Re: [PATCH v2 2/2] mm/memfd: Use strncpy_from_user() to read memfd name

[Isaac Manjarres <isaacmanjarres@google.com>]

On Wed, Jan 08, 2025 at 02:43:12PM +0100, Alice Ryhl wrote:
> On Tue, Jan 7, 2025 at 7:48 PM Isaac J. Manjarres
> <isaacmanjarres@google.com> wrote:
> >
> > The existing logic uses strnlen_user() to calculate the length of the
> > memfd name from userspace and then copies the string into a buffer using
> > copy_from_user(). This is error-prone, as the string length
> > could have changed between the time when it was calculated and when the
> > string was copied. The existing logic handles this by ensuring that the
> > last byte in the buffer is the terminating zero.
> >
> > This handling is contrived and can better be handled by using
> > strncpy_from_user(), which gets the length of the string and copies
> > it in one shot. Therefore, simplify the logic for copying the memfd
> > name by using strncpy_from_user().
> >
> > No functional change.
> >
> > Signed-off-by: Isaac J. Manjarres <isaacmanjarres@google.com>
> 
> Looks okay to me. One nit below, but:
> 
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> 
> > +       /* length does not include terminating zero */
> > +       len = strncpy_from_user(name + MFD_NAME_PREFIX_LEN, uname, MFD_NAME_MAX_LEN + 1);
> 
> Can we have this comment say "returned length" instead of just
> "length"? Or just remove the comment. Initially I thought you were
> talking about the last argument, and I was confused as that does
> include the nul-terminator.
> 
> Alice

Yes, I will update it to say returned length and add your "Reviewed-by"
tag. Thanks for this!

--Isaac