[LLVMdev] Weird msan problem

Sat Feb 1 15:44:27 PST 2014

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
>> >>
>> >>
>> >> _______________________________________________
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>> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>
>>
>
>
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