linux-kernel - [PATCH v2 5/5] ARM: kprobes-test: Workaround GAS .align bug

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Date:	Fri, 10 Jan 2014 19:58:26 +0200
From:	Taras Kondratiuk <taras.kondratiuk@...aro.org>
To:	Russell King <linux@....linux.org.uk>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Jon Medhurst <tixy@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Cc:	patches@...aro.org, linaro-networking@...aro.org,
	linaro-kernel@...ts.linaro.org,
	Taras Kondratiuk <taras.kondratiuk@...aro.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 5/5] ARM: kprobes-test: Workaround GAS .align bug

By default if no fill symbol is given to .align directive in a code
section it fills gap with NOPs. If previous fragment is not
instruction-aligned, additional pre-alignment is done by zero bytes
before NOPs. These zero bytes are marked as data by special symbol $d in
symbol table. Unfortunately GAS assumes that there is only code in the
code section so it "puts back" code symbol $a at the end of this
pre-alignment. So if there is some data after alignment it will be
interpreted as code and will be swapped back to LE for BE8 system during
a final linking.

If explicit fill value is given to .align, the NOP-padding code is
skipped and symbol table does not get messed-up.

So the workaround for this issue:
Use explicit fill value if data should be aligned in the code section.

Acked-by: Jon Medhurst <tixy@...aro.org>
Signed-off-by: Taras Kondratiuk <taras.kondratiuk@...aro.org>
---
 arch/arm/kernel/kprobes-test.c |    2 +-
 arch/arm/kernel/kprobes-test.h |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kernel/kprobes-test.c b/arch/arm/kernel/kprobes-test.c
index 0850c72..d2c2174 100644
--- a/arch/arm/kernel/kprobes-test.c
+++ b/arch/arm/kernel/kprobes-test.c
@@ -113,7 +113,7 @@
  *	@ start of inline data...
  *	.ascii "mov r0, r7"	@ text title for test case
  *	.byte	0
- *	.align	2
+ *	.align	2, 0
  *
  *	@ TEST_ARG_REG
  *	.byte	ARG_TYPE_REG
diff --git a/arch/arm/kernel/kprobes-test.h b/arch/arm/kernel/kprobes-test.h
index e28a869..eecc90a 100644
--- a/arch/arm/kernel/kprobes-test.h
+++ b/arch/arm/kernel/kprobes-test.h
@@ -115,7 +115,7 @@ struct test_arg_end {
 	/* multiple strings to be concatenated.  */		\
 	".ascii "#title"				\n\t"	\
 	".byte	0					\n\t"	\
-	".align	2					\n\t"
+	".align	2, 0					\n\t"

 #define	TEST_ARG_REG(reg, val)					\
 	".byte	"__stringify(ARG_TYPE_REG)"		\n\t"	\
-- 
1.7.9.5

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