[PATCH v20 08/11] ARM: kprobes: enable OPTPROBES for ARM 32

On Fri, 2015-01-09 at 16:35 +0000, Russell King - ARM Linux wrote:

On Fri, Jan 09, 2015 at 10:25:54AM +0000, Jon Medhurst (Tixy) wrote:

quoted

On Fri, 2015-01-09 at 14:37 +0800, Wang Nan wrote:

quoted

+			"	and	r4, sp, #4\n"
+			"	sub	sp, sp, r4\n"
+#if __LINUX_ARM_ARCH__ >= 5
+			"	blx	r2\n"
+#else
+			"	mov     lr, pc\n"
+			"	bx	r2\n"

I think the BX instruction is not supported for ARMv4 chips that don't
have Thumb support (e.g. SA110), at least an old ARM ARM I have says BX
is supported on "Version 5 and above, and T variants of version 4".

Correct.

quoted

Though building assabet_defconfig with kprobes enabled doesn't produce
an error for the BX instruction (!?)

Which config are you using?  Does it have CONFIG_CPU_32v4 enabled?

Yes

That should result in "-D__LINUX_ARM_ARCH__=4 -march=armv4" being
passed to the compiler (please check with make V=1).

I does have that, the arguments for compiling this source file
include...

-mno-thumb-interwork -marm -D__LINUX_ARM_ARCH__=4 -march=armv4 -mtune=strongarm1100

Using objdump I can see that the BX instruction does indeed end up in
the code, it hasn't been auto-magically turned into a MOV PC,R2.

Adding in a ".code 16" to the assembler produces "Error: selected
processor does not support THUMB opcodes", so at least it's got that
right. 

I have "gcc version 4.9.1 (Ubuntu/Linaro 4.9.1-16ubuntu6)"

Interestingly...

$ echo 'asm ("bx r2\n");' | arm-linux-gnueabihf-gcc -x c -S -march=armv4 -
<stdin>:1:0: warning: target CPU does not support THUMB instructions
$

but adding -marm gets rid of that error.

$ echo 'asm ("bx r2\n");' | arm-linux-gnueabihf-gcc -x c -S -marm -march=armv4 -
$

-- 
Tixy