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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 21 Nov 2023 11:11:56 -0500
Message-ID: <CAMj1kXHLT6PhT0v6=9DWS1bXDV+QSZDwnYDo=+KvpzrCBqPNrw@mail.gmail.com>
Subject: Re: [PATCH v3 1/1] ARM: kprobes: Explicitly reserve r7 for local variables
To:     Maria Yu <quic_aiquny@quicinc.com>
Cc:     Nathan Chancellor <nathan@kernel.org>, linux@armlinux.org.uk,
        mhiramat@kernel.org, kernel@quicinc.com,
        linux-arm-kernel@lists.infradead.org,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        quic_lijuang@quicinc.com, stable@vger.kernel.org
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On Sun, 19 Nov 2023 at 22:29, 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. This caused the runtime behavior wrong.
> While those kind of reserved registers cannot be set to clobber list
> because of error like "inline asm clobber list contains reserved
> registers".
> Explicitly reserve r7 by adding attribute no-omit-frame-pointer for this
> file, then in T32 asm code r7 is used as a frame pointer and is not
> available for use as a general-purpose register.
> Note that "no-omit-frame-pointer" will make the code size a little bigger
> to store the stack frame pointer.
>
> Fixes: dd12e97f3c72 ("ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds")
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Maria Yu <quic_aiquny@quicinc.com>
> Cc: stable@vger.kernel.org
> ---
>  arch/arm/probes/kprobes/Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/arm/probes/kprobes/Makefile b/arch/arm/probes/kprobes/Makefile
> index 6159010dac4a..b1f21e78950b 100644
> --- a/arch/arm/probes/kprobes/Makefile
> +++ b/arch/arm/probes/kprobes/Makefile
> @@ -8,6 +8,7 @@ test-kprobes-objs               := test-core.o
>
>  ifdef CONFIG_THUMB2_KERNEL
>  obj-$(CONFIG_KPROBES)          += actions-thumb.o checkers-thumb.o
> +CFLAGS_actions-thumb.o         += -fno-omit-frame-pointer
>  test-kprobes-objs              += test-thumb.o
>  else
>  obj-$(CONFIG_KPROBES)          += actions-arm.o checkers-arm.o
>

If Nathan is happy with this, I think we can drop this into the patch tracker.