arm64之系统调用原理
异常入口
应用调用系统调用后，cpu进入异常模式。内核根据异常类型以及异常向量表里面的地址，调用同步异常处理例程
/arch/arm64/kernel/entry.S
/*
* Exception vectors.
*/
.align 11
ENTRY(vectors)
ventry el1_sync_invalid // Synchronous EL1t
ventry el1_irq_invalid // IRQ EL1t
ventry el1_fiq_invalid // FIQ EL1t
ventry el1_error_invalid // Error EL1t
ventry el1_sync // Synchronous EL1h
ventry el1_irq // IRQ EL1h
ventry el1_fiq_invalid // FIQ EL1h
ventry el1_error_invalid // Error EL1h
ventry el0_sync // Synchronous 64-bit EL0
ventry el0_irq // IRQ 64-bit EL0
ventry el0_fiq_invalid // FIQ 64-bit EL0
ventry el0_error_invalid // Error 64-bit EL0
#ifdef CONFIG_COMPAT
ventry el0_sync_compat // Synchronous 32-bit EL0
ventry el0_irq_compat // IRQ 32-bit EL0
ventry el0_fiq_invalid_compat // FIQ 32-bit EL0
ventry el0_error_invalid_compat // Error 32-bit EL0
#else
ventry el0_sync_invalid // Synchronous 32-bit EL0
ventry el0_irq_invalid // IRQ 32-bit EL0
ventry el0_fiq_invalid // FIQ 32-bit EL0
ventry el0_error_invalid // Error 32-bit EL0
#endif
END(vectors)系统调用handler
读取异常特征寄存器，判断当前是那种异常，是系统调用就就进入el0_svc
/*
* EL0 mode handlers.
*/
.align 6
el0_sync:
kernel_entry 0
mrs x25, esr_el1 // read the syndrome register
lsr x24, x25, #ESR_EL1_EC_SHIFT // exception class
cmp x24, #ESR_EL1_EC_SVC64 // SVC in 64-bit state
b.eq el0_svc
adr lr, ret_from_exception
cmp x24, #ESR_EL1_EC_DABT_EL0 // data abort in EL0
b.eq el0_da
cmp x24, #ESR_EL1_EC_IABT_EL0 // instruction abort in EL0
b.eq el0_ia
cmp x24, #ESR_EL1_EC_FP_ASIMD // FP/ASIMD access
b.eq el0_fpsimd_acc
cmp x24, #ESR_EL1_EC_FP_EXC64 // FP/ASIMD exception
b.eq el0_fpsimd_exc
cmp x24, #ESR_EL1_EC_SYS64 // configurable trap
b.eq el0_undef
cmp x24, #ESR_EL1_EC_SP_ALIGN // stack alignment exception
b.eq el0_sp_pc
cmp x24, #ESR_EL1_EC_PC_ALIGN // pc alignment exception
b.eq el0_sp_pc
cmp x24, #ESR_EL1_EC_UNKNOWN // unknown exception in EL0
b.eq el0_undef
cmp x24, #ESR_EL1_EC_BREAKPT_EL0 // debug exception in EL0
b.ge el0_dbg
b el0_inv
#ifdef CONFIG_COMPAT
.align 6EL0_SVC
每句后面的英文注释很直接了当了，比如从上层调用open系统调用打开文件时，就会从从sys_call_table，根据系统调用号，找到对应的sys_call_table元素，也即sys_open；并执行
el0_svc:
adrp stbl, sys_call_table // load syscall table pointer
uxtw scno, w8 // syscall number in w8
mov sc_nr, #__NR_syscalls
el0_svc_naked: // compat entry point
stp x0, scno, [sp, #S_ORIG_X0] // save the original x0 and syscall number
disable_step x16
isb
enable_dbg
enable_irq
get_thread_info tsk
ldr x16, [tsk, #TI_FLAGS] // check for syscall tracing
tbnz x16, #TIF_SYSCALL_TRACE, __sys_trace // are we tracing syscalls?
adr lr, ret_fast_syscall // return address
cmp scno, sc_nr // check upper syscall limit
b.hs ni_sys
ldr x16, [stbl, scno, lsl #3] // address in the syscall table
br x16 // call sys_* routine
ni_sys:
mov x0, sp
b do_ni_syscall
ENDPROC(el0_svc)sys_call_table
sys_call_table作为系统调用号就是数组index,元素就是系统调用函数
/arch/arm64/kernel/sys.c
#include
#undef __SYSCALL
#define __SYSCALL(nr, sym) [nr] = sym,
/*
* The sys_call_table array must be 4K aligned to be accessible from
* kernel/entry.S.
*/
void *sys_call_table[__NR_syscalls] __aligned(4096) = {
[0 ... __NR_syscalls - 1] = sys_ni_syscall,
#include
} 上述asm/unistd.h里的内容作为sys_call_table数组的元素
include/uapi/asm-generic/unistd.h
#ifdef __ARCH_WANT_SYSCALL_NO_AT
#define __NR_open 1024
__SYSCALL(__NR_open, sys_open)
#define __NR_link 1025
__SYSCALL(__NR_link, sys_link)
#define __NR_unlink 1026
__SYSCALL(__NR_unlink, sys_unlink)
#define __NR_mknod 1027
__SYSCALL(__NR_mknod, sys_mknod)
#define __NR_chmod 1028
__SYSCALL(__NR_chmod, sys_chmod)
#define __NR_chown 1029
__SYSCALL(__NR_chown, sys_chown)
#define __NR_mkdir 1030
__SYSCALL(__NR_mkdir, sys_mkdir)
#define __NR_rmdir 1031
__SYSCALL(__NR_rmdir, sys_rmdir)系统调用定义
SYSCALL_DEFINE3(open，***）展开就是 sys_open，就是系统调用的内核实例
fs/open.c
long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
{
struct open_flags op;
int fd = build_open_flags(flags, mode, &op);
struct filename *tmp;
if (fd)
return fd;
tmp = getname(filename);
if (IS_ERR(tmp))
return PTR_ERR(tmp);
fd = get_unused_fd_flags(flags);
if (fd >= 0) {
struct file *f = do_filp_open(dfd, tmp, &op);
if (IS_ERR(f)) {
put_unused_fd(fd);
fd = PTR_ERR(f);
} else {
fsnotify_open(f);
fd_install(fd, f);
}
}
putname(tmp);
return fd;
}
SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
{
if (force_o_largefile())
flags |= O_LARGEFILE;
return do_sys_open(AT_FDCWD, filename, flags, mode);
}添加一个系统调用
步骤很简单，1.声明系统调用；2.绑定系统调用号跟声明；3.定义系统调用
Index: include/linux/syscalls.h
===================================================================
--- include/linux/syscalls.h (revision 2449)
+++ include/linux/syscalls.h (working copy)
@@ -940,5 +940,5 @@
asmlinkage long sys_pkey_free(int pkey);
asmlinkage long sys_statx(int dfd, const char __user *path, unsigned flags,
unsigned mask, struct statx __user *buffer);
-
+asmlinkage long sys_my_test(void);
#endif
//
Index: include/uapi/asm-generic/unistd.h
===================================================================
--- include/uapi/asm-generic/unistd.h (revision 2449)
+++ include/uapi/asm-generic/unistd.h (working copy)
@@ -732,9 +732,11 @@
__SYSCALL(__NR_pkey_free, sys_pkey_free)
#define __NR_statx 291
__SYSCALL(__NR_statx, sys_statx)
+#define __NR_my_test 292
+__SYSCALL(__NR_my_test, sys_my_test)
#undef __NR_syscalls
-#define __NR_syscalls 292
+#define __NR_syscalls 293
//
Index: kernel/sys.c
===================================================================
--- kernel/sys.c (revision 2449)
+++ kernel/sys.c (working copy)
@@ -2504,6 +2504,13 @@
return 0;
}
+SYSCALL_DEFINE0(my_test)
+{
+ printk("test syscall\n");
+ return 0;
+}
+
+
#ifdef CONFIG_COMPAT
struct compat_sysinfo {
s32 uptime;