Received-SPF: pass (google.com: domain of linux-kernel+bounces-52653-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Precedence: bulk
MIME-Version: 1.0
References: <20240205122916.it.909-kees@kernel.org> <20240205123525.1379299-2-keescook@chromium.org>
In-Reply-To: <20240205123525.1379299-2-keescook@chromium.org>
From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Mon, 5 Feb 2024 13:47:08 +0100
Message-ID: <CAMuHMdU2c75WDCX+ptQgB7h0taHjG2pwL9db6gE3LKxv5Vz04Q@mail.gmail.com>
Subject: Re: [PATCH v2 2/4] string: Allow 2-argument strscpy()
To: Kees Cook <keescook@chromium.org>
Cc: Andy Shevchenko <andy@kernel.org>, Justin Stitt <justinstitt@google.com>, 
	linux-hardening@vger.kernel.org, Richard Weinberger <richard@nod.at>, 
	Anton Ivanov <anton.ivanov@cambridgegreys.com>, Johannes Berg <johannes@sipsolutions.net>, 
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>, Jason Wang <jasowang@redhat.com>, 
	kernel test robot <lkp@intel.com>, Nathan Chancellor <nathan@kernel.org>, 
	Azeem Shaikh <azeemshaikh38@gmail.com>, linux-kernel@vger.kernel.org, 
	linux-um@lists.infradead.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

Hi Kees,

On Mon, Feb 5, 2024 at 1:37=E2=80=AFPM Kees Cook <keescook@chromium.org> wr=
ote:
> Using sizeof(dst) for the "size" argument in strscpy() is the
> overwhelmingly common case. Instead of requiring this everywhere, allow a
> 2-argument version to be used that will use the sizeof() internally. Ther=
e
> are other functions in the kernel with optional arguments[1], so this
> isn't unprecedented, and improves readability. Update and relocate the
> kern-doc for strscpy() too.
>
> Adjust ARCH=3Dum build to notice the changed export name, as it doesn't
> do full header includes for the string helpers.
>
> This could additionally let us save a few hundred lines of code:
>  1177 files changed, 2455 insertions(+), 3026 deletions(-)
> with a treewide cleanup using Coccinelle:
>
> @needless_arg@
> expression DST, SRC;
> @@
>
>         strscpy(DST, SRC
> -, sizeof(DST)
>         )
>
> Link: https://elixir.bootlin.com/linux/v6.7/source/include/linux/pci.h#L1=
517 [1]
> Reviewed-by: Justin Stitt <justinstitt@google.com>
> Cc: Andy Shevchenko <andy@kernel.org>
> Cc: linux-hardening@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>

Thanks for your patch!

> --- a/include/linux/string.h
> +++ b/include/linux/string.h

> +/*
> + * The 2 argument style can only be used when dst is an array with a
> + * known size.
> + */
> +#define __strscpy0(dst, src, ...)      \
> +       sized_strscpy(dst, src, sizeof(dst) + __must_be_array(dst))
> +#define __strscpy1(dst, src, size)     sized_strscpy(dst, src, size)

(dst), (src), (size) etc.


> +
> +/**
> + * strscpy - Copy a C-string into a sized buffer
> + * @dst: Where to copy the string to
> + * @src: Where to copy the string from
> + * @...: Size of destination buffer (optional)
> + *
> + * Copy the source string @src, or as much of it as fits, into the
> + * destination @dst buffer. The behavior is undefined if the string
> + * buffers overlap. The destination @dst buffer is always NUL terminated=
,
> + * unless it's zero-sized.
> + *
> + * The size argument @... is only required when @dst is not an array, or
> + * when the copy needs to be smaller than sizeof(@dst).
> + *
> + * Preferred to strncpy() since it always returns a valid string, and
> + * doesn't unnecessarily force the tail of the destination buffer to be
> + * zero padded. If padding is desired please use strscpy_pad().
> + *
> + * Returns the number of characters copied in @dst (not including the
> + * trailing %NUL) or -E2BIG if @size is 0 or the copy from @src was
> + * truncated.
> + */
> +#define strscpy(dst, src, ...) \
> +       CONCATENATE(__strscpy, COUNT_ARGS(__VA_ARGS__))(dst, src, __VA_AR=
GS__)

Likewise

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k=
org

In personal conversations with technical people, I call myself a hacker. Bu=
t
when I'm talking to journalists I just say "programmer" or something like t=
hat.
                                -- Linus Torvalds