On Mon, Jan 12, 2026 at 11:01 AM Fushuai Wang [off-list ref] wrote:

quoted

quoted

From: Fushuai Wang <redacted>

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 <redacted>

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