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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 27 Sep 2023 10:11:05 +0000
Message-ID: <CAMj1kXEUTOpATv_jbimGktSXBBweyfVNPpqWWJaJNp0U-zvg+w@mail.gmail.com>
Subject: Re: [PATCH] ARM: kprobes: Explicitly assign register for local variables
To:     "Aiqun(Maria) Yu" <quic_aiquny@quicinc.com>
Cc:     linux@armlinux.org.uk, mhiramat@kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        kernel@quicinc.com, quic_lijuang@quicinc.com,
        linux-arm-msm@vger.kernel.org
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On Wed, 27 Sept 2023 at 09:44, Aiqun(Maria) Yu <quic_aiquny@quicinc.com> wr=
ote:
>
> On 9/27/2023 5:26 PM, Ard Biesheuvel wrote:
> > Hello Maria,
> >
> > On Wed, 27 Sept 2023 at 06:00, Maria Yu <quic_aiquny@quicinc.com> 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=E2=80=83=E2=80=83r7, [r1, #16] //r7 used for store asi->insn_fn
> ...
> ldmia.w=E2=80=83=E2=80=83ip, {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               :=3D test-core.o
 ifdef CONFIG_THUMB2_KERNEL
 obj-$(CONFIG_KPROBES)          +=3D actions-thumb.o checkers-thumb.o
 test-kprobes-objs              +=3D test-thumb.o
+KBUILD_CFLAGS                  +=3D -fno-omit-frame-pointer
 else
 obj-$(CONFIG_KPROBES)          +=3D actions-arm.o checkers-arm.o
 obj-$(CONFIG_OPTPROBES)                +=3D opt-arm.o