linux-kernel - [PATCH] arch/tile: make sure to build memcpy_user

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Message-Id: <201203302105.q2UL53K2006282@farm-0012.internal.tilera.com>
Date:	Thu, 29 Mar 2012 14:05:04 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Chris Metcalf <cmetcalf@...era.com>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] arch/tile: make sure to build memcpy_user_64 without frame pointer

Add a comment explaining why this is important, and add a CFLAGS_REMOVE
clause to the Makefile to make sure it happens.

Signed-off-by: Chris Metcalf <cmetcalf@...era.com>
---
 arch/tile/lib/Makefile         |    1 +
 arch/tile/lib/memcpy_user_64.c |    8 +++++++-
 2 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/arch/tile/lib/Makefile b/arch/tile/lib/Makefile
index 0c26086..985f598 100644
--- a/arch/tile/lib/Makefile
+++ b/arch/tile/lib/Makefile
@@ -7,6 +7,7 @@ lib-y = cacheflush.o checksum.o cpumask.o delay.o uaccess.o \
 	strchr_$(BITS).o strlen_$(BITS).o
 
 ifeq ($(CONFIG_TILEGX),y)
+CFLAGS_REMOVE_memcpy_user_64.o = -fno-omit-frame-pointer
 lib-y += memcpy_user_64.o
 else
 lib-y += atomic_32.o atomic_asm_32.o memcpy_tile64.o
diff --git a/arch/tile/lib/memcpy_user_64.c b/arch/tile/lib/memcpy_user_64.c
index 4763b3a..37440ca 100644
--- a/arch/tile/lib/memcpy_user_64.c
+++ b/arch/tile/lib/memcpy_user_64.c
@@ -14,7 +14,13 @@
  * Do memcpy(), but trap and return "n" when a load or store faults.
  *
  * Note: this idiom only works when memcpy() compiles to a leaf function.
- * If "sp" is updated during memcpy, the "jrp lr" will be incorrect.
+ * Here leaf function not only means it does not have calls, but also
+ * requires no stack operations (sp, stack frame pointer) and no
+ * use of callee-saved registers, else "jrp lr" will be incorrect since
+ * unwinding stack frame is bypassed. Since memcpy() is not complex so
+ * these conditions are satisfied here, but we need to be careful when
+ * modifying this file. This is not a clean solution but is the best
+ * one so far.
  *
  * Also note that we are capturing "n" from the containing scope here.
  */
-- 
1.6.5.2

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