在文件读写之前，我们必须先打开文件。从应用程序的角度来看，这是通过标准库的open函数完成的，该函数返回一个文件描述符。内核中是由系统调用sys_open()函数完成。
/*sys_open*/
SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, int, mode)
{
long ret;
/*检查是否应该不考虑用户层传递的标志、总是强行设置
O_LARGEFILE标志。如果底层处理器的字长不是32位，就是这种
情况*/
if (force_o_largefile())
flags |= O_LARGEFILE;
/*实际工作*/
ret = do_sys_open(AT_FDCWD, filename, flags, mode);
/* avoid REGPARM breakage on x86: */
asmlinkage_protect(3, ret, filename, flags, mode);
return ret;
}
实际实现工作
<pre class="cpp" name="code">long do_sys_open(int dfd, const char __user *filename, int flags, int mode)
{
/*从进程地址空间读取该文件的路径名*/
char *tmp = getname(filename);
int fd = PTR_ERR(tmp);
if (!IS_ERR(tmp)) {
/*在内核中，每个打开的文件由一个文件描述符表示
该描述符在特定于进程的数组中充当位置索引(数组是
task_struct->files->fd_arry)，该数组的元素包含了file结构，其中
包括每个打开文件的所有必要信息。因此，调用下面
函数查找一个未使用的文件描述符,返回的是上面
说的数组的下标*/
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)) {
put_unused_fd(fd);
fd = PTR_ERR(f);
} else {
fsnotify_open(f->f_path.dentry);
fd_install(fd, f);
}
}
putname(tmp);
}
return fd;
}
打开文件主体实现
/*
* 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)
{
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;/*跳过下面的创建部分*/
}
/*
* Create - we need to know the parent.
*/
/*到此则是要创建文件*/
/* 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);
error = path_walk(pathname, &nd);
if (error) {
if (nd.root.mnt)
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;
/*获取文件指针*/
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);
/*从哈希表中查找目的文件对应的dentry,上面路径搜索的是父节点
也就是目的文件的上一层目录，为了得到目的文件的
path结构，我们用nd中的last结构和上一层目录的dentry结构
可以找到*/
path.dentry = lookup_hash(&nd);
path.mnt = nd.path.mnt;
/*到此目标节点的path结构已经找到*/
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*,主要是创建inode*/
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)
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;
if (flag & O_EXCL)
goto exit_dput;
/*如果path上安装了文件系统，则依次往下找，直到找到
的文件系统没有安装别的文件系统，更新path结构为
此文件系统的根目录信息*/
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;/*顺次遍历符号链接*/
/*路径转化为相应的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;
/*到这里，nd结构中存放的信息已经是最后的目的文件信息*/
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.
*/
will_write = open_will_write_to_fs(flag, nd.path.dentry->d_inode);
if (will_write) {
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;
}
/*将nameidata转化为file*/
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.
*/
if (will_write)
/*释放写权限*/
mnt_drop_write(nd.path.mnt);
if (nd.root.mnt)
/*释放引用计数*/
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);
/*允许遍历连接文件，则手工找到连接文件对应的文件*/
do_link:
error = -ELOOP;
if (flag & O_NOFOLLOW)
goto exit_dput;/*不允许遍历连接文件，返回错误*/
/*
* 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,即路径搜索，结果放入nd中*/
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;
}
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，关于文件路径查找在前面已经分析过相关的代码了。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。