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

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Jan 12, 2026 at 02:22:38PM +0100, Borislav Petkov wrote:
> On Mon, Jan 12, 2026 at 08:22:36PM +0800, Fushuai Wang wrote:
> > > strncpy_from_user() succeeds even if userspace data does not contain a
> > > nul. Then it reads length bytes.
> > 
> > Yes, but if there is no NUL byte in the user buf, whether you use
> > strncpy_from_user() or copy_from_user(), you need to manually add
> > a '\0' in the kernel buf to ensure it is properly NUL-terminated.
> 
> This looks like a bunch of churn to save a "= \0" line.
> 
> The more important question, IMO, would be whether there are cases in the
> kernel which *miss* a NUL termination, audit them and fix them.
> 
> That'll give you a better idea whether such a *_nul() helper is even needed.
> 
> Because converting only a handful of obvious places in the face of thousands
> of copy_from_user() invocations in the kernel is not doing anything useful.

I'm getting the impression that strncpy_from_user() is the real problem
here. It should append a nul byte.

That would also be a lot less error-prone than the suggested new API.
Requiring a length of N+1 is not the most immediately-obvious API.

Alice