From: Josh Stone <jist...@redhat.com>
-------------------
This is a commit scheduled for the next v2.6.34 longterm release.
http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
If you see a problem with using this for longterm, please comment.
-------------------
commit 315eb8a2a1b7f335d40ceeeb11b9e067475eb881 upstream.
When compiling an i386_defconfig kernel with gcc-4.6.1-9.fc15.i686, I
noticed a warning about the asm operand for test_bit in kprobes'
can_boost. I discovered that this caused only the first long of
twobyte_is_boostable[] to be output.
Jakub filed and fixed gcc PR50571 to correct the warning and this output
issue. But to solve it for less current gcc, we can make kprobes'
twobyte_is_boostable[] non-const, and it won't be optimized out.
Before:
CC arch/x86/kernel/kprobes.o
In file included from include/linux/bitops.h:22:0,
from include/linux/kernel.h:17,
from [...]/arch/x86/include/asm/percpu.h:44,
from [...]/arch/x86/include/asm/current.h:5,
from [...]/arch/x86/include/asm/processor.h:15,
from [...]/arch/x86/include/asm/atomic.h:6,
from include/linux/atomic.h:4,
from include/linux/mutex.h:18,
from include/linux/notifier.h:13,
from include/linux/kprobes.h:34,
from arch/x86/kernel/kprobes.c:43:
[...]/arch/x86/include/asm/bitops.h: In function ‘can_boost.part.1’:
[...]/arch/x86/include/asm/bitops.h:319:2: warning: use of memory input
without lvalue in asm operand 1 is deprecated [enabled by default]
$ objdump -rd arch/x86/kernel/kprobes.o | grep -A1 -w bt
551: 0f a3 05 00 00 00 00 bt %eax,0x0
554: R_386_32 .rodata.cst4
$ objdump -s -j .rodata.cst4 -j .data arch/x86/kernel/kprobes.o
arch/x86/kernel/kprobes.o: file format elf32-i386
Contents of section .data:
0000 48000000 00000000 00000000 00000000 H...............
Contents of section .rodata.cst4:
0000 4c030000 L...
Only a single long of twobyte_is_boostable[] is in the object file.
After, without the const on twobyte_is_boostable:
$ objdump -rd arch/x86/kernel/kprobes.o | grep -A1 -w bt
551: 0f a3 05 20 00 00 00 bt %eax,0x20
554: R_386_32 .data
$ objdump -s -j .rodata.cst4 -j .data arch/x86/kernel/kprobes.o
arch/x86/kernel/kprobes.o: file format elf32-i386
Contents of section .data:
0000 48000000 00000000 00000000 00000000 H...............
0010 00000000 00000000 00000000 00000000 ................
0020 4c030000 0f000200 ffff0000 ffcff0c0 L...............
0030 0000ffff 3bbbfff8 03ff2ebb 26bb2e77 ....;.......&..w
Now all 32 bytes are output into .data instead.
Signed-off-by: Josh Stone <jist...@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu...@hitachi.com>
Cc: Jakub Jelinek <ja...@redhat.com>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Paul Gortmaker <paul.gortma...@windriver.com>
---
arch/x86/kernel/kprobes.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index ac4ed92..9619315 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/arch/x86/kernel/kprobes.c
@@ -75,8 +75,10 @@ DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
/*
* Undefined/reserved opcodes, conditional jump, Opcode Extension
* Groups, and some special opcodes can not boost.
+ * This is non-const to keep gcc from statically optimizing it out, as
+ * variable_test_bit makes gcc think only *(unsigned long*) is used.
*/
-static const u32 twobyte_is_boostable[256 / 32] = {
+static u32 twobyte_is_boostable[256 / 32] = {
/* 0 1 2 3 4 5 6 7 8 9 a b c d e f */
/* ---------------------------------------------- */
W(0x00, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 0, 0) | /* 00 */
--
1.7.12.rc2
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