Get absolute path from dentry_path_raw - c

Question

I try to get the target of symlink dentries in a kernel module, i.e. the dentry on which my dentry points.
I use this approach:
int print_dentry(struct dentry *d) {
struct path p;
char *buffer, *path_name;
int ret;
buffer = (char *)__get_free_page(GFP_KERNEL);
if (!buffer)
return -ENOMEM;
path_name = dentry_path_raw(d, buffer, PAGE_SIZE);
if (IS_ERR(path_name))
printk(KERN_ERR "ERR");
if ((ret=kern_path(path_name, LOOKUP_FOLLOW, &p))) {
printk("kern_path returned %d for path_name \"%s\", inode %ld\n", ret, path_name, d->d_inode->i_ino);
return 0;
}
printk_once(KERN_INFO "Path %s -> dentry_uid %ld\n", path_name, p.dentry->d_inode->i_ino);
free_page((unsigned long)buffer);
return 0;
}
However, dentry_path_raw doesn't return the absolute path, but a path relative to the vfsmount.
Hence I get errors like this when there is a vfsmount e.g.
kern_path returned -2 for path_name "/self", inode 4026531841
Which corresponds to /proc/self
ls -ial /proc/self
4026531841 lrwxrwxrwx 1 root root 0 5 déc. 04:28 /proc/self -> 1341
Is there a way to get the absolute path so that I can give it to kern_path()? Or maybe another approach to follow the symlink and get the associated dentry?
I don't think I can use directly d_absolute_path or prepend_path since they take as input a struct path* or a char* and I only have a access to a struct dentry* or a char*

Answer 1

Looking at the source code here, it seems that when you call dentry_path_raw() it actually calls another function that does the following:
/* SPDX-License-Identifier: GPL-2.0 */
/*
* Write full pathname from the root of the filesystem into the buffer.
*/
static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
{
const struct dentry *dentry;
struct prepend_buffer b;
int seq = 0;
rcu_read_lock();
restart:
dentry = d;
b = *p;
read_seqbegin_or_lock(&rename_lock, &seq);
while (!IS_ROOT(dentry)) {
const struct dentry *parent = dentry->d_parent;
prefetch(parent);
if (!prepend_name(&b, &dentry->d_name))
break;
dentry = parent;
}
if (!(seq & 1))
rcu_read_unlock();
if (need_seqretry(&rename_lock, seq)) {
seq = 1;
goto restart;
}
done_seqretry(&rename_lock, seq);
if (b.len == p->len)
prepend_char(&b, '/');
return extract_string(&b);
}
And everything seems to be in the description of this function. It writes the full path into the buffer, not in its return value, which share it's return with dentry_path_raw().
So, you should use the value pushed into your buffer instead of file_name, which is just the name returned by the function dentry_path_raw().
Note: dentry_path_raw() doesn't resolve symlinks, if you really want to resolve them you should probably use dentry_path() instead.
All these functions are available since linux v2.6.26/v2.6.38