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Message-ID: <20140221132024.GD21483@n2100.arm.linux.org.uk>
Date: Fri, 21 Feb 2014 13:20:24 +0000
From: Russell King - ARM Linux <linux@....linux.org.uk>
To: Dave Martin <Dave.Martin@....com>
Cc: Kees Cook <keescook@...omium.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Larry Bassel <lbassel@...eaurora.org>,
Stephen Rothwell <sfr@...b.auug.org.au>,
Nicolas Pitre <nico@...aro.org>,
Ben Dooks <ben.dooks@...ethink.co.uk>,
Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>,
Grant Likely <grant.likely@...aro.org>,
Jiang Liu <liuj97@...il.com>,
Christoffer Dall <cdall@...columbia.edu>,
Laura Abbott <lauraa@...eaurora.org>,
Marc Zyngier <marc.zyngier@....com>,
Rob Herring <rob.herring@...xeda.com>,
Vitaly Andrianov <vitalya@...com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
Jonathan Austin <jonathan.austin@....com>,
Simon Baatz <gmbnomis@...il.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Santosh Shilimkar <santosh.shilimkar@...com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] ARM: mm: keep rodata non-executable
On Fri, Feb 21, 2014 at 12:37:04PM +0000, Dave Martin wrote:
> It would be good if someone who's more familiar with the parms and
> vmlinux.lds stuff could take a look at it, though I don't see any
> obvious problem yet.
The biggest issue with it is that we end up with:
- the .text section rounded up to 1MB
- the .rodata section rounded up to 1MB
That means we can end up wasting up to 1MB of memory for each (in the
worst case where we encroach into the next 1MB aligned region by a few
bytes) and this memory can't be re-used.
The alternative is to adjust the maps such that we end up mapping the
.text / .rodata overlap 1MB using 4K pages, taking the additional TLB
hit by doing so.
The .text is aligned to 1MB, so the majority of the first 0x8000 to
0x100000 is unused. The end of the .text section is aligned to 1MB,
and the start of the .data section is also aligned to 1MB.
So, the minimum kernel size is: 0x100000 + MB_ALIGN(sizeof(.text)) +
MB_ALIGN(sizeof(.rodata)) + MB_ALIGN(sizeof(init sections)) + sizeof(.data)
- 0x8000
So, looking at this kernel I've recently built:
Idx Name Size VMA LMA File off Algn
0 .head.text 00000204 c0008000 c0008000 00008000 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
--- .text this gets set to 0xc0100000, we lose 0xc0008240 to 0xc0100000
1 .text 006c4530 c0008240 c0008240 00008240 2**6
CONTENTS, ALLOC, LOAD, READONLY, CODE
2 .text.head 0000004c c06cc770 c06cc770 006cc770 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
--- sizeof(.text) + sizeof(.text.head) becomes 0x700000
--- .rodata starts at 0xc0800000 instead of 0xc06cd000
3 .rodata 0022f568 c06cd000 c06cd000 006cd000 2**6
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 __bug_table 0000873c c08fc568 c08fc568 008fc568 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .pci_fixup 00000030 c0904ca4 c0904ca4 00904ca4 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 __ksymtab 00008158 c0904cd4 c0904cd4 00904cd4 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 __ksymtab_gpl 00006858 c090ce2c c090ce2c 0090ce2c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
8 __kcrctab 000040ac c0913684 c0913684 00913684 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
9 __kcrctab_gpl 0000342c c0917730 c0917730 00917730 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 __ksymtab_strings 00022a08 c091ab5c c091ab5c 0091ab5c 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
11 __param 00000c70 c093d564 c093d564 0093d564 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
12 __modver 00000e2c c093e1d4 c093e1d4 0093e1d4 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
13 __ex_table 00000f18 c093f000 c093f000 0093f000 2**3
CONTENTS, ALLOC, LOAD, READONLY, DATA
14 .notes 00000024 c093ff18 c093ff18 0093ff18 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
15 .vectors 00000020 00000000 c0940000 00940000 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
16 .stubs 00000240 00001000 c0940020 00941000 2**5
CONTENTS, ALLOC, LOAD, READONLY, CODE
17 .init.text 00051760 c0940260 c0940260 00948260 2**5
CONTENTS, ALLOC, LOAD, READONLY, CODE
18 .exit.text 00002130 c09919c0 c09919c0 009999c0 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
19 .init.arch.info 00000108 c0993af0 c0993af0 0099baf0 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
20 .init.tagtable 00000048 c0993bf8 c0993bf8 0099bbf8 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
21 .init.smpalt 000032f8 c0993c40 c0993c40 0099bc40 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
22 .init.pv_table 00000314 c0996f38 c0996f38 0099ef38 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
23 .init.data 0000c19c c0997250 c0997250 0099f250 2**3
CONTENTS, ALLOC, LOAD, DATA
24 .data..percpu 000035c0 c09a4000 c09a4000 009ac000 2**6
CONTENTS, ALLOC, LOAD, DATA
--- sizeof previous sections is 0x2db000, which becomes 0x300000
--- start of .data becomes 0xc0b00000 instead of 0xc09a8000
25 .data 00062728 c09a8000 c09a8000 009b0000 2**6
CONTENTS, ALLOC, LOAD, DATA
26 .bss 00754870 c0a0a740 c0a0a740 00a12728 2**6
ALLOC
--- which means the kernel image finishes at 0xC12B6FB0 whereas it used
to finish at 0xC115EFB0.
27 .comment 00000011 00000000 00000000 00a12728 2**0
CONTENTS, READONLY
28 .ARM.attributes 00000010 00000000 00000000 00a12739 2**0
CONTENTS, READONLY
That's almost 1.5MB larger on an image size of 18MB. Percentage wise,
that sounds small, but the thing to realise is that growth is independent
of the image size, so a smaller image sees a larger %age wise growth in
its size.
People have already complained bitterly when I've said that stealing
memory and taking out out of memblock should always be 1MB aligned, so
/no one/ has the right to say "it's only 1.5MB, it doesn't matter"
because quite frankly they should've been saying that and supporting me
with the memblock issue. So, I really don't want to hear that argument!
However, if you look at where these boundaries are placed, they're not
quite in the right place. For example, the .init.data section is writable,
and so should be grouped with the .data section. So should .data..percpu.
Now, a few other things stand out from the above:
(a) .text.head - imx, sunxi and tegra need to fix that. There is no
specific meaning to it.
(b) .init.text is executable, and can't be in a NX region when it's
set as non-executable.
(c) we can't free the .init sections (sections 15 through up to and
including 23) anymore with this feature enabled because it's setup
as read-only memory.
--
FTTC broadband for 0.8mile line: 5.8Mbps down 500kbps up. Estimation
in database were 13.1 to 19Mbit for a good line, about 7.5+ for a bad.
Estimate before purchase was "up to 13.2Mbit".
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