[llvm-dev] Memory sanitizer porting

vit9696 via llvm-dev llvm-dev at lists.llvm.org
Sun Feb 25 01:21:23 PST 2018


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.

2. Thank you for this clarification. It makes good sense now.

3. That’s what I supposed to be the case, thanks.

Best regards,

> 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
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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