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Date:   Wed, 12 Apr 2023 14:43:58 +0200
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Subject: Re: [PATCH] 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>,
        linux-kernel@vger.kernel.org
Cc:     Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        xiaoming.yu@mediatek.com, Linus Walleij <linus.walleij@linaro.org>,
        Russell King <rmk+kernel@armlinux.org.uk>,
        Alex Sverdlin <alexander.sverdlin@nokia.com>,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
References: <20230407033341.5139-1-haibo.li@mediatek.com>
From:   Alexandre Mergnat <amergnat@baylibre.com>
In-Reply-To: <20230407033341.5139-1-haibo.li@mediatek.com>
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Precedence: bulk

On 07/04/2023 05:33, Haibo Li wrote:
> 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>
> ---
>   arch/arm/kernel/unwind.c | 19 +++++++++++++++++--
>   1 file changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index 53be7ea6181b..e5796a5acba1 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -20,7 +20,6 @@
>   #warning    Change compiler or disable ARM_UNWIND option.
>   #endif
>   #endif /* __CHECKER__ */
> -

Why delete this line ?

>   #include <linux/kernel.h>
>   #include <linux/init.h>
>   #include <linux/export.h>
> @@ -308,6 +307,22 @@ 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 result = 0;
> +	unsigned long insn;
> +	unsigned long bytes = 0;

Alphabetical order please.

> +
> +	do {
> +		insn = unwind_get_byte(ctrl);
> +		result |= (insn & 0x7f) << (bytes * 7);
> +		bytes++;
> +		if (bytes == sizeof(result))
> +			break;
> +	} while (!!(insn & 0x80));
> +
> +	return result;
> +}

Please add a blank line for readability.

>   /*
>    * Execute the current unwind instruction.
>    */
> @@ -361,7 +376,7 @@ static int unwind_exec_insn(struct unwind_ctrl_block *ctrl)
>   		if (ret)
>   			goto error;
>   	} else if (insn == 0xb2) {
> -		unsigned long uleb128 = unwind_get_byte(ctrl);
> +		unsigned long uleb128 = unwind_decode_uleb128(ctrl);
>   
>   		ctrl->vrs[SP] += 0x204 + (uleb128 << 2);
>   	} else {

Great job! I'm aligned with Linus Walleij's feedback about the need of 
few comments to explain the decode loop, even if your code is clear, 
light and robust.

-- 
Regards,
Alexandre