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

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Date:   Fri, 05 Mar 2021 22:58:02 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Will Deacon <will@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>
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.

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.

cheers