Re: [PATCH, i386, MPX, 2/X] Pointers Checker [21/25] Size relocation

[Uros Bizjak <ubizjak at gmail dot com>]

On Tue, Nov 26, 2013 at 12:30 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>>> CM_MEDIUM has unlimited data size.
>>>>>
>>>>> i386-opts.h:  CM_MEDIUM,        /* Assumes code fits in the low 31
>>>>> bits; data unlimited.  */
>>>>>
>>>>> The x86_64_zext_immediate_operand allows _address_ to be loaded by
>>>>> movl. The @SIZE reloc returns object size, which is unlimited and has
>>>>> no connection to its address. For CM_MEDIUM,
>>>>> x86_64_zext_immediate_operand allows:
>>>>>
>>>>>       return (ix86_cmodel == CM_SMALL
>>>>>           || (ix86_cmodel == CM_MEDIUM
>>>>>           && !SYMBOL_REF_FAR_ADDR_P (op)));
>>>>
>>>> Yes, but large symbols never have SYMBOL_FLAG_FAR_ADDR set.
>>>
>>> Sorry, I mean all large symbols have this flag set.
>>
>> This flag marks the fact that object size is bigger than
>> -mlarge-data-threshold and goes into large section. It is true that
>> UInt option argument currently overflows at 4G, and consequently all
>> sizes larger than 4G go to large sections, but IMO, we should not rely
>> on this. The specification says unlimited data size, so we have to be
>> prepared for this.
>>
>> The reason I am against checking "Z" constraint is that "@SIZE"
>> relocation returns _size_ of the object (which is only remotely
>> connected to its address), while "Z" checks the address of the object.
>> So, it looks to me that this check would be conceptually wrong.
>>
>> The right place to "fix" movabs is in a linker relaxation pass that
>> can check the real size and change movabs to movl if the size fits
>> 32bits.
>>
>> As a side note, perhaps generic option subsystem should reject
>> overflows as e.g. -mlarge-data-threshold=5368709120 when 32bit UInt is
>> used for option argument. The effects of overflow are surprising.
>
> Hi Uros,
>
> Thanks for explanations! I'll make a fix for the patch.
>
> BTW Pointer Bounds Checker feature does not go to 4.9 and therefore
> this size relocation instructions is not required in 4.9. Suppose it
> should go into 4.10 because doubt this feature is useful for anyone
> except me.
>
> This also affects all MPX instructions. With no checker in 4.9, no one
> actually uses them.  Please tell me if I should pull them back from
> trunk.

The instructions in *.md are usually well isolated, so I'd suggest to
remove them. Since all issues in this part ware resolved, they could
be added back without problems.

BR,
Uros.