[PATCH] liveupdate: add LUO_SESSION_MAGIC magic inode type

[PATCH] liveupdate: add LUO_SESSION_MAGIC magic inode type

[luca.boccassi]

From: Luca Boccassi <luca.boccassi@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 fstat().

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
---
This was requested by Lennart and Christian when discussing integration
with systemd for LUO management.

 include/uapi/linux/magic.h       |  1 +
 kernel/liveupdate/luo_core.c     |  6 +++
 kernel/liveupdate/luo_internal.h |  1 +
 kernel/liveupdate/luo_session.c  | 67 +++++++++++++++++++++++++++++---
 4 files changed, 70 insertions(+), 5 deletions(-)

diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index 4f2da935a76cc..4f51005522ffe 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 dda7bb57d421c..ec00c681a0f50 100644
--- a/kernel/liveupdate/luo_core.c
+++ b/kernel/liveupdate/luo_core.c
@@ -197,6 +197,12 @@ 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)
 		luo_global.enabled = false;
diff --git a/kernel/liveupdate/luo_internal.h b/kernel/liveupdate/luo_internal.h
index 8083d8739b093..223a9dc1ca29b 100644
--- a/kernel/liveupdate/luo_internal.h
+++ b/kernel/liveupdate/luo_internal.h
@@ -79,6 +79,7 @@ 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);
 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 af9e498206d24..fbc4b32b39cb8 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>
@@ -376,18 +378,58 @@ 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;
+
+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;
@@ -662,3 +704,18 @@ 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);
+	}
+
+	return 0;
+}
-- 
2.47.3