linux-kernel - Re: [PATCH v2 1/6] uaccess: Add copy_from_user

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Message-ID: <aWUWBfKhP8DvzhDl@google.com>
Date: Mon, 12 Jan 2026 15:40:53 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Fushuai Wang <fushuai.wang@...ux.dev>, akpm@...ux-foundation.org, brauner@...nel.org, 
	cyphar@...har.com, dave.hansen@...ux.intel.com, hpa@...or.com, jack@...e.cz, 
	kees@...nel.org, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, luto@...nel.org, 
	mathieu.desnoyers@...icios.com, mhiramat@...nel.org, mingo@...hat.com, 
	peterz@...radead.org, rostedt@...dmis.org, tglx@...nel.org, vmalik@...hat.com, 
	wangfushuai@...du.com, x86@...nel.org, yury.norov@...il.com
Subject: Re: [PATCH v2 1/6] uaccess: Add copy_from_user_nul helper

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