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Message-ID: <22bb4f8f-8f4b-6efb-74ab-b33eabc1fbb9@collabora.com>
Date:   Thu, 13 Apr 2023 15:28:07 +0200
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Subject: Re: [PATCH v2] ARM:unwind:fix unwind abort for uleb128 case
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To:     Haibo Li <haibo.li@mediatek.com>,
        Russell King <linux@armlinux.org.uk>,
        Linus Walleij <linus.walleij@linaro.org>,
        amergnat@baylibre.com, linux-kernel@vger.kernel.org
Cc:     Matthias Brugger <matthias.bgg@gmail.com>,
        xiaoming.yu@mediatek.com,
        Russell King <rmk+kernel@armlinux.org.uk>,
        Alex Sverdlin <alexander.sverdlin@nokia.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, ardb@kernel.org,
        catalin.marinas@arm.com, a.anurag@samsung.com
References: <20230413073429.40050-1-haibo.li@mediatek.com>
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <20230413073429.40050-1-haibo.li@mediatek.com>
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Precedence: bulk

Il 13/04/23 09:34, Haibo Li ha scritto:
> When unwind instruction is 0xb2,the subsequent instructions
> are uleb128 bytes.
> For now,it uses only the first uleb128 byte in code.
> 
> For vsp increments of 0x204~0x400,use one uleb128 byte like below:
> 0xc06a00e4 <unwind_test_work>: 0x80b27fac
>    Compact model index: 0
>    0xb2 0x7f vsp = vsp + 1024
>    0xac      pop {r4, r5, r6, r7, r8, r14}
> 
> For vsp increments larger than 0x400,use two uleb128 bytes like below:
> 0xc06a00e4 <unwind_test_work>: @0xc0cc9e0c
>    Compact model index: 1
>    0xb2 0x81 0x01 vsp = vsp + 1032
>    0xac      pop {r4, r5, r6, r7, r8, r14}
> The unwind works well since the decoded uleb128 byte is also 0x81.
> 
> For vsp increments larger than 0x600,use two uleb128 bytes like below:
> 0xc06a00e4 <unwind_test_work>: @0xc0cc9e0c
>    Compact model index: 1
>    0xb2 0x81 0x02 vsp = vsp + 1544
>    0xac      pop {r4, r5, r6, r7, r8, r14}
> In this case,the decoded uleb128 result is 0x101(vsp=0x204+(0x101<<2)).
> While the uleb128 used in code is 0x81(vsp=0x204+(0x81<<2)).
> The unwind aborts at this frame since it gets incorrect vsp.
> 
> To fix this,add uleb128 decode to cover all the above case.
> 
> Signed-off-by: Haibo Li <haibo.li@mediatek.com>
> ---
> v2:
> - As Linus Walleij and Alexandre Mergnat suggested,add comments for unwind_decode_uleb128
> - As Alexandre Mergnat suggested,change variables declaration in Alphabetical order
> ---
>   arch/arm/kernel/unwind.c | 25 ++++++++++++++++++++++++-
>   1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index 53be7ea6181b..f37e55fcf81d 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -308,6 +308,29 @@ static int unwind_exec_pop_subset_r0_to_r3(struct unwind_ctrl_block *ctrl,
>   	return URC_OK;
>   }
>   
> +static unsigned long unwind_decode_uleb128(struct unwind_ctrl_block *ctrl)
> +{
> +	unsigned long bytes = 0;
> +	unsigned long insn;
> +	unsigned long result = 0;
> +
> +	/* unwind_get_byte() will advance ctrl one instruction at a time,
> +	 * we loop until we get an instruction byte where bit 7 is not set.
> +	 * Note:It decodes max 4 bytes to output 28bits data.
> +	 * 28bits data(0xfffffff) covers vsp increments of 1073742336.
> +	 * It is sufficent for unwinding stack.
> +	 */

/*
  * unwind_get_byte() will advance `ctrl` one instruction at a time, so
  * loop until we get an instruction byte where bit 7 is not set.
  *
  * Note: This decodes a maximum of 4 bytes to output 28 bits data where
  * max is 0xfffffff: that will cover a vsp increment of 1073742336, hence
  * it is sufficient for unwinding the stack.
  */

> +	do {
> +		insn = unwind_get_byte(ctrl);
> +		result |= (insn & 0x7f) << (bytes * 7);
> +		bytes++;

also, I would do ...

	} while (!!(insn & 0x80) && bytes != sizeof(result));

...compressing the code and not creating any human readability concern.

after which, you can get my

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>