernel instructions, data, some stacks on which kernel code can execute, and more interestingly, a region in this segment is directly mapped to physical memory, so that the kernel can directly access physical memory locations without having to worry about address translation. The same kernel segment is mapped into every process, but processes can access it only when executing in protected kernel mode.

So, in user-mode, the process may only access addresses less than 0xC0000000; any access to an address higher than this results in a fault. However, when a user-mode process begins executing in the kernel (for instance, after having made a system call), the protection bit in the CPU is changed to supervisor mode (and some segmentation registers are changed), meaning that the process is thereby able to access addresses above 0xC0000000.

Refer ed from: HERE