linux-kernel - Re: [PATCH 1/5] arm: decompressor: set malloc pool size for the decompressor

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Date:   Mon, 1 Jun 2020 16:10:31 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Lukasz Stelmach <l.stelmach@...sung.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Enrico Weigelt <info@...ux.net>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Ben Dooks <ben-linux@...ff.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 1/5] arm: decompressor: set malloc pool size for the
 decompressor

On Mon, Jun 01, 2020 at 04:56:40PM +0200, Lukasz Stelmach wrote:
> It was <2020-06-01 pon 15:46>, when Russell King - ARM Linux admin wrote:
> > On Mon, Jun 01, 2020 at 04:27:50PM +0200, Łukasz Stelmach wrote:
> >> Move the definition of malloc pool size of the decompressor to
> >> a single place. This value will be exposed later for kexec_file loader.
> >> 
> >> Signed-off-by: Łukasz Stelmach <l.stelmach@...sung.com>
> >> ---
> >>  arch/arm/boot/compressed/Makefile | 2 ++
> >>  arch/arm/boot/compressed/head.S   | 6 ++++--
> >>  2 files changed, 6 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> >> index 9c11e7490292..b3594cd1588c 100644
> >> --- a/arch/arm/boot/compressed/Makefile
> >> +++ b/arch/arm/boot/compressed/Makefile
> >> @@ -125,6 +125,8 @@ KBSS_SZ = $(shell echo $$(($$($(NM) $(obj)/../../../../vmlinux | \
> >>  		sed -n -e 's/^\([^ ]*\) [AB] __bss_start$$/-0x\1/p' \
> >>  		       -e 's/^\([^ ]*\) [AB] __bss_stop$$/+0x\1/p') )) )
> >>  LDFLAGS_vmlinux = --defsym _kernel_bss_size=$(KBSS_SZ)
> >> +# malloc pool size
> >> +LDFLAGS_vmlinux += --defsym _malloc_size=0x10000
> >>  # Supply ZRELADDR to the decompressor via a linker symbol.
> >>  ifneq ($(CONFIG_AUTO_ZRELADDR),y)
> >>  LDFLAGS_vmlinux += --defsym zreladdr=$(ZRELADDR)
> >> diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
> >> index e8e1c866e413..dcc1afa60fb9 100644
> >> --- a/arch/arm/boot/compressed/head.S
> >> +++ b/arch/arm/boot/compressed/head.S
> >> @@ -309,7 +309,8 @@ restart:	adr	r0, LC0
> >>  #ifndef CONFIG_ZBOOT_ROM
> >>  		/* malloc space is above the relocated stack (64k max) */
> >>  		add	sp, sp, r0
> >> -		add	r10, sp, #0x10000
> >> +		ldr	r10, =_malloc_size
> >> +		add	r10, r10, sp
> >
> > This says "locate _malloc_size in a literal pool somewhere, and load it
> > using a PC-relative offset".  Are you sure that the literal pool is
> > sensibly located?
> >
> > Would it be better to use a definition for this?
> 
> I've followed ZRELADDR way. I think both values should be handled the
> same way (it makes it easier to comprehend IMHO). Is there any reason
> not to?  Should I change ZRELADDR for v2 too?

There's a good reason.  ZRELADDR can be a constant that does not fit
into the ARMs immediate constants (8 bits of significant data plus
a multiple of a 2-bit shift), whereas the size of the malloc space
is guaranteed to fit.  So, ZRELADDR has to use a literal load.
This doesn't.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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