Re: [PATCH v2 1/6] uaccess: Add copy_from_user_nul helper

[Alice Ryhl]

On Mon, Jan 12, 2026 at 11:01 AM Fushuai Wang <fushuai.wang@xxxxxxxxx> wrote:
>
> >> From: Fushuai Wang <wangfushuai@xxxxxxxxx>
> >>
> >> Many places call copy_from_user() to copy a buffer from user space,
> >> and then manually add a NULL terminator to the destination buffer,
> >> e.g.:
> >>
> >>      if (copy_from_user(dest, src, len))
> >>              return -EFAULT;
> >>      dest[len] = '\0';
> >>
> >> This is repetitive and error-prone. Add a copy_from_user_nul() helper to
> >> simplify such patterns. It copied n bytes from user space to kernel space,
> >> and NUL-terminates the destination buffer.
> >>
> >> Signed-off-by: Fushuai Wang <wangfushuai@xxxxxxxxx>
> >
> > Hmm, this function is very very similar to strncpy_from_user(). Should
> > they be using that instead?
> >
> > Alice
>
> The strncpy_from_user() is for NUL-terminated strings and stops at the
> first NUL in userspace. But copy_from_user_nul() always copies a fixed length
> and adds a NUL at the end in kernel space, even if userspace data doesn’t
> contain a NUL.
>
> So I think they are for different cases and can’t replace each other.

strncpy_from_user() succeeds even if userspace data does not contain a
nul. Then it reads length bytes.

As far as I can tell, when a nul byte is present, none of these kernel
use-cases use data after the nul byte. So the behavior is identical
except that copy_from_user_nul() may result in EFAULT if there are
unmapped bytes between the first nul byte in `src` and `src+len`.

Alice