On Wed, 27 Sept 2023 at 09:44, Aiqun(Maria) Yu <quic_aiquny@xxxxxxxxxxx> wrote:
>
> On 9/27/2023 5:26 PM, Ard Biesheuvel wrote:
> > Hello Maria,
> >
> > On Wed, 27 Sept 2023 at 06:00, Maria Yu <quic_aiquny@xxxxxxxxxxx> wrote:
> >>
> >> Registers r7 is removed in clobber list, so compiler may choose r7 for
> >> local variables usage, while r7 will be actually updated by the inline asm
> >> code.
> >
> > The inline asm does not update R7, it preserves and restores it.
> That is the asm is updating r7 purposely and compiler still choose r7
> for the asm local varialbe and use it inside the asm code.
> So the change is to fix the issue when "r7 is removed from the clobber
> list of current asm code while actually r7 shouldn't be choosed for the
> current asm local variables".
> The issue is only reproducible when ftrace is not enabled, and r7 is
> removed from the current clobber list.
>
> Let me have the assemble code that will make you understand better.
>
> --the original code:
> "mov r11, r7 \n\t"
> ...
> "ldmia %[regs], {r0-r7} \n\t"
> "blx %[fn] \n\t"
> ...
> "mov r7, r11 \n\t"
>
> --After compile to choose register for [fn] and [regs].
> mov r11, r7
> ldr r7, [r1, #16] //r7 used for store asi->insn_fn
> ...
> ldmia.w ip, {r0, r1, r2, r3, r4, r5, r6, r7}
> blx r7
> ...
> mov r7,r11
>
> The current change is to avoid by fix the registers for local variable
> usage and not choose r7 for [fn].
>
OK, I understand now, thanks.
Would it help if we just always enabled frame pointers for this source file?
--- a/arch/arm/probes/kprobes/Makefile
+++ b/arch/arm/probes/kprobes/Makefile
@@ -9,6 +9,7 @@ test-kprobes-objs := test-core.o
ifdef CONFIG_THUMB2_KERNEL
obj-$(CONFIG_KPROBES) += actions-thumb.o checkers-thumb.o
test-kprobes-objs += test-thumb.o
+KBUILD_CFLAGS += -fno-omit-frame-pointer
else
obj-$(CONFIG_KPROBES) += actions-arm.o checkers-arm.o
obj-$(CONFIG_OPTPROBES) += opt-arm.o
