llvm dev - Dec 2011 - [LLVMdev] load widening conflicts with AddressSanitizer

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 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.

-- 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<mailto: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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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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________________________________
From: Kostya Serebryany [kcc at google.com]
Sent: Wednesday, December 28, 2011 2:46 PM
To: Criswell, John T
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] load widening conflicts with AddressSanitizer



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

I believe it would remove memory unsafe behavior from the program (so the
program is still functionally incorrect, but it would not violate memory safety,
making memory safety attacks impossible).




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?)

If you have modified the clang driver with a -fasn option, then you could also
modify the driver so that it does not run the load-widening pass when the -fasn
option is given on the command-line.

This assumes, of course, that the load-widening transform is not part of some
other transform (like instcombine).  If it is part of another transform, it
would be split out into its own transform (separation of concerns and all that).

-- John T.


--kcc


-- John T.

________________________________
From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at
cs.uiuc.edu> [llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at
cs.uiuc.edu>] on behalf of Kostya Serebryany [kcc at google.com<mailto:kcc
at google.com>]
Sent: Tuesday, December 27, 2011 12:57 PM
To: Chris Lattner
Cc: llvmdev at cs.uiuc.edu<mailto: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<mailto: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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