llvm dev - Jan 2012 - [LLVMdev] load widening conflicts with AddressSanitizer

________________________________
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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Hi,
[resurrecting an old mail thread about AddressSanitizer false positive
caused by load widening]

Once the Attribute::AddressSafety is set by clang (a separate patch),
fixing this bug may look as simple as this:

--- lib/Analysis/MemoryDependenceAnalysis.cpp   (revision 148708)
+++ lib/Analysis/MemoryDependenceAnalysis.cpp   (working copy)
@@ -323,6 +323,14 @@
         !TD.fitsInLegalInteger(NewLoadByteSize*8))
       return 0;

+    if
(LI->getParent()->getParent()->hasFnAttr(Attribute::AddressSafety)
&&
+        LIOffs+NewLoadByteSize > MemLocEnd) {
+      // We will be reading past the location accessed by the original
program.
+      // While this is safe in a regular build, Address Safety analysys
tools
+      // may start reporting false warnings. So, do't do widening.
+      return 0;
+    }
+
     // If a load of this width would include all of MemLoc, then we
succeed.
     if (LIOffs+NewLoadByteSize >= MemLocEnd)
       return NewLoadByteSize;


Other suggestions, objections?

Thanks,
--kcc





On Wed, Dec 28, 2011 at 1:12 PM, Criswell, John T <criswell at
illinois.edu>wrote:
>
>  ------------------------------
> *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>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 [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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Hi Kostya,
> [resurrecting an old mail thread about AddressSanitizer false positive
caused by
> load widening]
>
> Once the Attribute::AddressSafety is set by clang (a separate patch),
fixing
> this bug may look as simple as this:
I don't get the point of an attribute.  There's plenty of code out there
that does wide loads like this directly (without them being created by the
optimizers) since, just like the optimizers, they know it is safe and a win.
The attribute won't help them.  It looks like a way of just hiding the real
problem, which seems to be that address sanitizer is overly strict.

Ciao, Duncan.
>
> --- lib/Analysis/MemoryDependenceAnalysis.cpp   (revision 148708)
> +++ lib/Analysis/MemoryDependenceAnalysis.cpp   (working copy)
> @@ -323,6 +323,14 @@
>           !TD.fitsInLegalInteger(NewLoadByteSize*8))
>         return 0;
> +    if
(LI->getParent()->getParent()->hasFnAttr(Attribute::AddressSafety)
&&
> +        LIOffs+NewLoadByteSize > MemLocEnd) {
> +      // We will be reading past the location accessed by the original
program.
> +      // While this is safe in a regular build, Address Safety analysys
tools
> +      // may start reporting false warnings. So, do't do widening.
> +      return 0;
> +    }
> +
>       // If a load of this width would include all of MemLoc, then we
succeed.
>       if (LIOffs+NewLoadByteSize >= MemLocEnd)
>         return NewLoadByteSize;
>
>
> Other suggestions, objections?
>
> Thanks,
> --kcc
>
>
>
>
>
> On Wed, Dec 28, 2011 at 1:12 PM, Criswell, John T <criswell at
illinois.edu
> <mailto:criswell at illinois.edu>> wrote:
>
>
>     *From:* Kostya Serebryany [kcc at google.com <mailto:kcc at
google.com>]
>     *Sent:* Wednesday, December 28, 2011 2:46 PM
>     *To:* Criswell, John T
>
>     *Cc:* llvmdev at cs.uiuc.edu <mailto: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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