[LLVMdev] Weird msan problem

[Evgeniy Stepanov]

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
>>