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文章目录
一、简介1.1 __dentry_path1.2 prepend_name1.3 d_path
二、dmeo参考资料
一、简介
// linux-5.4.18/fs/d_path.c
char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
{
return __dentry_path(dentry, buf, buflen);
}
EXPORT_SYMBOL(dentry_path_raw);
该函数根据给定dentry返回其文件的完整路径。使用EXPORT_SYMBOL导出，可以直接调用。 关于dentry介绍请参考：Linux文件系统 struct dentry 结构体解析
1.1 __dentry_path
/*
* Write full pathname from the root of the filesystem into the buffer.
*/
static char *__dentry_path(struct dentry *d, char *buf, int buflen)
{
struct dentry *dentry;
char *end, *retval;
int len, seq = 0;
int error = 0;
if (buflen
seq = 1;
goto restart;
}
done_seqretry(&rename_lock, seq);
if (error)
goto Elong;
return retval;
Elong:
return ERR_PTR(-ENAMETOOLONG);
}
函数 __dentry_path，用于将文件系统根目录到给定的 dentry 的完整路径名写入缓冲区。
函数的参数包括要获取路径名的 dentry，存储路径名的缓冲区 buf，以及缓冲区的长度 buflen。
static int prepend(char **buffer, int *buflen, const char *str, int namelen)
{
*buflen -= namelen;
if (*buflen d_parent)
在此期间，函数通过调用 prefetch 函数预取父节点，以提高性能。
_dentry_path 函数负责构建文件系统根目录到给定 dentry 对象的完整路径名，并将其存储在提供的缓冲区中。
注意_dentry_path 函数（即dentry_path_raw）对于传入的 buf 缓冲区，是从缓冲区的末尾开始构建dentry 对象的路径名，从buf的末尾往前构建dentry 对象的路径名，因此构建完成后，我们不能使用buf缓冲区来当作路径名，应该使用该函数的返回值来当作路径名，即调用者应该使用返回的指针来使用路径名，而不是传入的缓冲区指针。缓冲区的开头留下 0 字节（如果传入的缓冲区buf初始化为0），而路径名的开始通常会从缓冲区开始的部分字节偏移处开始的。
大概的过程就是，比如目录 /home/user01/c/：
//缓冲区buf末尾添加一个斜杠 (/) 和一个空字符 (\0)，形成一个以斜杠结尾的字符串，以确保路径名的正确格式。
/\0
/c/\0
/user01/c/\0
/home/user01/c/\0
buf retval
/
/c/
/user01/c/
/home/user01/c/
buf retval
应该使用retval作为文件名而不是buf。
这里有一个疑问？如果是获取文件的完整路径，比如文件 /home/user01/c/text.txt，那么需要在text.txt的末尾添加一个斜杠 (/)？
如下图所示： 
1.2 prepend_name
/**
* prepend_name - prepend a pathname in front of current buffer pointer
* @buffer: buffer pointer
* @buflen: allocated length of the buffer
* @name: name string and length qstr structure
*
* With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
* make sure that either the old or the new name pointer and length are
* fetched. However, there may be mismatch between length and pointer.
* The length cannot be trusted, we need to copy it byte-by-byte until
* the length is reached or a null byte is found. It also prepends "/" at
* the beginning of the name. The sequence number check at the caller will
* retry it again when a d_move() does happen. So any garbage in the buffer
* due to mismatched pointer and length will be discarded.
*
* Load acquire is needed to make sure that we see that terminating NUL.
*/
static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
{
const char *dname = smp_load_acquire(&name->name); /* ^^^ */
u32 dlen = READ_ONCE(name->len);
char *p;
*buflen -= dlen + 1;
if (*buflen
char *res = buf + buflen;
struct path root;
int error;
/*
* We have various synthetic filesystems that never get mounted. On
* these filesystems dentries are never used for lookup purposes, and
* thus don't need to be hashed. They also don't need a name until a
* user wants to identify the object in /proc/pid/fd/. The little hack
* below allows us to generate a name for these objects on demand:
*
* Some pseudo inodes are mountable. When they are mounted
* path->dentry == path->mnt->mnt_root. In that case don't call d_dname
* and instead have d_path return the mounted path.
*/
if (path->dentry->d_op && path->dentry->d_op->d_dname &&
(!IS_ROOT(path->dentry) || path->dentry != path->mnt->mnt_root))
return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
rcu_read_lock();
get_fs_root_rcu(current->fs, &root);
error = path_with_deleted(path, &root, &res, &buflen);
rcu_read_unlock();
if (error
return 0;
}
printk("path = %s\n", path);
free_page((unsigned long)buffer);
return 0;
}
static void __exit hello_exit(void)
{
}
module_init(hello_init);
module_exit(hello_exit);
MODULE_LICENSE("GPL");
需要注意的是，调用者调用dentry_path_raw应该使用返回的指针来作为路径名，而不是传入的缓冲区指针。路径名通常是从缓冲区的部分字节偏移处开始的，缓冲区的开头部分字节是无意义的。
/usr/bin/kmod
buffer path
应当使用返回的指针path来作为路径名，而不是传入的缓冲区指针buffer。
# dmesg -c
path = /usr/bin/kmod
参考资料
Linux 5.4.18
https://blog.csdn.net/qq_42931917/article/details/119803534
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