[LLVMdev] Weird msan problem

Sat Feb 1 21:26:49 PST 2014

Also, I was looking at the instrumented LLVM code and I noticed that the
LLVM global variable is generated with size 1000, while
kMsanParamTlsSizeInWords == 100, could there be some sort of overflow
happening?

On Sat, Feb 1, 2014 at 6:44 PM, Keno Fischer
<kfischer at college.harvard.edu>wrote:

> I have verified that both TLS implementations indeed find the same area of
> memory. Anything else I could look for?
>
>
> On Tue, Jan 28, 2014 at 4:28 PM, Keno Fischer <
> kfischer at college.harvard.edu> wrote:
>
>> Yes, both JIT code and the native runtime are instrumented. I am under
>> the impressions that the the C library should guarantee that from the way
>> the relocations are implemented as long as both native and JITed code are
>> on the same thread (but I will verify this and report back).
>>
>>
>> On Tue, Jan 28, 2014 at 2:41 AM, Evgeniy Stepanov <
>> eugeni.stepanov at gmail.com> wrote:
>>
>>> I assume there are transitions between JITted code and native helper
>>> functions. How are you handling them? Are native functions
>>> MSan-instrumented?
>>> MSan is passing shadow across function calls in TLS slots. Does your
>>> TLS implementation guarantee that accesses to __msan_param_tls from
>>> JITted and from native code map to the same memory?
>>>
>>>
>>> On Mon, Jan 27, 2014 at 11:36 PM, Evgeniy Stepanov
>>> <eugeni.stepanov at gmail.com> wrote:
>>> > This is really cool. I've not heard of anyone using MSan with MSJIT
>>> before.
>>> >
>>> >
>>> > On Mon, Jan 27, 2014 at 7:44 PM, Keno Fischer
>>> > <kfischer at college.harvard.edu> wrote:
>>> >> Hello everybody,
>>> >>
>>> >> I've run into some strange behavior with memory sanitizer that I can't
>>> >> explain and hope somebody with more knowledge of the implementation
>>> would be
>>> >> able to help me out or at least point me into the right direction.
>>> >>
>>> >> For background, I'm using memory sanitizer to check Julia (
>>> julialang.org),
>>> >> which uses (or at least will once I track down a few bugs) MCJIT for
>>> the
>>> >> code compilation. So far I have rebuilt the runtime and all
>>> dependencies
>>> >> (including LLVM, libcxx, etc.) with memory sanitizer enabled and
>>> added the
>>> >> instrumentation pass in the appropriate place in the julia code
>>> generator.
>>> >>
>>> >> I'm now going through the usual bootstrap which basically loads the
>>> standard
>>> >> library and compiles it, does inference, etc. This works fine for
>>> several
>>> >> hours (this is usually much faster - by which I mean several hundred
>>> time -
>>> >> I suspect the issue is with MCJIT having to process a ton more
>>> relocations
>>> >> and code and being inefficient at it, but I can't prove that). That's
>>> not
>>> >> the issue however. Eventually, I get
>>> >>
>>> >> ==17150== WARNING: MemorySanitizer: use-of-uninitialized-value
>>> >>     #0 0x7f417cea3189 in bitvector_any1
>>> >> /home/kfischer/julia-san/src/support/bitvector.c:177
>>> >> [ snip ]
>>> >>
>>> >>   Uninitialized value was created by a heap allocation
>>> >>     #0 0x7f41815de543 in __interceptor_malloc
>>> >>
>>> /home/kfischer/julia-san/deps/llvm-svn/projects/compiler-rt/lib/msan/msan_interceptors.cc:854
>>> >>     #1 0x7f417cc7d7f1 in alloc_big
>>> /home/kfischer/julia-san/src/gc.c:355
>>> >> [snip]
>>> >>
>>> >> Now, by going through it in the debugger, I see
>>> >>
>>> >> (gdb) f 3
>>> >> #3  0x00007f417cea318a in bitvector_any1 (b=0x60c000607240,
>>> >> b at entry=<optimized out>, offs=0, offs at entry=<optimized out>,
>>> nbits=256,
>>> >> nbits at entry=<optimized out>)
>>> >>     at bitvector.c:177
>>> >> 177         if ((b[0] & mask) != 0) return 1;
>>> >> (gdb) p __msan_print_shadow(&b,8)
>>> >> ff ff ff ff ff ff ff ff
>>> >>  o: 3f0010a6  o: 80007666
>>> >>
>>> >> which seems to indicate that the local variable b has uninitialized
>>> data.
>>> >> I'm having a hard time believing that though, since if I look at the
>>> >> functions before it, the place where it's coming from is initialized:
>>> >>
>>> >> #4  0x00007f41755208a8 in julia_isempty248 ()
>>> >> #5  0x00007f417c163e3d in jl_apply (f=0x606000984d60, f at entry
>>> =<optimized
>>> >> out>, args=0x7fff9132da20, args at entry=<optimized out>, nargs=1,
>>> >>     nargs at entry=<optimized out>) at ./julia.h:1043
>>> >>
>>> >> (here's the code of that julia function for reference)
>>> >>
>>> >> isempty(s::IntSet) =
>>> >>     !s.fill1s && ccall(:bitvector_any1, Uint32, (Ptr{Uint32}, Uint64,
>>> >> Uint64), s.bits, 0, s.limit)==0
>>> >>
>>> >> Looking at where that value is coming from:
>>> >>
>>> >> (gdb) f 5
>>> >> #5  0x00007f417c163e3d in jl_apply (f=0x606000984d60, f at entry
>>> =<optimized
>>> >> out>, args=0x7fff9132da20, args at entry=<optimized out>, nargs=1,
>>> >>     nargs at entry=<optimized out>) at ./julia.h:1043
>>> >> 1043        return f->fptr((jl_value_t*)f, args, nargs);
>>> >> (gdb) p ((jl_array_t*)((void**)args[0])[1])->data
>>> >> $43 = (void *) 0x60c000607240
>>> >> (gdb) p __msan_print_shadow(((jl_array_t*)((void**)args[0])[1]),0x30)
>>> >> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>>> 00 00
>>> >> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>>> >>  o: d800496  o: d800496  o: d800496  o: d800496  o: d800496  o:
>>> d800496  o:
>>> >> d800496  o: d800496  o: d800496  o: d800496  o: d800496  o: d800496
>>> >>
>>> >> There are no uninitialized values to be seen anywhere and the `b`
>>> value
>>> >> isn't touched before that line, so I'm a little stumped.
>>> >>
>>> >> One note I should make is that I did have to implement TLS support
>>> myself in
>>> >> MCJIT for this to work (I'll upstream the patch soon), so I may have
>>> made a
>>> >> mistake, but I haven't found anything wrong yet. If nothing looks
>>> unusual,
>>> >> I'd also appreciate pointers on what to look for in the TLS variables.
>>> >>
>>> >> Thank you for your help,
>>> >> Keno
>>> >>
>>> >>
>>> >> _______________________________________________
>>> >> LLVM Developers mailing list
>>> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> >>
>>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140202/55614e18/attachment.html>