[PATCH v4 1/2] liveupdate: add LUO_SESSION_MAGIC magic inode type

[PATCH v4 1/2] 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.

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

 include/uapi/linux/magic.h       |  1 +
 kernel/liveupdate/luo_core.c     | 10 ++++-
 kernel/liveupdate/luo_internal.h |  2 +
 kernel/liveupdate/luo_session.c  | 73 +++++++++++++++++++++++++++++---
 4 files changed, 80 insertions(+), 6 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..f1a63ebe4fa44 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 8083d8739b093..d4ac7b4c58829 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 af9e498206d24..f30151c37bd43 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 +704,24 @@ 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;
+}
+
+void __init luo_session_fs_cleanup(void)
+{
+	iput(luo_session_inode);
+	kern_unmount(luo_session_mnt);
+}
-- 
2.47.3

[PATCH v4 2/2] selftests/liveupdate: add test case for LUO_SESSION_MAGIC

[luca.boccassi]

From: Luca Boccassi <luca.boccassi@gmail.com>

Verify that fstat works as expected after the switch from anon_inode
to the new magic number.

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
---
v4: split test case in separate patch, reorder variable declarations

 .../testing/selftests/liveupdate/liveupdate.c | 40 +++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/tools/testing/selftests/liveupdate/liveupdate.c b/tools/testing/selftests/liveupdate/liveupdate.c
index bdc1cedc4f944..3c21352f023a7 100644
--- a/tools/testing/selftests/liveupdate/liveupdate.c
+++ b/tools/testing/selftests/liveupdate/liveupdate.c
@@ -22,9 +22,12 @@
 #include <fcntl.h>
 #include <string.h>
 #include <sys/ioctl.h>
+#include <sys/stat.h>
+#include <sys/vfs.h>
 #include <unistd.h>
 
 #include <linux/liveupdate.h>
+#include <linux/magic.h>
 
 #include "../kselftest.h"
 #include "../kselftest_harness.h"
@@ -416,4 +419,41 @@ TEST_F(liveupdate_device, get_session_name_max_length)
 	ASSERT_EQ(close(session_fd), 0);
 }
 
+/*
+ * 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).
+ */
+TEST_F(liveupdate_device, session_fstat)
+{
+	int session_fd1, session_fd2;
+	struct stat st1, st2;
+	struct statfs sfs;
+
+	self->fd1 = open(LIVEUPDATE_DEV, O_RDWR);
+	if (self->fd1 < 0 && errno == ENOENT)
+		SKIP(return, "%s does not exist", LIVEUPDATE_DEV);
+	ASSERT_GE(self->fd1, 0);
+
+	session_fd1 = create_session(self->fd1, "fstat-session-1");
+	ASSERT_GE(session_fd1, 0);
+
+	session_fd2 = create_session(self->fd1, "fstat-session-2");
+	ASSERT_GE(session_fd2, 0);
+
+	/* Verify the filesystem type is LUO_SESSION_MAGIC */
+	ASSERT_EQ(fstatfs(session_fd1, &sfs), 0);
+	EXPECT_EQ(sfs.f_type, LUO_SESSION_MAGIC);
+
+	/* Verify both sessions share the same inode number */
+	ASSERT_EQ(fstat(session_fd1, &st1), 0);
+	ASSERT_EQ(fstat(session_fd2, &st2), 0);
+	EXPECT_EQ(st1.st_ino, st2.st_ino);
+
+	ASSERT_EQ(close(session_fd1), 0);
+	ASSERT_EQ(close(session_fd2), 0);
+}
+
 TEST_HARNESS_MAIN
-- 
2.47.3

Re: [PATCH v4 1/2] liveupdate: add LUO_SESSION_MAGIC magic inode type

[Pasha Tatashin]

On 04-17 21:57, luca.boccassi@gmail.com wrote:
> 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>

Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>

Re: [PATCH v4 2/2] selftests/liveupdate: add test case for LUO_SESSION_MAGIC

[Pasha Tatashin]

On 04-17 21:57, luca.boccassi@gmail.com wrote:
> From: Luca Boccassi <luca.boccassi@gmail.com>
> 
> Verify that fstat works as expected after the switch from anon_inode
> to the new magic number.
> 
> Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>

Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>

> ---
> v4: split test case in separate patch, reorder variable declarations
> 
>  .../testing/selftests/liveupdate/liveupdate.c | 40 +++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/tools/testing/selftests/liveupdate/liveupdate.c b/tools/testing/selftests/liveupdate/liveupdate.c
> index bdc1cedc4f944..3c21352f023a7 100644
> --- a/tools/testing/selftests/liveupdate/liveupdate.c
> +++ b/tools/testing/selftests/liveupdate/liveupdate.c
> @@ -22,9 +22,12 @@
>  #include <fcntl.h>
>  #include <string.h>
>  #include <sys/ioctl.h>
> +#include <sys/stat.h>
> +#include <sys/vfs.h>
>  #include <unistd.h>
>  
>  #include <linux/liveupdate.h>
> +#include <linux/magic.h>
>  
>  #include "../kselftest.h"
>  #include "../kselftest_harness.h"
> @@ -416,4 +419,41 @@ TEST_F(liveupdate_device, get_session_name_max_length)
>  	ASSERT_EQ(close(session_fd), 0);
>  }
>  
> +/*
> + * 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).
> + */
> +TEST_F(liveupdate_device, session_fstat)
> +{
> +	int session_fd1, session_fd2;
> +	struct stat st1, st2;
> +	struct statfs sfs;
> +
> +	self->fd1 = open(LIVEUPDATE_DEV, O_RDWR);
> +	if (self->fd1 < 0 && errno == ENOENT)
> +		SKIP(return, "%s does not exist", LIVEUPDATE_DEV);
> +	ASSERT_GE(self->fd1, 0);
> +
> +	session_fd1 = create_session(self->fd1, "fstat-session-1");
> +	ASSERT_GE(session_fd1, 0);
> +
> +	session_fd2 = create_session(self->fd1, "fstat-session-2");
> +	ASSERT_GE(session_fd2, 0);
> +
> +	/* Verify the filesystem type is LUO_SESSION_MAGIC */
> +	ASSERT_EQ(fstatfs(session_fd1, &sfs), 0);
> +	EXPECT_EQ(sfs.f_type, LUO_SESSION_MAGIC);
> +
> +	/* Verify both sessions share the same inode number */
> +	ASSERT_EQ(fstat(session_fd1, &st1), 0);
> +	ASSERT_EQ(fstat(session_fd2, &st2), 0);
> +	EXPECT_EQ(st1.st_ino, st2.st_ino);
> +
> +	ASSERT_EQ(close(session_fd1), 0);
> +	ASSERT_EQ(close(session_fd2), 0);
> +}
> +
>  TEST_HARNESS_MAIN
> -- 
> 2.47.3
>