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Subject: Re: [PATCH v2] objtool,ftrace: Implement UNWIND_HINT_RET_OFFSET
From:   Julien Thierry <jthierry@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, x86@kernel.org, mhiramat@kernel.org,
        mbenes@suse.cz, Steven Rostedt <rostedt@goodmis.org>
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Date:   Thu, 2 Apr 2020 07:56:18 +0100
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On 4/2/20 7:41 AM, Julien Thierry wrote:
> 
> 
> On 4/1/20 6:09 PM, Peter Zijlstra wrote:
>> On Wed, Apr 01, 2020 at 04:43:35PM +0100, Julien Thierry wrote:
>>>> +        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
> 

And the x86 decode could also lookup the symbol containing an IRET and 
chose whether its type should be INSN_CONTEXT_SWITCH or INSN_OTHER 
depending on whether the symbol is a function or not.

This would avoid having the arch specific pattern detected the generic 
stack validation part of objtool.

-- 
Julien Thierry