kernel stack vs user-mode application stack

Question

Is the kernel stack a different structure to the user-mode stack that is used by applications we (programmers) write?

Can you explain the differences?

Answer 1

Conceptually, both are the same data structure: a stack.

The reason why there are two different stack per thread is because in user mode, code must not be allowed to mess up kernel memory. When switching to kernel mode, a different stack in memory only accessible in kernel mode is used for return addresses an so on.

If the user mode had access to the kernel stack, it could modify a jump address (for instance), then do a system call; when the kernel jumps to the previously modified address, your code is executed in kernel mode!

Also, security-related information/information about other processes (for synchronisation) might be on the kernel stack, so the user mode should not have read access to it either.

Answer 2

The stack of a typical modern operating system is just a region of memory used for storing return addresses and local data. It has the same structure in both the kernel and user-mode, but each thread gets its own memory area for storing its stack. Context switches restore the stack pointer, so no thread sees another thread's stack even though they may be able to share other memory (if the threads are in the same process).

A thread doesn't have to use the stack by the way. The operating system makes assumptions about how it will be used, but the thread doesn't have to follow them.