[PATCH] ARM: unwind: Fix handling of "Pop r4-r[4+nnn], r14" opcode

[Anurag Aggarwal]

I must of missed the wrong opcode, as my major focus was on preventing
the Data abort.


On Wed, Apr 30, 2014 at 4:22 PM, Nikolay Borisov
<Nikolay.Borisov at arm.com> wrote:
> The arm EABI states that opcode 10100nnn means pop register r4-4[4+nnn],
> aditionally there is a similar opcode: 10101nnn which means the same thing plus
> popping r14. Those two cases are handled by the unwind_exec_pop_r4_to_rN
> function which checks whether the 4th bit is set and does r14 popping.
>
> However, up until now it has been checking whether the 8th was set (mask & 0x80)
> instead of the 4th (mask & 0x8), a simple to make typo but this meant that we
> were always popping r14 even if we had the former opcode.
>
> This patch changes the mask so that the 2 opcodes are being handled correctly.
>
> Signed-off-by: Nikolay Borisov <Nikolay.Borisov at arm.com>
Reviewed-by: Anurag Aggarwal <anurag19aggarwal at gmail.com>
> ---
>
> I tested this patch using magic sysrq to generate backtrace of every process and
> it was working. However, inspecting the unwind table for vmlinuz it seems that
> the majority of the instructions do in fact require popping r14 so that's why
> this has gone unnoticed for a long period, however I'm open to suggestions how
> to more thoroughly test this.
>
> The first thing that comes to mind is manually fiddle with the unwind table of a
> well-known function so that it no longer requires r14 being popped.
>
>  arch/arm/kernel/unwind.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index 3c21769..cb791ac 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -285,7 +285,7 @@ static int unwind_exec_pop_r4_to_rN(struct unwind_ctrl_block *ctrl,
>                 if (unwind_pop_register(ctrl, &vsp, reg))
>                                 return -URC_FAILURE;
>
> -       if (insn & 0x80)
> +       if (insn & 0x8)
>                 if (unwind_pop_register(ctrl, &vsp, 14))
>                                 return -URC_FAILURE;
>
> --
> 1.8.1.5
>
>
>
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-- 
Anurag Aggarwal