目录
0. 引言 1. open() syscall 2. close() syscall
0. 引言
在linux的哲学中，所有的磁盘文件、目录、外设设备、驱动设备全部被抽象为了"文件"这个概念，所以本文提到的"File IO"适用于linux下所有的IO操作，需要明白的的，本文分析的是linux下的IO系统调用对应的内核源代码，linux下每一个系统调用都有对应的内核源代码，而我们在ring3常用的glib c的编程所有的c库API，它们只是对系统调用的一个封装，最终还是要通过系统调用实现功能
0x1: SYSCALL_DEFINE宏定义
我们在学习内核源代码的时候经常会遇到一个宏定义: SYSCALL_DEFINE，所有的系统调用的声明都通过它来实现
linux-2.6.32.63includelinuxsyscalls.h
#define SYSCALL_DEFINE0(sname) SYSCALL_TRACE_ENTER_EVENT(_##sname); SYSCALL_TRACE_EXIT_EVENT(_##sname); static const struct syscall_metadata __used __attribute__((__aligned__(4))) __attribute__((section("__syscalls_metadata"))) __syscall_meta_##sname = { .name = "sys_"#sname, .nb_args = 0, .enter_event = &event_enter__##sname, .exit_event = &event_exit__##sname, }; asmlinkage long sys_##sname(void) #else #define SYSCALL_DEFINE0(name) asmlinkage long sys_##name(void) #endif #define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__) #define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__) #define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__) #define SYSCALL_DEFINE4(name, ...) SYSCALL_DEFINEx(4, _##name, __VA_ARGS__) #define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__) #define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)
...
#ifdef CONFIG_FTRACE_SYSCALLS #define SYSCALL_DEFINEx(x, sname, ...) static const char *types_##sname[] = { __SC_STR_TDECL##x(__VA_ARGS__) }; static const char *args_##sname[] = { __SC_STR_ADECL##x(__VA_ARGS__) }; SYSCALL_METADATA(sname, x); __SYSCALL_DEFINEx(x, sname, __VA_ARGS__) #else #define SYSCALL_DEFINEx(x, sname, ...) __SYSCALL_DEFINEx(x, sname, __VA_ARGS__) #endif #ifdef CONFIG_HAVE_SYSCALL_WRAPPERS #define SYSCALL_DEFINE(name) static inline long SYSC_##name #define __SYSCALL_DEFINEx(x, name, ...) asmlinkage long sys##name(__SC_DECL##x(__VA_ARGS__)); static inline long SYSC##name(__SC_DECL##x(__VA_ARGS__)); asmlinkage long SyS##name(__SC_LONG##x(__VA_ARGS__)) { __SC_TEST##x(__VA_ARGS__); return (long) SYSC##name(__SC_CAST##x(__VA_ARGS__)); } SYSCALL_ALIAS(sys##name, SyS##name); static inline long SYSC##name(__SC_DECL##x(__VA_ARGS__)) #else /* CONFIG_HAVE_SYSCALL_WRAPPERS */ #define SYSCALL_DEFINE(name) asmlinkage long sys_##name #define __SYSCALL_DEFINEx(x, name, ...) asmlinkage long sys##name(__SC_DECL##x(__VA_ARGS__)) #endif /* CONFIG_HAVE_SYSCALL_WRAPPERS */
所以对函数定义
SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol)就等于
asmlinkage long sys_socket(int family, int type, int protocol)
Relevant Link:
http://blog.csdn.net/p_panyuch/article/details/5648007
1. open() syscall
open()系统调用在kernel中对应的是sys_open()
linux-2.6.32.63fsopen.c
SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode) { long ret; if (force_o_largefile()) { flags |= O_LARGEFILE; } //调用do_sys_open完成实际功能 ret = do_sys_open(AT_FDCWD, filename, flags, mode); /* avoid REGPARM breakage on x86: */ asmlinkage_protect(3, ret, filename, flags, mode); return ret; }
继续跟进do_sys_open()函数
long do_sys_open(int dfd, const char __user *filename, int flags, int mode) { /*获取文件名称，由getname()函数完成，其内部首先创建存取文件名称的空间，然后从用户空间把文件名拷贝过来*/ char *tmp = getname(filename); int fd = PTR_ERR(tmp); if (!IS_ERR(tmp)) { /*获取一个可用的fd，此函数调用alloc_fd()函数从fd_table中获取一个可用fd,并进行初始化*/ fd = get_unused_fd_flags(flags); if (fd >= 0) { /*fd获取成功则开始打开文件，此函数是主要完成打开功能的函数*/ struct file *f = do_filp_open(dfd, tmp, flags, mode, 0); if (IS_ERR(f)) { /*打开失败，释放fd*/ put_unused_fd(fd); fd = PTR_ERR(f); } else { //文件如果已经被打开了，调用fsnotify_open()函数 fsnotify_open(f->f_path.dentry); //将文件指针安装在fd数组中，每个进程都会将打开的文件句柄保存在fd_array[]数组中 fd_install(fd, f); } } //释放放置从用户空间拷贝过来的文件名的存储空间 putname(tmp); } return fd; }
继续跟进do_file_open()函数
/* * Note that the low bits of the passed in "open_flag" * are not the same as in the local variable "flag". See * open_to_namei_flags() for more details. */ struct file *do_filp_open(int dfd, const char *pathname, int open_flag, int mode, int acc_mode) { /* 若干变量声明 */ struct file *filp; struct nameidata nd; int error; struct path path; struct dentry *dir; int count = 0; int will_write; /*改变参数flag的值，具体做法是flag+1*/ int flag = open_to_namei_flags(open_flag); /*设置访问权限*/ if (!acc_mode) { acc_mode = MAY_OPEN | ACC_MODE(flag); } /* O_TRUNC implies we need access checks for write permissions */ /* 根据O_TRUNC标志设置写权限 */ if (flag & O_TRUNC) { acc_mode |= MAY_WRITE; } /* Allow the LSM permission hook to distinguish append access from general write access. */ /* 设置O_APPEND标志 */ if (flag & O_APPEND) { acc_mode |= MAY_APPEND; } /* The simplest case - just a plain lookup. */ /* 如果不是创建文件 */ if (!(flag & O_CREAT)) { /* 当内核要访问一个文件的时候，第一步要做的是找到这个文件，而查找文件的过程在vfs里面是由path_lookup或者path_lookup_open函数来完成的 这两个函数将用户传进来的字符串表示的文件路径转换成一个dentry结构，并建立好相应的inode和file结构，将指向file的描述符返回用户 用户随后通过文件描述符，来访问这些数据结构 */ error = path_lookup_open(dfd, pathname, lookup_flags(flag), &nd, flag); if (error) { return ERR_PTR(error); } goto ok; } /* * Create - we need to know the parent. */ //path-init为查找作准备工作，path_walk真正上路查找，这两个函数联合起来根据一段路径名找到对应的dentry error = path_init(dfd, pathname, LOOKUP_PARENT, &nd); if (error) { return ERR_PTR(error); } /* 这个函数相当重要，是整个NFS的名字解析函数，其实也是NFS得以构筑的函数 该函数采用一个for循环，对name路径根据目录的层次，一层一层推进，直到终点或失败。在推进的过程中，一步步建立了目录树的dentry和对应的inode */ error = path_walk(pathname, &nd); if (error) { if (nd.root.mnt) { /*减少dentry和vsmount得计数*/ path_put(&nd.root); } return ERR_PTR(error); } if (unlikely(!audit_dummy_context())) { /*保存inode节点信息*/ audit_inode(pathname, nd.path.dentry); } /* * We have the parent and last component. First of all, check * that we are not asked to creat(2) an obvious directory - that * will not do. */ error = -EISDIR; /*父节点信息*/ if (nd.last_type != LAST_NORM || nd.last.name[nd.last.len]) { goto exit_parent; } error = -ENFILE; /* 返回特定的file结构体指针 */ filp = get_empty_filp(); if (filp == NULL) { goto exit_parent; } /* 填充nameidata结构 */ nd.intent.open.file = filp; nd.intent.open.flags = flag; nd.intent.open.create_mode = mode; dir = nd.path.dentry; nd.flags &= ~LOOKUP_PARENT; nd.flags |= LOOKUP_CREATE | LOOKUP_OPEN; if (flag & O_EXCL) { nd.flags |= LOOKUP_EXCL; } mutex_lock(&dir->d_inode->i_mutex); /*从哈希表中查找nd对应的dentry*/ path.dentry = lookup_hash(&nd); path.mnt = nd.path.mnt; do_last: error = PTR_ERR(path.dentry); if (IS_ERR(path.dentry)) { mutex_unlock(&dir->d_inode->i_mutex); goto exit; } if (IS_ERR(nd.intent.open.file)) { error = PTR_ERR(nd.intent.open.file); goto exit_mutex_unlock; } /* Negative dentry, just create the file */ /*如果此dentry结构没有对应的inode节点，说明是无效的，应该创建文件节点 */ if (!path.dentry->d_inode) { /* * This write is needed to ensure that a * ro->rw transition does not occur between * the time when the file is created and when * a permanent write count is taken through * the 'struct file' in nameidata_to_filp(). */ /*write权限是必需的*/ error = mnt_want_write(nd.path.mnt); if (error) { goto exit_mutex_unlock; } /*按照namei格式的flag open*/ error = __open_namei_create(&nd, &path, flag, mode); if (error) { mnt_drop_write(nd.path.mnt); goto exit; } /*根据nameidata 得到相应的file结构*/ filp = nameidata_to_filp(&nd, open_flag); if (IS_ERR(filp)) { ima_counts_put(&nd.path, acc_mode & (MAY_READ | MAY_WRITE | MAY_EXEC)); } /*放弃写权限*/ mnt_drop_write(nd.path.mnt); if (nd.root.mnt) { /*计数减一*/ path_put(&nd.root); } return filp; } /* * It already exists. */ /*要打开的文件已经存在*/ mutex_unlock(&dir->d_inode->i_mutex); /*保存inode节点*/ audit_inode(pathname, path.dentry); error = -EEXIST; /*flag标志检查代码*/ if (flag & O_EXCL) { goto exit_dput; } if (__follow_mount(&path)) { error = -ELOOP; if (flag & O_NOFOLLOW) { goto exit_dput; } } error = -ENOENT; if (!path.dentry->d_inode) { goto exit_dput; } if (path.dentry->d_inode->i_op->follow_link) { goto do_link; } /*路径装化为相应的nameidata结构*/ path_to_nameidata(&path, &nd); error = -EISDIR; /*如果是文件夹*/ if (path.dentry->d_inode && S_ISDIR(path.dentry->d_inode->i_mode)) { goto exit; } ok: /* * Consider: * 1. may_open() truncates a file * 2. a rw->ro mount transition occurs * 3. nameidata_to_filp() fails due to * the ro mount. * That would be inconsistent, and should * be avoided. Taking this mnt write here * ensures that (2) can not occur. */ /*检测是否截断文件标志*/ will_write = open_will_write_to_fs(flag, nd.path.dentry->d_inode); if (will_write) { /*要截断的话就要获取写权限*/ error = mnt_want_write(nd.path.mnt); if (error) { goto exit; } } //may_open执行权限检测、文件打开和truncate的操作 error = may_open(&nd.path, acc_mode, flag); if (error) { if (will_write) { mnt_drop_write(nd.path.mnt); } goto exit; } filp = nameidata_to_filp(&nd, open_flag); if (IS_ERR(filp)) { ima_counts_put(&nd.path, acc_mode & (MAY_READ | MAY_WRITE | MAY_EXEC)); } /* * It is now safe to drop the mnt write * because the filp has had a write taken * on its behalf. */ //安全的放弃写权限 if (will_write) { mnt_drop_write(nd.path.mnt); } if (nd.root.mnt) { path_put(&nd.root); } return filp; exit_mutex_unlock: mutex_unlock(&dir->d_inode->i_mutex); exit_dput: path_put_conditional(&path, &nd); exit: if (!IS_ERR(nd.intent.open.file)) { release_open_intent(&nd); } exit_parent: if (nd.root.mnt) { path_put(&nd.root); } path_put(&nd.path); return ERR_PTR(error); do_link: //允许遍历连接文件，则手工找到连接文件对应的文件 error = -ELOOP; if (flag & O_NOFOLLOW) { //不允许遍历连接文件，返回错误 goto exit_dput; } /* * This is subtle. Instead of calling do_follow_link() we do the * thing by hands. The reason is that this way we have zero link_count * and path_walk() (called from ->follow_link) honoring LOOKUP_PARENT. * After that we have the parent and last component, i.e. * we are in the same situation as after the first path_walk(). * Well, almost - if the last component is normal we get its copy * stored in nd->last.name and we will have to putname() it when we * are done. Procfs-like symlinks just set LAST_BIND. */ /* 以下是手工找到链接文件对应的文件dentry结构代码 */ //设置查找LOOKUP_PARENT标志 nd.flags |= LOOKUP_PARENT; //判断操作是否安全 error = security_inode_follow_link(path.dentry, &nd); if (error) { goto exit_dput; } //处理符号链接 error = __do_follow_link(&path, &nd); if (error) { /* Does someone understand code flow here? Or it is only * me so stupid? Anathema to whoever designed this non-sense * with "intent.open". */ release_open_intent(&nd); if (nd.root.mnt) { path_put(&nd.root); } return ERR_PTR(error); } nd.flags &= ~LOOKUP_PARENT; //检查最后一段文件或目录名的属性情况 if (nd.last_type == LAST_BIND) { goto ok; } error = -EISDIR; if (nd.last_type != LAST_NORM) { goto exit; } if (nd.last.name[nd.last.len]) { __putname(nd.last.name); goto exit; } error = -ELOOP; //出现回环标志: 循环超过32次 if (count++==32) { __putname(nd.last.name); goto exit; } dir = nd.path.dentry; mutex_lock(&dir->d_inode->i_mutex); //更新路径的挂接点和dentry path.dentry = lookup_hash(&nd); path.mnt = nd.path.mnt; __putname(nd.last.name); goto do_last; }
总结一下流程
1. open系统调用访问SYSCALL_DEFINE3函数 2. 在open系统调用中，调用do_sys_open函数完成主要功能 3. 在do_sys_open函数中，调用函数do_filp_open完成主要的打开功能 4. 在内核中要打开一个文件，首先应该找到这个文件，而查找文件的过程在vfs里面是由do_path_lookup或者path_lookup_open函数来完成的 4.1 设置nd->root=根路径(绝对地址)或者当前工作目录(相对地址) 4.2 这一步做完了后，内核会建立一些数据结构(dentry,inode)来初始化查找的起点 if(!retval){ retval = path_walk(name,nd);} 4.3 path_walk会遍历路径的每一节点分量，也就是用"/"分隔开的每一部分，最终找到name指向的文件 int path_walk(const char *name,struct nameidata *nd) { return link_path_walk(name,nd); //path_walk其实相当于直接调用link_path_walk来完成工作 } 4.4 link_path_walk的主要工作是有其内部函数__link_path_walk 来完成的 result = __link_path_walk(name,nd) 4.5 __link_walk_path,该函数把传进来的字符串name，也就是用户指定的路径，按路径分隔符分解成一系列小的component。比如用户说，我要找"/path/to/dest"这个文件，那么我们的文件系统就会按path、to、dest一个
一个来找，知道最后一个分量是文件或者查找完成。他找的时候，会先用path_init初始化过的根路径去找第一个分量，也就是path。然后用path的dentry->d_inode去找to，这样循环到最后一个。注意，内核会缓存找到的路径分量，
所以往往只有第一次访问一个路径的时候，才会去访问磁盘，后面的访问会直接从缓存里找，下面会看到，很多与页告诉缓存打交道的代码。但不管怎样，第一遍查找总是会访问磁盘的 static int __link_path_walk(const char *name,strucy nameidata *nd){..} 至此，按照每一个component查找完成之后，就会找到相应的文件，然后相应的打开工作就基本完成了
Relevant Link:
http://oss.org.cn/kernel-book/ http://blog.csdn.net/f413933206/article/details/5701913
2. close() syscall
close()系统调用对应内核中的函数为: sys_close()
linux-2.6.32.63fsopen.c
/* * Careful here! We test whether the file pointer is NULL before * releasing the fd. This ensures that one clone task can't release * an fd while another clone is opening it. */ SYSCALL_DEFINE1(close, unsigned int, fd) { struct file * filp; struct files_struct *files = current->files; struct fdtable *fdt; int retval; spin_lock(&files->file_lock); /* 获取指向struct fdtable结构体的指针 linux-2.6.32.63includelinuxfdtable.h #define files_fdtable(files) (rcu_dereference((files)->fdt)) */ fdt = files_fdtable(files); if (fd >= fdt->max_fds) { goto out_unlock; } //获取需要关闭的文件描述符编号 filp = fdt->fd[fd]; if (!filp) { goto out_unlock; } /* 将fd_array[]中的的指定元素值置null */ rcu_assign_pointer(fdt->fd[fd], NULL); FD_CLR(fd, fdt->close_on_exec); /* 调用__put_unused_fd函数，将当前fd回收，则下一次打开新的文件又可以用这个fd了 static void __put_unused_fd(struct files_struct *files, unsigned int fd) { struct fdtable *fdt = files_fdtable(files); __FD_CLR(fd, fdt->open_fds); if (fd < files->next_fd) { files->next_fd = fd; } } */ __put_unused_fd(files, fd); spin_unlock(&files->file_lock); retval = filp_close(filp, files); /* can't restart close syscall because file table entry was cleared */ if (unlikely(retval == -ERESTARTSYS || retval == -ERESTARTNOINTR || retval == -ERESTARTNOHAND || retval == -ERESTART_RESTARTBLOCK)) { retval = -EINTR; } return retval; out_unlock: spin_unlock(&files->file_lock); return -EBADF; } EXPORT_SYMBOL(sys_close);
对于，我们需要重点跟进2个函数: rcu_assign_pointer(fdt->fd[fd], NULL);、retval = filp_close(filp, files);
linux-2.6.32.63fs cupdate.h
/** * rcu_assign_pointer - assign (publicize) a pointer to a newly * initialized structure that will be dereferenced by RCU read-side * critical sections. Returns the value assigned. * * Inserts memory barriers on architectures that require them * (pretty much all of them other than x86), and also prevents * the compiler from reordering the code that initializes the * structure after the pointer assignment. More importantly, this * call documents which pointers will be dereferenced by RCU read-side * code. */ #define rcu_assign_pointer(p, v) ({ if (!__builtin_constant_p(v) || ((v) != NULL)) smp_wmb(); (p) = (v); })
我们知道，每个进程在kernel中都有一个对应的task_struct与之对应，而通过task_struct可以间接地获得一个fd_array[]数组，表示当前进程已经打开的文件，每一个元素都是一个文件描述符的值，只有通过这个fd_array[x]才能获取当前进程打开的文件的struc file*，而rcu_assign_pointer(fdt->fd[fd], NULL)的作用就在于将将这个数组的指定元素置空，即断开了这个引用的关系，至于之后内核栈中的那个struct file*是否释放，那内存回收的事，至少现在进程想通过task_stuct是无法再引用到之前打开过的文件了，这里面的关系图可以参阅:
http://www.cnblogs.com/LittleHann/p/3865490.html //搜索: 用一张图表示task_struct、fs_struct、files_struct、fdtable、file的关系
我们继续分析etval = filp_close(filp, files);
linux-2.6.32.63fsopen.c
/* * "id" is the POSIX thread ID. We use the * files pointer for this.. */ int filp_close(struct file *filp, fl_owner_t id) { int retval = 0; if (!file_count(filp)) { printk(KERN_ERR "VFS: Close: file count is 0 "); return 0; } if (filp->f_op && filp->f_op->flush) { retval = filp->f_op->flush(filp, id); } dnotify_flush(filp, id); locks_remove_posix(filp, id); fput(filp); return retval; }
filp_close()负责将表示打开的文件的struct file*内存空间进行释放，至此，内核栈中就再也没有之前打开过的文件的任何痕迹了
Relevant Link:
http://blog.csdn.net/ce123_zhouwei/article/details/8459794
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