linux-kernel - Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET

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Date:   Thu, 2 Apr 2020 07:41:46 +0100
From:   Julien Thierry <jthierry@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, x86@...nel.org, mhiramat@...nel.org,
        mbenes@...e.cz, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET



On 4/1/20 6:09 PM, Peter Zijlstra wrote:
> On Wed, Apr 01, 2020 at 04:43:35PM +0100, Julien Thierry wrote:
> 
>>> +static bool has_modified_stack_frame(struct instruction *insn, struct insn_state *state)
>>>    {
>>> +	u8 ret_offset = insn->ret_offset;
>>>    	int i;
>>>
>>> -	if (state->cfa.base != initial_func_cfi.cfa.base ||
>>> -	    state->cfa.offset != initial_func_cfi.cfa.offset ||
>>> -	    state->stack_size != initial_func_cfi.cfa.offset ||
>>> -	    state->drap)
>>> +	if (state->cfa.base != initial_func_cfi.cfa.base || state->drap)
>>> +		return true;
>>> +
>>> +	if (state->cfa.offset != initial_func_cfi.cfa.offset &&
>>> +	    !(ret_offset && state->cfa.offset == initial_func_cfi.cfa.offset + ret_offset))
>>
>> Isn't that the same thing as "state->cfa.offset !=
>> initial_func_cfi.cfa.offset + ret_offset" ?
> 
> I'm confused on what cfa.offset is, sometimes it increase with
> stack_size, sometimes it doesn't.
> 

Steven already replied for me about that :) .

> ISTR that for the ftrace case it was indeed cfa.offset + 8, but for the
> IRET case below (where it is now not used anymore) it was cfa.offset
> (not cfa.offset + 40, which I was expecting).
> 
>>> +		return true;
>>> +
>>> +	if (state->stack_size != initial_func_cfi.cfa.offset + ret_offset)
>>>    		return true;
>>>
>>> -	for (i = 0; i < CFI_NUM_REGS; i++)
>>> +	for (i = 0; i < CFI_NUM_REGS; i++) {
>>>    		if (state->regs[i].base != initial_func_cfi.regs[i].base ||
>>>    		    state->regs[i].offset != initial_func_cfi.regs[i].offset)
>>>    			return true;
>>> +	}
>>>
>>>    	return false;
>>>    }
> 
>>> @@ -2185,6 +2148,13 @@ static int validate_branch(struct objtoo
>>>
>>>    			break;
>>>
>>> +		case INSN_EXCEPTION_RETURN:
>>> +			if (func) {
>>> +				state.stack_size -= arch_exception_frame_size;
>>> +				break;
>>
>> Why break instead of returning? Shouldn't an exception return mark the end
>> of a branch (whether inside or outside a function) ?
>>
>> Here it seems it will continue to the next instruction which might have been
>> unreachable.
> 
> The code in question (x86's sync_core()), is an exception return to
> self. It pushes an exception frame that points to right after the
> exception return instruction.
> 
> This is the only usage of IRET in STT_FUNC symbols.
> 
> So rather than teaching objtool how to interpret the whole
> push;push;push;push;push;iret sequence, teach it how big the frame is
> (arch_exception_frame_size) and let it continue.
> 
> All the other (real) IRETs are in STT_NOTYPE in the entry assembly.
> 

Right, I see.. However I'm not completely convinced by this. I must 
admit I haven't followed the whole conversation, but what was the issue 
with the HINT_IRET_SELF? It seemed more elegant, but I might be missing 
some context.

Otherwise, it might be worth having a comment in the code to point that 
this only handles the sync_core() case.


Also, instead of adding a special "arch_exception_frame_size", I could 
suggest:
- Picking this patch [1] from a completely arbitrary source
- Getting rid of INSN_STACK type, any instruction could then include 
stack ops on top of their existing semantics, they can just have an 
empty list if they don't touch SP/BP
- x86 decoder adds a stack_op to the iret to modify the stack pointer by 
the right amount

[1] https://www.spinics.net/lists/kernel/msg3453725.html

Thanks,

-- 
Julien Thierry