Syscall
|---------------------------------------------------| |---------------------------------------------------|
| |----------------------| |----------------------| | | |----------------------| |----------------------| |
| | | | | | | | | | | |
| | | | | | | | | | | |
| | read ---------------> read API --------------> syscall ------------> sys_read | |
| | | | | | | | | | | |
| | | | | | | | | | | |
| | | | | | | | | | | |
| |----------------------| |----------------------| | | |----------------------| |----------------------| |
| App C lib | | syscall layer read operation |
|---------------------------------------------------| |---------------------------------------------------|
用户空间 内核空间
除异常和陷入外，它是用户空间访问内核空间的唯一合法入口（dev/proc/sys文件系统均通过syscall实现）。
1 系统调用机制
1） 系统调用号
每个系统调用有唯一且不可回收的系统调用号。
其中arm进入内核态是通过SWI指令（USR-->SVC）实现的， 详见《ARM Architecture Reference Manual》
A2.6.4 Software Interrupt exception(S3C2440 属于ARM9TDMI ARMv4T)。
R14_svc = address of next instruction after the SWI instruction
SPSR_svc = CPSR
CPSR[4:0] = 0b10011 /* Enter Supervisor mode */
CPSR[5] = 0 /* Execute in ARM state */
/* CPSR[6] is unchanged */
CPSR[7] = 1 /* Disable normal interrupts */
/* CPSR[8] is unchanged */
CPSR[9] = CP15_reg1_EEbit
/* Endianness on exception entry */
if high vectors configured then
PC = 0xFFFF0008
else
PC = 0x00000008
To return after performing the SWI operation, use the following instruction to restore the PC
(from R14_svc) and CPSR (from SPSR_svc) and return to the instruction following the SWI:
MOVS PC,R14
arch/arm/kernel/calls.S
/* 0 */ CALL(sys_restart_syscall) CALL(sys_exit) CALL(sys_fork_wrapper) CALL(sys_read) CALL(sys_write) /* 5 */ CALL(sys_open)
2）调用号和参数传递
关于如何传递对应的参数 struct pt_regs *regs，如何从C中调用汇编，会在Compliers中讨论。
arch/arm/include/asm/ptrace.h
/*
* This struct defines the way the registers are stored on the
* stack during a system call. Note that sizeof(struct pt_regs)
* has to be a multiple of 8.
*/
struct pt_regs {
long uregs[18];
};
#define ARM_cpsr uregs[16]
#define ARM_pc uregs[15]
#define ARM_lr uregs[14]
#define ARM_sp uregs[13]
#define ARM_ip uregs[12]
#define ARM_fp uregs[11]
#define ARM_r10 uregs[10]
#define ARM_r9 uregs[9]
#define ARM_r8 uregs[8]
#define ARM_r7 uregs[7]
#define ARM_r6 uregs[6]
#define ARM_r5 uregs[5]
#define ARM_r4 uregs[4]
#define ARM_r3 uregs[3]
#define ARM_r2 uregs[2]
#define ARM_r1 uregs[1]
#define ARM_r0 uregs[0]
#define ARM_ORIG_r0 uregs[17]
arch/arm/kernel/entry-common.S
这个R0是struct pt_regs *regs？自己没看明白，希望哪位大侠告知下 （ATPCS）
entry-header.S (arch\arm\kernel):#define S_OFF 8
sys_fork_wrapper: add r0, sp, #S_OFF b sys_fork
2 系统调用实现（策略）
arch/arm/kernle/sys_arm.c
/* Fork a new task - this creates a new program thread.
* This is called indirectly via a small wrapper
*/
asmlinkage int sys_fork(struct pt_regs *regs)
{
#ifdef CONFIG_MMU
return do_fork(SIGCHLD, regs->ARM_sp, regs, 0, NULL, NULL);
#else
/* can not support in nommu mode */
return(-EINVAL);
#endif
}
kernel/fork.c
/*
* Ok, this is the main fork-routine.
*
* It copies the process, and if successful kick-starts
* it and waits for it to finish using the VM if required.
*/
long do_fork(unsigned long clone_flags,
unsigned long stack_start,
struct pt_regs *regs,
unsigned long stack_size,
int __user *parent_tidptr,
int __user *child_tidptr)
{
struct task_struct *p;
int trace = 0;
long nr;
/*
* Do some preliminary argument and permissions checking before we
* actually start allocating stuff
*/
if (clone_flags & CLONE_NEWUSER) {
if (clone_flags & CLONE_THREAD)
return -EINVAL;
/* hopefully this check will go away when userns support is
* complete
*/
if (!capable(CAP_SYS_ADMIN) || !capable(CAP_SETUID) ||
!capable(CAP_SETGID))
return -EPERM;
}
/*
* We hope to recycle these flags after 2.6.26
*/
if (unlikely(clone_flags & CLONE_STOPPED)) {
static int __read_mostly count = 100;
if (count > 0 && printk_ratelimit()) {
char comm[TASK_COMM_LEN];
count--;
printk(KERN_INFO "fork(): process `%s' used deprecated "
"clone flags 0x%lx\n",
get_task_comm(comm, current),
clone_flags & CLONE_STOPPED);
}
}
/*
* When called from kernel_thread, don't do user tracing stuff.
*/
if (likely(user_mode(regs)))
trace = tracehook_prepare_clone(clone_flags);
p = copy_process(clone_flags, stack_start, regs, stack_size,
child_tidptr, NULL, trace);
/*
* Do this prior waking up the new thread - the thread pointer
* might get invalid after that point, if the thread exits quickly.
*/
if (!IS_ERR(p)) {
struct completion vfork;
trace_sched_process_fork(current, p);
nr = task_pid_vnr(p);
if (clone_flags & CLONE_PARENT_SETTID)
put_user(nr, parent_tidptr);
if (clone_flags & CLONE_VFORK) {
p->vfork_done = &vfork;
init_completion(&vfork);
}
audit_finish_fork(p);
tracehook_report_clone(regs, clone_flags, nr, p);
/*
* We set PF_STARTING at creation in case tracing wants to
* use this to distinguish a fully live task from one that
* hasn't gotten to tracehook_report_clone() yet. Now we
* clear it and set the child going.
*/
p->flags &= ~PF_STARTING;
if (unlikely(clone_flags & CLONE_STOPPED)) {
/*
* We'll start up with an immediate SIGSTOP.
*/
sigaddset(&p->pending.signal, SIGSTOP);
set_tsk_thread_flag(p, TIF_SIGPENDING);
__set_task_state(p, TASK_STOPPED);
} else {
wake_up_new_task(p, clone_flags);
}
tracehook_report_clone_complete(trace, regs,
clone_flags, nr, p);
if (clone_flags & CLONE_VFORK) {
freezer_do_not_count();
wait_for_completion(&vfork);
freezer_count();
tracehook_report_vfork_done(p, nr);
}
} else {
nr = PTR_ERR(p);
}
return nr;
}
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