linux-kernel - Re: [PATCH v2 2/2] mm/memfd: Use strncpy_from

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Message-ID: <CAH5fLghH+637BDpEYpRZ+qMF=WdiLoJ6T93vPwRCaPvEdh_mNw@mail.gmail.com>
Date: Wed, 8 Jan 2025 14:43:12 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: "Isaac J. Manjarres" <isaacmanjarres@...gle.com>
Cc: lorenzo.stoakes@...cle.com, Andrew Morton <akpm@...ux-foundation.org>, 
	kaleshsingh@...gle.com, jstultz@...gle.com, surenb@...gle.com, 
	kernel-team@...roid.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm/memfd: Use strncpy_from_user() to read memfd name

On Tue, Jan 7, 2025 at 7:48 PM Isaac J. Manjarres
<isaacmanjarres@...gle.com> wrote:
>
> The existing logic uses strnlen_user() to calculate the length of the
> memfd name from userspace and then copies the string into a buffer using
> copy_from_user(). This is error-prone, as the string length
> could have changed between the time when it was calculated and when the
> string was copied. The existing logic handles this by ensuring that the
> last byte in the buffer is the terminating zero.
>
> This handling is contrived and can better be handled by using
> strncpy_from_user(), which gets the length of the string and copies
> it in one shot. Therefore, simplify the logic for copying the memfd
> name by using strncpy_from_user().
>
> No functional change.
>
> Signed-off-by: Isaac J. Manjarres <isaacmanjarres@...gle.com>

Looks okay to me. One nit below, but:

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>

> +       /* length does not include terminating zero */
> +       len = strncpy_from_user(name + MFD_NAME_PREFIX_LEN, uname, MFD_NAME_MAX_LEN + 1);

Can we have this comment say "returned length" instead of just
"length"? Or just remove the comment. Initially I thought you were
talking about the last argument, and I was confused as that does
include the nul-terminator.

Alice