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

["Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>]

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