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

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Message-ID: <20260112132238.GDaWT1ntPxY8VD1hlH@fat_crate.local>
Date: Mon, 12 Jan 2026 14:22:38 +0100
From: Borislav Petkov <bp@...en8.de>
To: Fushuai Wang <fushuai.wang@...ux.dev>
Cc: aliceryhl@...gle.com, 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 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.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette