Linux內核分析之系統調用

在內核入口函數start_kernel中調用trap_init實現系統調用的初始化工作

1. void __init trap_init(void)
2. {
3. ...
4. set_system_trap_gate(SYSCALL_VECTOR, &system_call);
5. ....
6. }

也就是IDT中0x80用來實現系統調用，實現系統調用的函數為system_call，為匯編實現

1. ENTRY(system_call)
2. RING0_INT_FRAME # can't unwind into user space anyway
3. pushl %eax # save orig_eax
4. CFI_ADJUST_CFA_OFFSET 4
5. SAVE_ALL
6. GET_THREAD_INFO(%ebp)
7. # system call tracing in operation / emulation
8. testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%ebp)
9. jnz syscall_trace_entry
10. cmpl $(nr_syscalls), %eax
11. jae syscall_badsys
12. syscall_call:
13. call *sys_call_table(,%eax,4)/*跳轉到具體的系統調用函數*/
14. movl %eax,PT_EAX(%esp) # store the return value
15. syscall_exit:
16. LOCKDEP_SYS_EXIT
17. DISABLE_INTERRUPTS(CLBR_ANY) # make sure we don't miss an interrupt
18. # setting need_resched or sigpending
19. # between sampling and the iret
20. TRACE_IRQS_OFF
21. movl TI_flags(%ebp), %ecx
22. testl $_TIF_ALLWORK_MASK, %ecx # current->work
23. jne syscall_exit_work

........

具體的系統調用函數存放在sys_call_table表中，在調用system_call具體的系統調用號存放在eax寄存器中。