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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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.

> 
>> +/*
>> + * 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