[llvm-dev] Memory sanitizer porting

Evgenii Stepanov via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 26 13:09:11 PST 2018

On Sun, Feb 25, 2018 at 1:21 AM, vit9696 <vit9696 at avp.su> wrote:
> Hi,
> 1. No, there is no custom triple for the platform. It currently uses Linux triple, and I do not think there is a possibility of upstreaming not so many changes in such a way. On the other side Apple uses the mllvm asan option to implement KASAN in XNU, so I think it will be fine to upstream a similar option, which I guess, could also be used for debugging, and may be helpful to other people prototyping their runtimes.

I see. So you want a flag or a set of LLVM flags to specify custom
MemoryMapParams. This is fine, feel free to send the patch for review.

> 2. Thank you for this clarification. It makes good sense now.
> 3. That’s what I supposed to be the case, thanks.
> Best regards,
> Vit
>> 23 февр. 2018 г., в 2:50, Evgenii Stepanov <eugeni.stepanov at gmail.com> написал(а):
>> Hi,
>> 1. This patch adds an internal (-mllvm) option, which is basically
>> meant for debugging. If your custom platform has a target triple, you
>> could submit changes to llvm, clang and compiler-rt to specify any
>> platform-specific offsets and other details.
>> 2. Blacklist is meant to disable checking for bugs in certain
>> functions, not to remove all instrumentation. With ASan, these are the
>> same. With MSan, it places instrumentation in a "safe" mode where, for
>> example, a function that reads from A and stores to B will (1) not
>> check A and (2) make B fully initialized even if data being stored
>> comes from an uninitialized location.
>> Building MSan runtime library with MSan is not going to work, blacklist or not.
>> 3. No, not really. If you manage to allocate shadow and origin, which
>> must be the same size as the application-accessible region each, and
>> define a mapping function between them, it should be possible to make
>> MSan work.
>> On Thu, Feb 22, 2018 at 6:38 AM, vit9696 via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>> Hello,
>>> I am currently porting memory sanitizer to a custom platform, and discovered
>>> some strange things in the existing implementation.
>>> 1. clang/llvm currently hardcode the list of supported platforms and
>>> disallow the use of a standalone msan implementation.
>>> I suppose the solution here is to submit a patch similar to
>>> https://reviews.llvm.org/D18865, which will provide the necessary arguments
>>> to configure the layout.
>>> I have it ready here. Will this approach be fine for the llvm dev team, and
>>> may I post it for the review then?
>>> 2. There exists a concept of -fsanitize-blacklist argument, which is
>>> supposed to exclude source locations from being instrumented by the
>>> sanitizers.
>>> Normally this works without issues, but I discovered that you cannot compile
>>> compiler-rt msan implementation with -fsanitize=memory even if the whole
>>> location is blacklisted (that much I can confirm from the invocations of
>>> CodeGenModule::isInSanitizerBlacklist function).
>>> What happens is that for some reason memory sanitizer:
>>> — still tries to partially instrument the blacklisted code;
>>> — does not check whether its global memory storage variables are already
>>> defined.
>>> The second issue, present in MemorySanitizer::initializeCallbacks, adds a
>>> second copy of storage global variables when compiling msan.cc (e.g.
>>> __emutls_v.__msan_retval_tls.63, __emutls_v.__msan_param_tls.65), and this
>>> results in an undefined reference during the linkage.
>>> Here the question is what was initially intended to be done. I know that
>>> compiling blacklisted asan runtime code with -fsanitize=address is just
>>> fine, and this is what Apple actually does in XNU KASAN implementation.
>>> I kind of expected it to be the right way to do for msan as well, is this
>>> just a bug? If it is not, should I compile msan runtime without
>>> -fsanitize=memory in this case, and in fact asan runtime too?
>>> 3. Other than that, I see that memory sanitizer is currently implemented
>>> only for 64-bit platforms. While I am aware of the issues behind requiring a
>>> lot of memory to use msan, are there any other issues for not supporting
>>> 32-bit?
>>> Best regards,
>>> Vit
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