In fs/tracefs/inode.c, tracefs_syscall_mkdir() and tracefs_syscall_rmdir()
previously used a local helper, get_dname(), which allocated a temporary
buffer on the heap via kmalloc() to hold the dentry name. This introduced
unnecessary overhead, an ENOMEM failure path, and required manual memory
cleanup via kfree().

As suggested by Al Viro, replace this heap allocation with the VFS dentry
name snapshot API. By stack-allocating a `struct name_snapshot` and using
take_dentry_name_snapshot() and release_dentry_name_snapshot(), we safely
capture the dentry name locklessly, eliminate the heap allocation entirely,
and remove the now-obsolete error handling paths. The get_dname() helper
is completely removed.

Testing:
Booted a custom kernel natively in virtme-ng (ARM64). Triggered tracefs
inode and dentry allocation by creating and removing a custom directory
under a temporary tracefs mount. Verified that the instance is created
successfully and that no memory errors or warnings are emitted in dmesg.

Signed-off-by: AnishMulay <anishm7030@gmail.com>
---
Changes in v3:
- Rebased into a single clean commit against upstream.

Changes in v2:
- Link to v1: https://lore.kernel.org/linux-trace-kernel/20260227194453.213095-1-anishm7030@gmail.com/
- Use the helper functions take/release_dentry_name_snapshot() instead of allocating the name. (Al Viro)

 fs/tracefs/inode.c | 39 ++++++++-------------------------------
 1 file changed, 8 insertions(+), 31 deletions(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index d9d8932a7b9c9..ad322e8f9e2ad 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -94,33 +94,14 @@ static struct tracefs_dir_ops {
 	int (*rmdir)(const char *name);
 } tracefs_ops __ro_after_init;
 
-static char *get_dname(struct dentry *dentry)
-{
-	const char *dname;
-	char *name;
-	int len = dentry->d_name.len;
-
-	dname = dentry->d_name.name;
-	name = kmalloc(len + 1, GFP_KERNEL);
-	if (!name)
-		return NULL;
-	memcpy(name, dname, len);
-	name[len] = 0;
-	return name;
-}
-
 static struct dentry *tracefs_syscall_mkdir(struct mnt_idmap *idmap,
 					    struct inode *inode, struct dentry *dentry,
 					    umode_t mode)
 {
 	struct tracefs_inode *ti;
-	char *name;
+	struct name_snapshot name;
 	int ret;
 
-	name = get_dname(dentry);
-	if (!name)
-		return ERR_PTR(-ENOMEM);
-
 	/*
 	 * This is a new directory that does not take the default of
 	 * the rootfs. It becomes the default permissions for all the
@@ -135,24 +116,20 @@ static struct dentry *tracefs_syscall_mkdir(struct mnt_idmap *idmap,
 	 * the files within the tracefs system. It is up to the individual
 	 * mkdir routine to handle races.
 	 */
+	take_dentry_name_snapshot(&name, dentry);
 	inode_unlock(inode);
-	ret = tracefs_ops.mkdir(name);
+	ret = tracefs_ops.mkdir(name.name.name);
 	inode_lock(inode);
-
-	kfree(name);
+	release_dentry_name_snapshot(&name);
 
 	return ERR_PTR(ret);
 }
 
 static int tracefs_syscall_rmdir(struct inode *inode, struct dentry *dentry)
 {
-	char *name;
+	struct name_snapshot name;
 	int ret;
 
-	name = get_dname(dentry);
-	if (!name)
-		return -ENOMEM;
-
 	/*
 	 * The rmdir call can call the generic functions that create
 	 * the files within the tracefs system. It is up to the individual
@@ -160,15 +137,15 @@ static int tracefs_syscall_rmdir(struct inode *inode, struct dentry *dentry)
 	 * This time we need to unlock not only the parent (inode) but
 	 * also the directory that is being deleted.
 	 */
+	take_dentry_name_snapshot(&name, dentry);
 	inode_unlock(inode);
 	inode_unlock(d_inode(dentry));
 
-	ret = tracefs_ops.rmdir(name);
+	ret = tracefs_ops.rmdir(name.name.name);
 
 	inode_lock_nested(inode, I_MUTEX_PARENT);
 	inode_lock(d_inode(dentry));
-
-	kfree(name);
+	release_dentry_name_snapshot(&name);
 
 	return ret;
 }
-- 
2.51.0

On Fri,  6 Mar 2026 15:04:58 -0500
AnishMulay <anishm7030@gmail.com> wrote:

Thanks for the update, but please do not have a new patch as a reply to the
old one. It makes it harder to find. As I'll only look at top level emails
for patches in my inbox which means all new versions should be their own
thread.

> In fs/tracefs/inode.c, tracefs_syscall_mkdir() and tracefs_syscall_rmdir()
> previously used a local helper, get_dname(), which allocated a temporary
> buffer on the heap via kmalloc() to hold the dentry name. This introduced
> unnecessary overhead, an ENOMEM failure path, and required manual memory
> cleanup via kfree().
> 
> As suggested by Al Viro, replace this heap allocation with the VFS dentry
> name snapshot API. By stack-allocating a `struct name_snapshot` and using
> take_dentry_name_snapshot() and release_dentry_name_snapshot(), we safely
> capture the dentry name locklessly, eliminate the heap allocation entirely,
> and remove the now-obsolete error handling paths. The get_dname() helper
> is completely removed.
> 
> Testing:
> Booted a custom kernel natively in virtme-ng (ARM64). Triggered tracefs
> inode and dentry allocation by creating and removing a custom directory
> under a temporary tracefs mount. Verified that the instance is created
> successfully and that no memory errors or warnings are emitted in dmesg.
> 
> Signed-off-by: AnishMulay <anishm7030@gmail.com>
> ---
> Changes in v3:

The above should have been:

Changes since v2: https://lore.kernel.org/all/20260227211505.226643-1-anishm7030@gmail.com/

> - Rebased into a single clean commit against upstream.
> 


> Changes in v2:
> - Link to v1: https://lore.kernel.org/linux-trace-kernel/20260227194453.213095-1-anishm7030@gmail.com/
> - Use the helper functions take/release_dentry_name_snapshot() instead of allocating the name. (Al Viro)

The above isn't needed as the above link will give it to you.

No need to resend (unless I find something wrong with the code of the
patch).

-- Steve