符号链接也是一种文件，只不过其内容是另一个文件的路径名。对于符号链接的处理，要注意避免死循环的产生，如一个符号链接指向其本身。
符号链接可能包含了符号链接，因此内核采用递归的方式来处理这个问题，为了避免符号链接的死循环而导致无穷递归，内核采用link_count和total_link_count来跟踪符号链接的处理，其中前者表示连续的符号链接数，后者表示总共的符号链接数，两者都不能超过各自的限制，否则内核会放弃解析。
staticinlineint do_follow_link(struct path *path, struct nameidata *nd)
{
int err = -ELOOP;
/*如果处理的链接数超过下面的限制则会放弃搜索(因为有可能陷入死循环)*/
/*link_count表示连续的符号链接数，其值不能超过8，total_link_count表示总共的
链接数，气值不能超过40*/
if (current->link_count >= MAX_NESTED_LINKS)
goto loop;
if (current->total_link_count >= 40)
goto loop;
BUG_ON(nd->depth >= MAX_NESTED_LINKS);
cond_resched();
err = security_inode_follow_link(path->dentry, nd);
if (err)
goto loop;
/*每处理一次符号链接，link_count,total_link_count和depth都要加1*/
current->link_count++;
current->total_link_count++;//
nd->depth++;
/*主要是调用特定于文件系统的follow_link函数进行符号链接的跟踪*/
err = __do_follow_link(path, nd);
/*每执行完一次符号链接，link_count和depth都要减1*/
current->link_count--;
nd->depth--;
return err;
loop:
path_put_conditional(path, nd);
path_put(&nd->path);
return err;
}
static __always_inline int __do_follow_link(struct path *path, struct nameidata *nd)
{
int error;
void *cookie;
struct dentry *dentry = path->dentry;
touch_atime(path->mnt, dentry);
nd_set_link(nd, NULL);//在saved_names中清除当前解析的符号链接路径
if (path->mnt != nd->path.mnt) {
path_to_nameidata(path, nd);
dget(dentry);
}
mntget(path->mnt);
/*调用特定于文件系统的follow_link方法来跟踪符号链接，并将符号链接中保存的路径名提取出来
存放在nd的saved_names中*/
cookie = dentry->d_inode->i_op->follow_link(dentry, nd);
error = PTR_ERR(cookie);
if (!IS_ERR(cookie)) {
char *s = nd_get_link(nd);//提取saved_names中的路径
error = 0;
if (s)//路径存在，则继续解析，__vfs_follow_link将会调用link_path_walk,因此对于
//路径中存在多个符号链接的情况，这是一个递归的过程
error = __vfs_follow_link(nd, s);
if (dentry->d_inode->i_op->put_link)
dentry->d_inode->i_op->put_link(dentry, nd, cookie);
}
path_put(path);
return error;
}
static __always_inline int __vfs_follow_link(struct nameidata *nd, constchar *link)
{
int res = 0;
char *name;
if (IS_ERR(link))
goto fail;
if (*link == '/') {
set_root(nd);
path_put(&nd->path);
nd->path = nd->root;
path_get(&nd->root);
}
res = link_path_walk(link, nd);
if (nd->depth || res || nd->last_type!=LAST_NORM)
return res;
/*
* If it is an iterative symlinks resolution in open_namei() we
* have to copy the last component. And all that crap because of
* bloody create() on broken symlinks. Furrfu...
*/
name = __getname();
if (unlikely(!name)) {
path_put(&nd->path);
return -ENOMEM;
}
strcpy(name, nd->last.name);
nd->last.name = name;
return 0;
fail:
path_put(&nd->path);
return PTR_ERR(link);
}