[PATCH v20 08/11] ARM: kprobes: enable OPTPROBES for ARM 32
Jon Medhurst (Tixy)
tixy at linaro.org
Fri Jan 9 09:28:22 PST 2015
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:
> > On Fri, 2015-01-09 at 14:37 +0800, Wang Nan wrote:
> > > + " 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.
>
> > 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
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