[LLVMdev] load widening conflicts with AddressSanitizer

[Kostya Serebryany]

On Wed, Dec 28, 2011 at 12:40 PM, Criswell, John T <criswell at illinois.edu>wrote:

>  Dear All,
>
>  I think adding metadata and expecting transforms to repect it is a bad
> idea.  It is just too easy for someone who does not know about the metadata
> to add a transform that ignores it.
>
>  As for SAFECode, I think we have one of several options for handling
> load-widening.  The most obvious one is to have a pass that just boosts the
> allocation size of any alloca with an align 16 attribute;
>

This may lead to real bugs being lost (false negatives)


> this pass would only be scheduled to execute when the other SAFECode
> instrumentation passes are scheduled to execute.  Another option would be
> to just disable the load-widening transform or to use a specialized version
> that only widens when the allocation size does not cause a problem.
>

To disable load widening you need to pass some flag to the load widening
phase.
Passing it through metadata is one of the possible solutions.

Or we can disable load widening from the clang driver, but then we will
need a flag for that (do we have it now?)

--kcc


>
>  -- John T.
>
>  ------------------------------
> *From:* llvmdev-bounces at cs.uiuc.edu [llvmdev-bounces at cs.uiuc.edu] on
> behalf of Kostya Serebryany [kcc at google.com]
> *Sent:* Tuesday, December 27, 2011 12:57 PM
> *To:* Chris Lattner
> *Cc:* llvmdev at cs.uiuc.edu
> *Subject:* Re: [LLVMdev] load widening conflicts with AddressSanitizer
>
>
>
> On Mon, Dec 19, 2011 at 4:27 PM, Chris Lattner <clattner at apple.com> wrote:
>
>>
>> On Dec 17, 2011, at 7:40 AM, Rafael Ávila de Espíndola wrote:
>>
>> > On 16/12/11 08:46 PM, Chris Lattner wrote:
>> >> I'm not opposed to disabling this transformation when asan is on, we
>> just need a clean way to express this in the IR.
>> >
>> > Could clang be aware of asan being on and introduce a "please don't
>> > widen" metadata on local variable accesses?
>>
>>  Yes, "we just need a clean way to express this in the IR."
>>
>> LLVM can't have a global "bool ASANIsOn;" that the optimizers listen to.
>>
>
>
>  A global is bad.
> What about a metadata attached to a Function saying that transformations
> which will read out of bounds (even "safely") are illegal?
> asan and SAFEcode will add this metadata, optimizers will listen to it.
>
>  Any other suggestion?
>
>
>  --kcc
>
>
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