Linux kernel notes

refs: learningjourney

Syscalls

refs: s0

int 0x80/iret (legacy syscalls)

linux needs 32-bit emulation parameter CONFIG_IA32_EMULATION it's a slow path to execute a system call. syscall and sysenter should be preferred unless not available.

sysenter/sysexit

you should always use __kernel_vsyscall (vdso) to call sysenter as the convention is likely to change.

kernel must write msr (segment selector, instruction pointer, stack pointer) to specify how sysenter works.

syscall/sysret

syscall loads rip from LSTAR msr register.

virtual dynamic shared object (VDSO)

special memory area that is mapped into the address space of a user-space process at a fixed address. It is used to reduce the cost of making system calls. libc searches for the VDSO and uses it to make system calls if present. VDSO allows for vsyscalls, which are "system calls" that run in user space. examples are gettimeofday and getpid. these are faster than traditional system calls because they don't require a context switch.

uefi

refs: osdev/uefi