[LLVMdev] load widening conflicts with AddressSanitizer

[Duncan Sands]

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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