[LLVMdev] Weird msan problem

Tue Jan 28 13:28:42 PST 2014

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
> >>
>
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