[PATCH v9 3/6] liveupdate: add LUO_SESSION_MAGIC magic inode type
Christian Brauner
brauner at kernel.org
Wed Apr 22 04:58:08 PDT 2026
On Mon, Apr 20, 2026 at 03:15:11PM +0100, luca.boccassi at gmail.com wrote:
> From: Luca Boccassi <luca.boccassi at gmail.com>
>
> In userspace when managing LUO sessions we want to be able to identify
> a FD as a LUO session, in order to be able to do the special handling
> that they require in order to function as intended on kexec.
>
> Currently this requires scraping procfs and doing string matching on
> the prefix of the dname, which is not an ideal interface.
>
> Add a singleton inode type with a magic value, so that we can
> programmatically identify a fd as a LUO session via fstatfs().
>
> Signed-off-by: Luca Boccassi <luca.boccassi at gmail.com>
> Reviewed-by: Pasha Tatashin <pasha.tatashin at soleen.com>
> ---
Fold the following diff [1] into this commit (and the test), please.
Btw, I see that currently you only allow to retrieve one file per
session. If you ever want to allow multiple files for a session
but single shared inode amongst the same session so that different files
for the same session still compare as identical I added a mechanism
called path_from_stashed() which would easily allow for this. The luo
session just needs to stash a dentry pointer to achieve this. Just a
heads-up.
[1]:
>From 7a11a931eaf4766e70c81d32093668ecd60f4952 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner at kernel.org>
Date: Wed, 22 Apr 2026 12:41:25 +0200
Subject: [PATCH] liveupdate: allocate per-session inodes
The luo_session pseudo-fs handed every session the same singleton inode,
so fstat() on two session fds could not tell them apart. Give each
session its own inode via new_inode_pseudo(), with a unique cookie for
i_ino (and the upper bits in i_generation so 32-bit configs still get
distinct inode numbers).
Leave inode->i_fop unset. Session file descriptors must only come from
the LIVEUPDATE_IOCTL_{CREATE,RETRIEVE}_SESSION ioctls, so an unset i_fop
makes open() on /proc/<pid>/fd/N fail.
Lifetime continues to hang off file->private_data and the .release
callback; this patch only changes where the inode comes from.
Rename LUO_SESSION_MAGIC to LUO_FS_MAGIC and flip the session_fstat
selftest to assert distinct inode numbers.
Signed-off-by: Christian Brauner <brauner at kernel.org>
---
include/uapi/linux/magic.h | 2 +-
kernel/liveupdate/luo_session.c | 59 +++++++++++++------
.../testing/selftests/liveupdate/liveupdate.c | 13 ++--
3 files changed, 48 insertions(+), 26 deletions(-)
diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index 4f51005522ff..afc3f935d00d 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -105,6 +105,6 @@
#define PID_FS_MAGIC 0x50494446 /* "PIDF" */
#define GUEST_MEMFD_MAGIC 0x474d454d /* "GMEM" */
#define NULL_FS_MAGIC 0x4E554C4C /* "NULL" */
-#define LUO_SESSION_MAGIC 0x4c554f53 /* "LUOS" */
+#define LUO_FS_MAGIC 0x4c554f53 /* "LUOS" */
#endif /* __LINUX_MAGIC_H__ */
diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
index 885e1f346caa..2484feaf102a 100644
--- a/kernel/liveupdate/luo_session.c
+++ b/kernel/liveupdate/luo_session.c
@@ -51,6 +51,7 @@
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/cleanup.h>
+#include <linux/cookie.h>
#include <linux/err.h>
#include <linux/errno.h>
#include <linux/file.h>
@@ -380,17 +381,43 @@ static const struct file_operations luo_session_fops = {
};
static struct vfsmount *luo_session_mnt __ro_after_init;
-static struct inode *luo_session_inode __ro_after_init;
-/*
- * Reject all attribute changes on the singleton session inode.
- * Without this the VFS falls back to simple_setattr(), allowing
- * fchmod()/fchown() to modify the shared inode.
- */
+DEFINE_COOKIE(luo_session_cookie);
+
+static u64 luo_session_alloc_ino(void)
+{
+ guard(preempt)();
+ return gen_cookie_next(&luo_session_cookie);
+}
+
+/* .setattr rejects attribute changes to prevent fchmod()/fchown(). */
static const struct inode_operations luo_session_inode_operations = {
.setattr = anon_inode_setattr,
};
+static struct inode *luo_new_inode(struct super_block *sb)
+{
+ struct inode *inode;
+ u64 ino;
+
+ inode = new_inode_pseudo(sb);
+ if (!inode)
+ return ERR_PTR(-ENOMEM);
+
+ ino = luo_session_alloc_ino();
+ inode->i_ino = ino;
+ inode->i_generation = ino >> 32;
+ inode->i_flags |= S_IMMUTABLE | S_PRIVATE | S_ANON_INODE;
+ inode->i_mode |= S_IRUSR | S_IWUSR;
+ inode->i_uid = current_fsuid();
+ inode->i_gid = current_fsgid();
+ inode->i_op = &luo_session_inode_operations;
+ /* i_fop left unset: session fds must not be reopenable via procfs. */
+ simple_inode_init_ts(inode);
+
+ return inode;
+}
+
static char *luo_session_dname(struct dentry *dentry, char *buffer, int buflen)
{
return dynamic_dname(buffer, buflen, "luo_session:[%s]",
@@ -410,7 +437,7 @@ static int luo_session_init_fs_context(struct fs_context *fc)
{
struct pseudo_fs_context *ctx;
- ctx = init_pseudo(fc, LUO_SESSION_MAGIC);
+ ctx = init_pseudo(fc, LUO_FS_MAGIC);
if (!ctx)
return -ENOMEM;
@@ -432,17 +459,20 @@ static struct file_system_type luo_session_fs_type = {
static int luo_session_getfile(struct luo_session *session, struct file **filep)
{
char name_buf[LIVEUPDATE_SESSION_NAME_LENGTH + 1];
+ struct inode *inode;
struct file *file;
lockdep_assert_held(&session->mutex);
- ihold(luo_session_inode);
+ inode = luo_new_inode(luo_session_mnt->mnt_sb);
+ if (IS_ERR(inode))
+ return PTR_ERR(inode);
snprintf(name_buf, sizeof(name_buf), "%s", session->name);
- file = alloc_file_pseudo(luo_session_inode, luo_session_mnt, name_buf,
+ file = alloc_file_pseudo(inode, luo_session_mnt, name_buf,
O_RDWR, &luo_session_fops);
if (IS_ERR(file)) {
- iput(luo_session_inode);
+ iput(inode);
return PTR_ERR(file);
}
@@ -731,19 +761,10 @@ int __init luo_session_fs_init(void)
luo_session_mnt = kern_mount(&luo_session_fs_type);
if (IS_ERR(luo_session_mnt))
panic("Cannot create LUO Session pseudo-fs");
-
- luo_session_inode = alloc_anon_inode(luo_session_mnt->mnt_sb);
- if (IS_ERR(luo_session_inode)) {
- kern_unmount(luo_session_mnt);
- return PTR_ERR(luo_session_inode);
- }
- luo_session_inode->i_op = &luo_session_inode_operations;
-
return 0;
}
void __init luo_session_fs_cleanup(void)
{
- iput(luo_session_inode);
kern_unmount(luo_session_mnt);
}
diff --git a/tools/testing/selftests/liveupdate/liveupdate.c b/tools/testing/selftests/liveupdate/liveupdate.c
index c21354dc9b93..34f3ecb21396 100644
--- a/tools/testing/selftests/liveupdate/liveupdate.c
+++ b/tools/testing/selftests/liveupdate/liveupdate.c
@@ -410,8 +410,9 @@ TEST_F(liveupdate_device, create_session_empty_name)
* Test Case: Session fstat
*
* Verifies that fstatfs() on a session file descriptor reports the
- * LUO_SESSION_MAGIC filesystem type, and that fstat() returns consistent
- * inode numbers across different sessions (shared singleton inode).
+ * LUO_FS_MAGIC filesystem type, and that fstat() returns distinct inode
+ * numbers across different sessions so userspace can compare two session
+ * file descriptors for equality using stat().
*/
TEST_F(liveupdate_device, session_fstat)
{
@@ -430,14 +431,14 @@ TEST_F(liveupdate_device, session_fstat)
session_fd2 = create_session(self->fd1, "fstat-session-2");
ASSERT_GE(session_fd2, 0);
- /* Verify the filesystem type is LUO_SESSION_MAGIC */
+ /* Verify the filesystem type is LUO_FS_MAGIC */
ASSERT_EQ(fstatfs(session_fd1, &sfs), 0);
- EXPECT_EQ(sfs.f_type, LUO_SESSION_MAGIC);
+ EXPECT_EQ(sfs.f_type, LUO_FS_MAGIC);
- /* Verify both sessions share the same inode number */
+ /* Each session has its own inode, so inode numbers must differ. */
ASSERT_EQ(fstat(session_fd1, &st1), 0);
ASSERT_EQ(fstat(session_fd2, &st2), 0);
- EXPECT_EQ(st1.st_ino, st2.st_ino);
+ EXPECT_NE(st1.st_ino, st2.st_ino);
ASSERT_EQ(close(session_fd1), 0);
ASSERT_EQ(close(session_fd2), 0);
--
2.47.3
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