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

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

Le 03/03/2021 à 18:46, Will Deacon a écrit :
> 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.

I don't think it is possible to setup an earlier early shadow.

At the point we are in prom_init, the code is not yet relocated at the address it was linked for, 
and it is running with the MMU set by the bootloader, I can't imagine being able to setup MMU 
entries for the early shadow KASAN yet without breaking everything.

Is it really worth trying to point to the normal library routines by default ? It is really only a 
few lines of code hence only not many bytes, and anyway they are in __init section so they get 
discarded at the end.

> 
>>>> +/*
>>>> + * 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).

Ok I'll simplify that when I re-spin.

Christophe