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

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Date:   Fri, 5 Mar 2021 13:49:03 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will@...nel.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, 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.



Le 05/03/2021 à 12:58, Michael Ellerman a écrit :
> Will Deacon <will@...nel.org> writes:
>> On Wed, Mar 03, 2021 at 06:57:09PM +0100, Christophe Leroy wrote:
>>> 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@...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?
>>>>>
>>>>> 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.
>>
>> That's very similar to us; we're not relocated, although we are at least
>> in control of the MMU (which is using a temporary set of page-tables).
> 
> prom_init runs as an OF client, with the MMU off (except on some Apple
> machines), and we don't own the MMU. So there's really nothing we can do :)
> 
> Though now that I look at it, I don't think we should be doing this
> level of commandline handling in prom_init. It should just grab the
> value from firmware and pass it to the kernel proper, and then all the
> prepend/append/force etc. logic should happen there.

But then, how do you handle the command line parameters that are needed by prom_init ?

For instance, prom_init_mem() use 'prom_memory_limit', which comes from the "mem=" option in the 
command line.

Christophe