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

[Will Deacon <will@kernel.org>]

On Wed, Mar 03, 2021 at 06:38:16PM +0100, Christophe Leroy wrote:
> 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@csgroup.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?
> 
> 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.

Hmm. We deliberately setup a _really_ early shadow on arm64 for this, can
you not do something similar? Failing that, I think it would be better to
offer the option for an arch to implement cmdline_*, but have then point to
the normal library routines by default.

> > > +/*
> > > + * 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 ?

If CONFIG_CMDLINE is empty, I don't think you can select
CONFIG_CMDLINE_EXTEND (but even if you could, I don't think it matters).

Will