Re: [PATCH v2] ARM:unwind:fix unwind abort for uleb128 case

[AngeloGioacchino Del Regno]

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@xxxxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxx>