鲁达 在上面几篇文章中已经介绍了文件描述符的创建、fd的创建、路径查找的内容，接下来介绍do_sys_open的最后一个操作，即文件打开接口do_last的分析。本小节主要涉及do_last、finish_open、may_open、lookup_open这几个接口的介绍。
do_last接口主要用于打开文件，包括文件创建等操作。
do_last接口实现如下功能：
1.若为目录（链接目录、根目录、当前目录、当前目录的父目录），则返回失败
2.若为链接文件，返回1，由上层函数对链接文件进行查找操作
3.若文件不存在且拥有创建文件权限，则创建文件对应的inode节点，并打开文件
4.若文件不存在且不拥有创建文件权限，则返回失败
5.若文件存在，则打开文件（若对该文件没有相应的权限，则返回失败）
这个接口函数设置的函数调用比较多，不过其主要实现的功能即为上面说明的几个。
static int do_last(struct nameidata *nd, struct path *path,
struct file *file, const struct open_flags *op,
int *opened, struct filename *name)
{
struct dentry *dir = nd->;/*获取到文件所在父目录对应的dentry变量*/
int open_flag = op->open_flag;/*获取open的flag标识*/
bool will_truncate = (open_flag & O_TRUNC) != 0;/*标识是否可修改文件大小*/
bool got_write = false;
int acc_mode = op->acc_mode;
struct inode *inode;
bool symlink_ok = false;/*标识文件是否为链接文件*/
struct path save_parent = { .dentry = NULL, .mnt = NULL };
bool retried = false;
int error;
/*nd->flags 去除LOOKUP_PARENT*/
nd->flags &= ~LOOKUP_PARENT;
nd->flags |= op->intent;
/*判断last_type,若为如下三种，则会打开文件失败：
1.若为LAST_DOTDOT、LAST_DOT，则说明路径查找的最后一级分量为目录，且不是文件，更新nd->path相关的变量，然后进入finish_open
,在may_open中，即会返回失败
2.若为LAST_ROOT，则说明亦是目录，同样在may_open时会返回失败
3.若为链接目录，则同样在may_open时，会返回失败*/
switch (nd->last_type) {
case LAST_DOTDOT:
case LAST_DOT:
error = handle_dots(nd, nd->last_type);
if (error)
return error;
/* fallthrough */
case LAST_ROOT:
error = complete_walk(nd);
if (error)
return error;
/*审计相关的信息*/
audit_inode(name, nd->, 0);
if (open_flag & O_CREAT) {
error = -EISDIR;
goto out;
}
goto finish_open;
case LAST_BIND:
error = complete_walk(nd);
if (error)
return error;
audit_inode(name, dir, 0);
goto finish_open;
}
/*当打开文件的flag中，没有创建文件的权限时，则调用lookup_fast,根据文件名称，在其父dentry的hash链表中，获取文件对应dentry与inode
若查找成功，则调用finish_lookup进行后续操作*/
if (!(open_flag & O_CREAT)) {
if (nd->la[nd->la])
nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
if (open_flag & O_PATH && !(nd->flags & LOOKUP_FOLLOW))
symlink_ok = true;
/* we _can_ be in RCU mode here */
error = lookup_fast(nd, path, &inode);
if (likely(!error))
goto finish_lookup;
if (error < 0)
goto out;
BUG_ON(nd->inode != dir->d_inode);
} else {
/* create side of things */
/*
* This will *only* deal with leaving RCU mode - LOOKUP_JUMPED
* has been cleared when we got to the last component we are
* about to look up
*/
error = complete_walk(nd);
if (error)
return error;
audit_inode(name, dir, LOOKUP_PARENT);
error = -EISDIR;
/* trailing slashes? */
if (nd->la[nd->la])
goto out;
}
retry_lookup:
/*若可以创建文件，则设置got_write*/
if (op->open_flag & (O_CREAT | O_TRUNC | O_WRONLY | O_RDWR)) {
error = mnt_want_write(nd->);
if (!error)
got_write = true;
/*
* do _not_ fail yet - we might not need that or fail with
* a different error; let lookup_open() decide; we'll be
* dropping this one anyway.
*/
}
mutex_lock(&dir->d_inode->i_mutex);
error = lookup_open(nd, path, file, op, got_write, opened);
mutex_unlock(&dir->d_inode->i_mutex);
if (error <= 0) {
if (error)
goto out;
if ((*opened & FILE_CREATED) ||
!S_ISREG(file_inode(file)->i_mode))
will_truncate = false;
audit_inode(name, file->f_, 0);
goto opened;
}
/*若为新创建的文件，则去除O_TRUN权限，并更新nd->path变量，并跳转至finish_open_created继续进行*/
if (*opened & FILE_CREATED) {
/* Don't check for write permission, don't truncate */
open_flag &= ~O_TRUNC;
will_truncate = false;
acc_mode = MAY_OPEN;
path_to_nameidata(path, nd);
goto finish_open_created;
}
/*
* create/update audit record if it already exists.
*/
/*执行审计相关的操作*/
if (path->dentry->d_inode)
audit_inode(name, path->dentry, 0);
/*
* If atomic_open() acquired write access it is dropped now due to
* possible mount and symlink following (this might be optimized away if
* necessary...)
*/
/*针对atomic_open相关的操作*/
if (got_write) {
mnt_drop_write(nd->);
got_write = false;
}
error = -EEXIST;
if ((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))
goto exit_dput;
error = follow_managed(path, nd->flags);
if (error < 0)
goto exit_dput;
if (error)
nd->flags |= LOOKUP_JUMPED;
BUG_ON(nd->flags & LOOKUP_RCU);
inode = path->dentry->d_inode;
finish_lookup:
/* we _can_ be in RCU mode here */
/*若文件不存在（即inode节点不存在），则更新nd->path，返回文件不存在错误*/
error = -ENOENT;
if (!inode) {
path_to_nameidata(path, nd);
goto out;
}
/*若文件存在，则且为链接文件，则返回1，调用链接文件的处理代码*/
if (should_follow_link(inode, !symlink_ok)) {
if (nd->flags & LOOKUP_RCU) {
if (unlikely(unlazy_walk(nd, path->dentry))) {
error = -ECHILD;
goto out;
}
}
BUG_ON(inode != path->dentry->d_inode);
return 1;
}
/*更新nd->path等变量*/
if ((nd->flags & LOOKUP_RCU) || nd-> != path->mnt) {
path_to_nameidata(path, nd);
} else {
= nd->;
= mntget(path->mnt);
nd-> = path->dentry;
}
nd->inode = inode;
/* Why this, you ask? _Now_ we might have grown LOOKUP_JUMPED... */
error = complete_walk(nd);
if (error) {
path_put(&save_parent);
return error;
}
/*若为目录，返回失败*/
error = -EISDIR;
if ((open_flag & O_CREAT) && S_ISDIR(nd->inode->i_mode))
goto out;
error = -ENOTDIR;
if ((nd->flags & LOOKUP_DIRECTORY) && !can_lookup(nd->inode))
goto out;
/*审计相关*/
audit_inode(name, nd->, 0);
finish_open:
/*若不是普通文件，则will_truncate设置为false，即不能调整文件大小*/
if (!S_ISREG(nd->inode->i_mode))
will_truncate = false;
/*若可以修改文件大小，则调用mnt_want_write，确认是否*/
if (will_truncate) {
error = mnt_want_write(nd->);
if (error)
goto out;
got_write = true;
}
finish_open_created:
/*调用may_open接口，判断是否可打开该文件*/
error = may_open(&nd->path, acc_mode, open_flag);
if (error)
goto out;
/*至此，可正常打开一个文件，则设置struct file*指针的f_变量*/
file->f_ = nd->;
error = finish_open(file, nd->, NULL, opened);
if (error) {
if (error == -EOPENSTALE)
goto stale_open;
goto out;
}
opened:
error = open_check_o_direct(file);
if (error)
goto exit_fput;
error = ima_file_check(file, op->acc_mode);
if (error)
goto exit_fput;
/*调整文件大小*/
if (will_truncate) {
error = handle_truncate(file);
if (error)
goto exit_fput;
}
out:
if (got_write)
mnt_drop_write(nd->);
path_put(&save_parent);
terminate_walk(nd);
return error;
exit_dput:
path_put_conditional(path, nd);
goto out;
exit_fput:
fput(file);
goto out;
stale_open:
/* If no saved parent or already retried then can't retry */
if (! || retried)
goto out;
BUG_ON( != dir);
path_put(&nd->path);
nd->path = save_parent;
nd->inode = dir->d_inode;
= NULL;
= NULL;
if (got_write) {
mnt_drop_write(nd->);
got_write = false;
}
retried = true;
goto retry_lookup;
}
下面针对上面调用的lookup_open、may_open、finish_open这几个接口进行分析
lookup_open接口分析
该接口包括文件的查找以及文件创建等功能
该接口主要用于查找指定名称的文件，主要包括如下几种可能
1. 从dentry的dcache中查找文件，若查找成功，则更新path变量，返回成功
2.若从dentry 的dcache中没有查找到文件，则调用dentry的looku接口，查找dentry的子dentry是否存在符合条件的子dentry
若查找成功，更新path变量，返回成功；
若查找失败，则判断是否有创建文件的权限：
若有创建文件的权限，则调用vfs_create接口创建文件，更新path变量，返回成功；
若没有创建文件的权限，则更新path变量后，返回成功
static int lookup_open(struct nameidata *nd, struct path *path,
struct file *file,
const struct open_flags *op,
bool got_write, int *opened)
{
struct dentry *dir = nd->;
struct inode *dir_inode = dir->d_inode;
struct dentry *dentry;
int error;
bool need_lookup;
/*调用lookup_dcache，从dcache中查找文件（文件名为nd->last）,若没有查找到，则设置need_lookup为true*/
*opened &= ~FILE_CREATED;
dentry = lookup_dcache(&nd->last, dir, nd->flags, &need_lookup);
if (IS_ERR(dentry))
return PTR_ERR(dentry);
/*若查找到dentr，且dentry->inode存在，则更新path变量，并返回1*/
/* Cached positive dentry: will open in f_op->open */
if (!need_lookup && dentry->d_inode)
goto out_no_open;
if ((nd->flags & LOOKUP_OPEN) && dir_inode->i_op->atomic_open) {
return atomic_open(nd, dentry, path, file, op, got_write,
need_lookup, opened);
}
/*若在dcache中没有查找到，则调用lookup_real进行查找（调用parennt dentry的lookup接口进行查找）*/
if (need_lookup) {
BUG_ON(dentry->d_inode);
dentry = lookup_real(dir_inode, dentry, nd->flags);
if (IS_ERR(dentry))
return PTR_ERR(dentry);
}
/* Negative dentry, just create the file */
/*若文件不存在，且open flag中包含了create权限，则调用vfs_create创建文件*/
if (!dentry->d_inode && (op->open_flag & O_CREAT)) {
umode_t mode = op->mode;
if (!IS_POSIXACL(dir->d_inode))
mode &= ~current_umask();
/*
* This write is needed to ensure that a
* rw->ro 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 finish_open().
*/
if (!got_write) {
error = -EROFS;
goto out_dput;
}
*opened |= FILE_CREATED;
error = security_path_mknod(&nd->path, dentry, mode, 0);
if (error)
goto out_dput;
error = vfs_create(dir->d_inode, dentry, mode,
nd->flags & LOOKUP_EXCL);
if (error)
goto out_dput;
}
out_no_open:
path->dentry = dentry;
path->mnt = nd->;
return 1;
out_dput:
dput(dentry);
return error;
}
其中针对vfs_create接口，其定义如下，主要调用dentry的create接口进行inode节点的创建，这就涉及到具体文件系统的create接口调用
int vfs_create(struct inode *dir, struct dentry *dentry, umode_t mode,
bool want_excl)
{
int error = may_create(dir, dentry);
if (error)
return error;
if (!dir->i_op->create)
return -EACCES; /* shouldn't it be ENOSYS? */
mode &= S_IALLUGO;
mode |= S_IFREG;
error = security_inode_create(dir, dentry, mode);
if (error)
return error;
error = dir->i_op->create(dir, dentry, mode, want_excl);
if (!error)
fsnotify_create(dir, dentry);
return error;
}
may_open接口分析
该接口主要用于判断是否可以打开一个文件，主要涉及以下几个方面的判断：
1.若文件的flag设置了O_PATH标签，则返回可以；
2.若为链接文件，则返回失败；
3.若为字符或块设备，且该文件对应的文件系统不允许访问设备文件，则返回失败
4.若打开文件的标签与超级块或者inode的权限不匹配，返回失败
5.针对inode标记为S_APPEND或者打开文件标签为O_NOATIME时，做相应的合法性判断
static int may_open(struct path *path, int acc_mode, int flag)
{
struct dentry *dentry = path->dentry;
struct inode *inode = dentry->d_inode;
int error;
/* 若设置了O_PATH，则直接返回0，即可以打开文件，因O_PATH并不真正打开文件，因此若为O_PATH，则此时直接返回0*/
if (!acc_mode)
return 0;
/*若文件的inode节点不存在，返回ENOENT失败*/
if (!inode)
return -ENOENT;
/*判断文件的类型，主要包括如下判断：
1.若为链接文件，返回失败（失败原因为链接数太多）
2.若为字符设备或块设备，且该文件对应的文件系统不支持访问设备文件，则返回失败
3.若要打开的文件为目录，则返回失败
4.若为fifo或socket文件，则不能修改文件大小，去除S_IFSOCK标签*/
switch (inode->i_mode & S_IFMT) {
case S_IFLNK:
return -ELOOP;
case S_IFDIR:
if (acc_mode & MAY_WRITE)
return -EISDIR;
break;
case S_IFBLK:
case S_IFCHR:
if (path->mnt->mnt_flags & MNT_NODEV)
return -EACCES;
/*FALLTHRU*/
case S_IFIFO:
case S_IFSOCK:
flag &= ~O_TRUNC;
break;
}
/*调用inode_permission，判断超级块、inode节点是否允许acc_mode标识的权限*/
error = inode_permission(inode, acc_mode);
if (error)
return error;
/*
* An append-only file must be opened in append mode for writing.
*/
/*若inode节点标识了S_APPEND，则对文件只能以readonly方式或者追加的方式打开（且不能设置O_TRUNC）*/
if (IS_APPEND(inode)) {
if ((flag & O_ACCMODE) != O_RDONLY && !(flag & O_APPEND))
return -EPERM;
if (flag & O_TRUNC)
return -EPERM;
}
/*仅在进程所有者是文件的所有者或者超级用户权限，方可不记录访问文件的atime*/
/* O_NOATIME can only be set by the owner or superuser */
if (flag & O_NOATIME && !inode_owner_or_capable(inode))
return -EPERM;
return 0;
}
finish_open接口分析
该接口主要调用do_dentry_open接口，实现文件的打开操作
int finish_open(struct file *file, struct dentry *dentry,
int (*open)(struct inode *, struct file *),
int *opened)
{
int error;
/*若文件已经打开，则抛出异常*/
BUG_ON(*opened & FILE_OPENED); /* once it's opened, it's opened */
/*设置struct file *类型指针变量的f_*/
file->f_ = dentry;
/*调用do_dentry_open执行打开文件操作，期间会调用各文件系统具体的open接口等*/
error = do_dentry_open(file, open, current_cred());
if (!error)
*opened |= FILE_OPENED;
return error;
}
do_dentry_open接口的定义如下，该接口通过调用inode->i_fop获取文件操作的接口函数指针。
static int do_dentry_open(struct file *f,
int (*open)(struct inode *, struct file *),
const struct cred *cred)
{
static const struct file_operations empty_fops = {};
struct inode *inode;
int error;
/*设置文件打开的mode*/
f->f_mode = OPEN_FMODE(f->f_flags) | FMODE_LSEEK |
FMODE_PREAD | FMODE_PWRITE;
/*若为O_PATH，则设置f->f_mode为FMODE_PATH*/
if (unlikely(f->f_flags & O_PATH))
f->f_mode = FMODE_PATH;
/*增加对mnt、dentry的引用计数*/
path_get(&f->f_path);
/*设置文件描述符的f_inode，至此完成了文件描述符与该文件对应的dentry、mnt、inode的关联*/
inode = f->f_inode = f->f_->d_inode;
/*若设置了文件的写访问权限，则判断当前用户是否对该文件拥有写权限*/
if (f->f_mode & FMODE_WRITE) {
error = __get_file_write_access(inode, f->f_);
if (error)
goto cleanup_file;
if (!special_file(inode->i_mode))
file_take_write(f);
}
f->f_mapping = inode->i_mapping;
file_sb_list_add(f, inode->i_sb);
if (unlikely(f->f_mode & FMODE_PATH)) {
f->f_op = &empty_fops;
return 0;
}
/*通过该操作，实现将文件inode节点对应i_fop指针函数赋值给file->f_op*/
f->f_op = fops_get(inode->i_fop);
error = security_file_open(f, cred);
if (error)
goto cleanup_all;
error = break_lease(inode, f->f_flags);
if (error)
goto cleanup_all;
if (!open && f->f_op)
open = f->f_op->open;
if (open) {
error = open(inode, f);
if (error)
goto cleanup_all;
}
if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
i_readcount_inc(inode);
f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
file_ra_state_init(&f->f_ra, f->f_mapping->host->i_mapping);
return 0;
cleanup_all:
fops_put(f->f_op);
file_sb_list_del(f);
if (f->f_mode & FMODE_WRITE) {
put_write_access(inode);
if (!special_file(inode->i_mode)) {
/*
* We don't consider this a real
* mnt_want/drop_write() pair
* because it all happenend right
* here, so just reset the state.
*/
file_reset_write(f);
__mnt_drop_write(f->f_);
}
}
cleanup_file:
path_put(&f->f_path);
f->f_ = NULL;
f->f_ = NULL;
f->f_inode = NULL;
return error;
}
以上即为do_sys_open接口的分析，主要涉及路径查找、链接目录、链接文件的查找、文件的打开等操作，简单来说就是
实现了将文件系统相关的结构体（dentry、inode、task、fs_struct、file等结构体的关联）。针对内核模块的分析，建议
首先进行结构体的分析，当结构体之间的关联图理清后，则基本上对内核模块的实现有了大致的了解。