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

[Christophe Leroy]

Le 03/03/2021 à 18:28, Will Deacon a écrit :
> 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@xxxxxxxxxx>
> > ---
> >   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?

Because on powerpc (at least), it will be used in prom_init, it is very early in the boot and KASAN 
shadow memory is not set up yet so calling generic string functions would crash the board.

> > +/*
> > + * 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?

Ah yes, since cbe46bd4f510 ("powerpc: remove CONFIG_CMDLINE #ifdef mess") it is feasible. I can 
change that now.

> > +	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.

It is to avoid adding a white space at the end of the command line when CONFIG_CMDLINE is empty. But 
maybe it doesn't matter ?

Christophe