sys_xxx都是什么玩意儿？

sys_write的声明

所有的sys_xxx都在linux/include/linux/syscalls.h，例如sys_write。

asmlinkage long sys_write(unsigned int fd, const char __user *buf, size_t count);

write的定义

定义不太容易直接找到，这里直接给出位置linux/fs/read_write.c。

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf, size_t, count)
{
    ...
}

展开write定义

声明sys_write，却定义write，它们之间有什么联系呢？下面根据linux/include/linux/syscalls.h、linux/include/linux/stringify.h和linux/include/linux/linkage.h里面的宏对其进行展开。

SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf, size_t, count)

======>

SYSCALL_DEFINEx(3, _write, unsigned int, fd, const char __user *, buf, size_t, count)

======>

SYSCALL_METADATA(_write, 3, unsigned int, fd, const char __user *, buf, size_t, count)  \
__SYSCALL_DEFINEx(3, sname, unsigned int, fd, const char __user *, buf, size_t, count)

defined(CONFIG_FTRACE_SYSCALLS) = 1 ======>

    asmlinkage long sys_write(__MAP(3,__SC_DECL,unsigned int, fd, const char __user *, buf, size_t, count)) \
        __attribute__((alias(__stringify(SyS_write))));     \
    static inline long SYSC_write(__MAP(3,__SC_DECL,unsigned int, fd, const char __user *, buf, size_t, count));    \
    asmlinkage long SyS_write(__MAP(3,__SC_LONG,unsigned int, fd, const char __user *, buf, size_t, count));    \
    asmlinkage long SyS_write(__MAP(3,__SC_LONG,unsigned int, fd, const char __user *, buf, size_t, count)) \
    {                               \
        long ret = SYSC_write(__MAP(3,__SC_CAST,unsigned int, fd, const char __user *, buf, size_t, count));    \
        __MAP(3,__SC_TEST,unsigned int, fd, const char __user *, buf, size_t, count);               \
        __PROTECT(3, ret,__MAP(3,__SC_ARGS,unsigned int, fd, const char __user *, buf, size_t, count)); \
        return ret;                     \
    }                               \
    static inline long SYSC_write(__MAP(3,__SC_DECL,unsigned int, fd, const char __user *, buf, size_t, count))

省略一些不关注内容======>

    asmlinkage long sys_write(__MAP3(__SC_DECL,unsigned int, fd, const char __user *, buf, size_t, count))  \
        __attribute__((alias("SyS_write")));        \
    static inline long SYSC_write(__MAP3(__SC_DECL,unsigned int, fd, const char __user *, buf, size_t, count)); \
    asmlinkage long SyS_write(__MAP3(__SC_LONG,unsigned int, fd, const char __user *, buf, size_t, count)); \
    asmlinkage long SyS_write(__MAP3(__SC_LONG,unsigned int, fd, const char __user *, buf, size_t, count))  \
    {                               \
        return SYSC_write(__MAP3(__SC_CAST,unsigned int, fd, const char __user *, buf, size_t, count)); \                       \
    }                               \
    static inline long SYSC_write(__MAP3(__SC_DECL,unsigned int, fd, const char __user *, buf, size_t, count))

省略一些不关注内容======>

    asmlinkage long sys_write(unsigned int fd, const char __user * buf, size_t count)   \
        __attribute__((alias("SyS_write")));        \
    static inline long SYSC_write(unsigned int fd, const char __user * buf, size_t count);  \
    asmlinkage long SyS_write(unsigned int fd, const char __user * buf, size_t count);  \
    asmlinkage long SyS_write(unsigned int fd, const char __user * buf, size_t count)   \
    {                               \
        return SYSC_write(unsigned int fd, const char __user * buf, size_t count);  \
    }                               \
    static inline long SYSC_write(unsigned int fd, const char __user * buf, size_t count)

加上函数体，最终结果就是

    asmlinkage long sys_write(unsigned int fd, const char __user * buf, size_t count)   \
        __attribute__((alias("SyS_write")));        \
    static inline long SYSC_write(unsigned int fd, const char __user * buf, size_t count);  \
    asmlinkage long SyS_write(unsigned int fd, const char __user * buf, size_t count);  \
    asmlinkage long SyS_write(unsigned int fd, const char __user * buf, size_t count)   \
    {                               \
        return SYSC_write(unsigned int fd, const char __user * buf, size_t count);  \
    }                               \
    static inline long SYSC_write(unsigned int fd, const char __user * buf, size_t count)
    {
        ......
    }

因此实际上调用关系是sys_write(SyS_write)=>SYSC_write

内核为什么要整这个套路呢？