[PATCH v2 2/4] string: Allow 2-argument strscpy()

[Geert Uytterhoeven]

Hi Kees,

On Mon, Feb 5, 2024 at 1:37 PM Kees Cook <keescook at chromium.org> wrote:
> 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. There
> 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=um 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#L1517 [1]
> Reviewed-by: Justin Stitt <justinstitt at google.com>
> Cc: Andy Shevchenko <andy at kernel.org>
> Cc: linux-hardening at vger.kernel.org
> Signed-off-by: Kees Cook <keescook at 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_ARGS__)

Likewise

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds