ARM system call

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本文详细阐述了Linux内核如何通过系统调用接口与用户空间进行交互，包括系统调用向量的定义、内核初始化时如何设置系统调用表，以及特殊系统调用的实现方式。特别强调了系统调用表中的特殊系统调用包装器及其工作原理，同时介绍了中断处理程序如何根据系统调用编号调用相应的系统调用处理函数。

1. The system call vector numbers are defined in arch/arm/include/asm/unistd.h

#define __NR_restart_syscall(__NR_SYSCALL_BASE+ 0)
#define __NR_exit (__NR_SYSCALL_BASE+ 1)
#define __NR_fork (__NR_SYSCALL_BASE+ 2)
#define __NR_read (__NR_SYSCALL_BASE+ 3)

...

2. Inarch/arm/kernel/entry-common.S, defined a variable, sys_call_table, then put syscall handlers' address (which defined inarch/arm/kernel/calls.S) here:

.type sys_call_table, #object
ENTRY(sys_call_table)
#include "calls.S"

...

/* Special system call wrappers*/

...

sys_mmap2:

#if PAGE_SHIFT > 12

...

#else

str r5, [sp, #4]

b do_mmap2

#endif

ENDPROC(sys_mmap2)

Be noted:

A. each entry of the sys_call_table is the syscall handler's address.

B. Some system call handler (the wrappers) are defined in this file.

3. Most of the syscall routines are declared ininclude/linux/syscalls.h and defined in separate modules. For several special syscalls, for example, sys_mmap2(), system defines wrappers for them just below the sys_call_table. See above. That is, the system call, mmap2's handler is just defined in this assembly source file, and it calls do_mmap2().

3. Also the swi software interrupt handler is defined inarch/arm/kernel/entry-common.S. It's a quite long routine but basically, it does following:

ENTRY(vector_swi)
Get syscall number

...
enable_irq
...
ldrcc pc, [tbl, scno, lsl #2]@ call sys_* routine
ENDPROC(vector_swi)