Re: [RFC PATCH v2 11/20] objtool: arm64: Walk instructions and compute CFI for each instruction

Madhavan T. Venkataraman
2022-05-29T15:18:08Z

On 5/24/22 08:45, Chen Zhongjin wrote:
> Hi,
>
> On 2022/5/24 8:16, madvenka@linux.microsoft.com wrote:
>> From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
>>
>> Implement arch_initial_func_cfi_state() to initialize the CFI for a
>> function.
>>
>> Add code to fpv_decode() to walk the instructions in every function and
>> compute the CFI information for each instruction.
>>
>> Implement special handling for cases like jump tables.
>>
>> Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
>> ---
>> tools/objtool/arch/arm64/decode.c | 15 +++
>> tools/objtool/fpv.c | 204 ++++++++++++++++++++++++++++++
>> 2 files changed, 219 insertions(+)
> ...
>> +static void update_cfi_state(struct cfi_state *cfi, struct stack_op *op)
>> +{
>> + struct cfi_reg *cfa = &cfi->cfa;
>> + struct cfi_reg *regs = cfi->regs;
>> +
>> + if (op->src.reg == CFI_SP) {
>> + if (op->dest.reg == CFI_SP)
>> + cfa->offset -= op->src.offset;
>> + else
>> + regs[CFI_FP].offset = -cfa->offset + op->src.offset;
> Seems wrong here, we don't have any op->src.offset for [mov x29, sp] so here we
> get: fp->offset = -cfa->offset. The dumped info also proves this.

See the example below.

>
>> + case UNWIND_HINT_TYPE_CALL:
>> + /* Normal call */
>> + frame->cfa += orc->sp_offset;
>> + fp = frame->cfa + orc->fp_offset;
>> + break;
> Obviously this is not conform to the reliability check because we get
> frame->cfa == fp here.
>

See the example below:

> IIUC your sp_offset equals to stack length, and fp_offset is offset from next
> x29 to next CFA. So maybe here we should have
> regs[CFI_FP].offset = regs[CFI_SP].offset for [mov x29, sp].
>
> Anyway, in original objtool sp_offset and fp_offset both represents the offset
> from CFA to REGs. I think it's better not spoiling their original meaning and
> just extending.
>
>

I am not spoiling anything.

Let us take an example:

ffff800008010000 <bcm2835_handle_irq>:
ffff800008010000: d503201f nop
ffff800008010004: d503201f nop
ffff800008010008: d503233f paciasp
ffff80000801000c: a9be7bfd stp x29, x30, [sp, #-32]!
ffff800008010010: 910003fd mov x29, sp
ffff800008010014: f9000bf3 str x19, [sp, #16]

The stack pointer is first moved by -32 and the FP and LR are stored there.
At this point, SP is pointing to the frame. The CFA is:

CFA = SP + 32

The frame pointer has been stored at the location pointed to by the SP.
So, FP should be:

FP = CFA - 32

Therefore, at instruction address ffff800008010014:

frame->cfa = SP + 32;
frame->fp = frame->cfa - 32 = SP;

So, if a call/interrupt happens after this instruction, the frame pointer computed
from the above data will match with the actual frame pointer.

I have verified this using the DWARF data generated by the compiler. It is correct.
I have also verified that the stack trace through such code passes the reliability
check. That is, it computes the frame pointer correctly which matches with the
actual frame pointer.

Madhavan