[PATCH] liveupdate: truncate getfile name to NAME_MAX

[luca.boccassi@gmail.com]

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

dynamic_dname() harcodes its limit to NAME_MAX bytes, which includes a
NUL terminator and a 'anon_inode:' prefix. If the name passed on goes
above the limit it results in userspace getting a ENAMETOOLONG error.

Truncate the name to NAME_MAX - 12 - 1 characters to ensure this never
fails (accounting for prefix and NUL).

Fixes: 0153094d03df ("liveupdate: luo_session: add sessions support")

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
---
Builds on top of this just sent by Aleksa:

https://lore.kernel.org/linux-fsdevel/20260401-dynamic-dname-name_max-v1-1-8ca20ab2642e@amutable.com/

 kernel/liveupdate/luo_session.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
index 7836772956406..4a40c7fdfb44f 100644
--- a/kernel/liveupdate/luo_session.c
+++ b/kernel/liveupdate/luo_session.c
@@ -366,11 +366,14 @@ static const struct file_operations luo_session_fops = {
 /* 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[NAME_MAX];
 	struct file *file;
 
 	lockdep_assert_held(&session->mutex);
-	snprintf(name_buf, sizeof(name_buf), "[luo_session] %s", session->name);
+	/* dynamic_dname() rejects names above NAME_MAX bytes, including NUL terminator
+	 * and a 'anon_inode:' prefix. Truncate to NAME_MAX - 12 - 1 to avoid
+	 * ENAMETOOLONG. */
+	snprintf(name_buf, NAME_MAX - 12 - 1, "[luo_session] %s", session->name);
 	file = anon_inode_getfile(name_buf, &luo_session_fops, session, O_RDWR);
 	if (IS_ERR(file))
 		return PTR_ERR(file);
-- 
2.47.3