[PATCH v7 1/4] liveupdate: add LUO_SESSION_MAGIC magic inode type

luca.boccassi
2026-04-18T14:11:31Z

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@gmail.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
This was requested by Lennart and Christian when discussing integration
with systemd for LUO management.

v2: apply one fix from bot review about cleanup on error path:
https://sashiko.dev/#/patchset/20260415184536.1155220-1-luca.boccassi%40gmail.com
the other comments are invalid: luo is not a kmod, and the write hooks are not set up
v3: add test case to liveupdate selftest
v4: split test case in separate follow-up patch
v5: add r-b tag, merge series with LIVEUPDATE_SESSION_GET_NAME ioctl
as they both change the same unit test source file, to avoid merge conflicts
v7: apply suggestion from review bot to stub out setattr ops,
like it was done in 22bdf3d6581a

include/uapi/linux/magic.h | 1 +
kernel/liveupdate/luo_core.c | 10 +++-
kernel/liveupdate/luo_internal.h | 2 +
kernel/liveupdate/luo_session.c | 89 ++++++++++++++++++++++++++++++--
4 files changed, 96 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index 4f2da935a76c..4f51005522ff 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -105,5 +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" */

#endif /* __LINUX_MAGIC_H__ */
diff --git a/kernel/liveupdate/luo_core.c b/kernel/liveupdate/luo_core.c
index dda7bb57d421..f1a63ebe4fa4 100644
--- a/kernel/liveupdate/luo_core.c
+++ b/kernel/liveupdate/luo_core.c
@@ -197,9 +197,17 @@ static int __init luo_late_startup(void)
if (!liveupdate_enabled())
return 0;

+ err = luo_session_fs_init();
+ if (err) {
+ luo_global.enabled = false;
+ return err;
+ }
+
err = luo_fdt_setup();
- if (err)
+ if (err) {
+ luo_session_fs_cleanup();
luo_global.enabled = false;
+ }

return err;
}
diff --git a/kernel/liveupdate/luo_internal.h b/kernel/liveupdate/luo_internal.h
index 8083d8739b09..d4ac7b4c5882 100644
--- a/kernel/liveupdate/luo_internal.h
+++ b/kernel/liveupdate/luo_internal.h
@@ -79,6 +79,8 @@ struct luo_session {

int luo_session_create(const char *name, struct file **filep);
int luo_session_retrieve(const char *name, struct file **filep);
+int __init luo_session_fs_init(void);
+void __init luo_session_fs_cleanup(void);
int __init luo_session_setup_outgoing(void *fdt);
int __init luo_session_setup_incoming(void *fdt);
int luo_session_serialize(void);
diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
index 25ae704d7787..1d35379e4e52 100644
--- a/kernel/liveupdate/luo_session.c
+++ b/kernel/liveupdate/luo_session.c
@@ -50,7 +50,6 @@

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

-#include <linux/anon_inodes.h>
#include <linux/cleanup.h>
#include <linux/err.h>
#include <linux/errno.h>
@@ -62,7 +61,10 @@
#include <linux/libfdt.h>
#include <linux/list.h>
#include <linux/liveupdate.h>
+#include <linux/magic.h>
+#include <linux/mount.h>
#include <linux/mutex.h>
+#include <linux/pseudo_fs.h>
#include <linux/rwsem.h>
#include <linux/slab.h>
#include <linux/unaligned.h>
@@ -363,18 +365,73 @@ static const struct file_operations luo_session_fops = {
.unlocked_ioctl = luo_session_ioctl,
};

+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.
+ */
+static int luo_session_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
+ struct iattr *attr)
+{
+ return -EOPNOTSUPP;
+}
+
+static const struct inode_operations luo_session_inode_operations = {
+ .setattr = luo_session_setattr,
+};
+
+static char *luo_session_dname(struct dentry *dentry, char *buffer, int buflen)
+{
+ return dynamic_dname(buffer, buflen, "luo_session:%s",
+ dentry->d_name.name);
+}
+
+static const struct dentry_operations luo_session_dentry_operations = {
+ .d_dname = luo_session_dname,
+};
+
+static int luo_session_init_fs_context(struct fs_context *fc)
+{
+ struct pseudo_fs_context *ctx;
+
+ ctx = init_pseudo(fc, LUO_SESSION_MAGIC);
+ if (!ctx)
+ return -ENOMEM;
+
+ fc->s_iflags |= SB_I_NOEXEC;
+ fc->s_iflags |= SB_I_NODEV;
+ ctx->dops = &luo_session_dentry_operations;
+ return 0;
+}
+
+static struct file_system_type luo_session_fs_type = {
+ .name = "luo_session",
+ .init_fs_context = luo_session_init_fs_context,
+ .kill_sb = kill_anon_super,
+};
+
/* Create a "struct file" for session */
static int luo_session_getfile(struct luo_session *session, struct file **filep)
{
- char name_buf[128];
+ char name_buf[LIVEUPDATE_SESSION_NAME_LENGTH + 1];
struct file *file;

lockdep_assert_held(&session->mutex);
- snprintf(name_buf, sizeof(name_buf), "[luo_session] %s", session->name);
- file = anon_inode_getfile(name_buf, &luo_session_fops, session, O_RDWR);
- if (IS_ERR(file))
+
+ ihold(luo_session_inode);
+
+ snprintf(name_buf, sizeof(name_buf), "%s", session->name);
+ file = alloc_file_pseudo(luo_session_inode, luo_session_mnt, name_buf,
+ O_RDWR, &luo_session_fops);
+ if (IS_ERR(file)) {
+ iput(luo_session_inode);
return PTR_ERR(file);
+ }

+ file->private_data = session;
*filep = file;

return 0;
@@ -649,3 +706,25 @@ void luo_session_resume(void)
up_write(&luo_session_global.outgoing.rwsem);
up_write(&luo_session_global.incoming.rwsem);
}
+
+int __init luo_session_fs_init(void)
+{
+ luo_session_mnt = kern_mount(&luo_session_fs_type);
+ if (IS_ERR(luo_session_mnt))
+ return PTR_ERR(luo_session_mnt);
+
+ 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);
+}
--
2.47.3