On Tue, Mar 31, 2026 at 1:55 PM <luca.boccassi@gmail.com> wrote:
>
> 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);

I am confused by this change, the maximum session name length is: 64,
as defined in:
include/uapi/linux/liveupdate.h
/* The maximum length of session name including null termination */
#define LIVEUPDATE_SESSION_NAME_LENGTH 64

So, 128 should be enough to include:
"[luo_session] %s", session->name

Or am I missing something?

Pasha