[PATCH AUTOSEL 4.9 37/98] ARM: 8677/1: boot/compressed: fix decompressor header layout for v7-M

From: Sasha Levin
Date: Thu Oct 25 2018 - 10:15:25 EST

From: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

[ Upstream commit 06a4b6d009a1b74a6ec46c5418b46cc53a79fcb8 ]

As reported by Patrice, the header layout of the decompressor is
incorrect when building for v7-M. In this case, the __nop macro
resolves to 'mov r0, r0', which is emitted as a narrow encoding,
resulting in the header data fields to end up at lower offsets than

Given the variety of targets we need to support with the same code,
the startup sequence is a bit of a jumble, and uses instructions
and macros whose encoding widths cannot be specified (badr), or only
exist in a narrow encoding (bx)

So force the use of a wide encoding in __nop, and replace the start
sequence with a simple jump to the label marking the start of code,
preceded by a Thumb2 mode switch if required (using explicit wide
encodings where appropriate). The label itself can be moved to the
start of code [where it belongs] due to the larger range of branch
instructions as compared to adr instructions.

Reported-by: Patrice CHOTARD <patrice.chotard@xxxxxx>
Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
arch/arm/boot/compressed/efi-header.S | 4 +---
arch/arm/boot/compressed/head.S | 17 ++++++++++-------
2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/arch/arm/boot/compressed/efi-header.S b/arch/arm/boot/compressed/efi-header.S
index 9d5dc4fda3c1..3f7d1b74c5e0 100644
--- a/arch/arm/boot/compressed/efi-header.S
+++ b/arch/arm/boot/compressed/efi-header.S
@@ -17,14 +17,12 @@
@ there.
.inst 'M' | ('Z' << 8) | (0x1310 << 16) @ tstne r0, #0x4d000
- mov r0, r0
+ W(mov) r0, r0

.macro __EFI_HEADER
- b __efi_start
.set start_offset, __efi_start - start
.org start + 0x3c
diff --git a/arch/arm/boot/compressed/head.S b/arch/arm/boot/compressed/head.S
index fc6d541549a2..2d7f2bb0d66a 100644
--- a/arch/arm/boot/compressed/head.S
+++ b/arch/arm/boot/compressed/head.S
@@ -130,19 +130,22 @@ start:
.rept 7
- ARM( mov r0, r0 )
- ARM( b 1f )
- THUMB( badr r12, 1f )
- THUMB( bx r12 )
+ mov r0, r0
+ AR_CLASS( sub pc, pc, #3 ) @ A/R: switch to Thumb2 mode
+ M_CLASS( nop.w ) @ M: already in Thumb2 mode
+ .thumb
+ W(b) 1f

.word _magic_sig @ Magic numbers to help the loader
.word _magic_start @ absolute load/run zImage address
.word _magic_end @ zImage end address
.word 0x04030201 @ endianness flag

- THUMB( .thumb )
ARM_BE8( setend be ) @ go BE8 if compiled for BE8
AR_CLASS( mrs r9, cpsr )