linux-kernel - Re: [PATCH v2 1/7] cmdline: Add generic function to build command line.

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Date:   Wed, 3 Mar 2021 17:28:10 +0000
From:   Will Deacon <will@...nel.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, danielwa@...co.com,
        robh@...nel.org, daniel@...pelevich.san-francisco.ca.us,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arch@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 1/7] cmdline: Add generic function to build command
 line.

On Tue, Mar 02, 2021 at 05:25:17PM +0000, Christophe Leroy wrote:
> This code provides architectures with a way to build command line
> based on what is built in the kernel and what is handed over by the
> bootloader, based on selected compile-time options.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  include/linux/cmdline.h | 62 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 62 insertions(+)
>  create mode 100644 include/linux/cmdline.h
> 
> diff --git a/include/linux/cmdline.h b/include/linux/cmdline.h
> new file mode 100644
> index 000000000000..ae3610bb0ee2
> --- /dev/null
> +++ b/include/linux/cmdline.h
> @@ -0,0 +1,62 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_CMDLINE_H
> +#define _LINUX_CMDLINE_H
> +
> +static __always_inline size_t cmdline_strlen(const char *s)
> +{
> +	const char *sc;
> +
> +	for (sc = s; *sc != '\0'; ++sc)
> +		; /* nothing */
> +	return sc - s;
> +}
> +
> +static __always_inline size_t cmdline_strlcat(char *dest, const char *src, size_t count)
> +{
> +	size_t dsize = cmdline_strlen(dest);
> +	size_t len = cmdline_strlen(src);
> +	size_t res = dsize + len;
> +
> +	/* This would be a bug */
> +	if (dsize >= count)
> +		return count;
> +
> +	dest += dsize;
> +	count -= dsize;
> +	if (len >= count)
> +		len = count - 1;
> +	memcpy(dest, src, len);
> +	dest[len] = 0;
> +	return res;
> +}

Why are these needed instead of using strlen and strlcat directly?

> +/*
> + * This function will append a builtin command line to the command
> + * line provided by the bootloader. Kconfig options can be used to alter
> + * the behavior of this builtin command line.
> + * @dest: The destination of the final appended/prepended string.
> + * @src: The starting string or NULL if there isn't one. Must not equal dest.
> + * @length: the length of dest buffer.
> + */
> +static __always_inline void cmdline_build(char *dest, const char *src, size_t length)
> +{
> +	if (length <= 0)
> +		return;
> +
> +	dest[0] = 0;
> +
> +#ifdef CONFIG_CMDLINE
> +	if (IS_ENABLED(CONFIG_CMDLINE_FORCE) || !src || !src[0]) {
> +		cmdline_strlcat(dest, CONFIG_CMDLINE, length);
> +		return;
> +	}
> +#endif

CONFIG_CMDLINE_FORCE implies CONFIG_CMDLINE, and even if it didn't,
CONFIG_CMDLINE is at worst an empty string. Can you drop the #ifdef?

> +	if (dest != src)
> +		cmdline_strlcat(dest, src, length);
> +#ifdef CONFIG_CMDLINE
> +	if (IS_ENABLED(CONFIG_CMDLINE_EXTEND) && sizeof(CONFIG_CMDLINE) > 1)
> +		cmdline_strlcat(dest, " " CONFIG_CMDLINE, length);
> +#endif

Likewise, but also I'm not sure why the sizeof() is required.

Will