Re: [PATCH] liveupdate: truncate getfile name to NAME_MAX

[Luca Boccassi <luca.boccassi@gmail.com>]

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On Tue, 31 Mar 2026, 19:21 Pasha Tatashin, <pasha.tatashin@soleen.com>
wrote:

> 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
>

Yeah we need to to bump that size, it's too small - see the shared doc.
This is anyway needed for safety, as the user space call falls apart
completely if it goes over the hard coded limit, so it should always be
used as a max value

>

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