[LLVMdev] Weird msan problem
Evgeniy Stepanov
eugeni.stepanov at gmail.com
Wed Feb 5 13:09:30 PST 2014
On Thu, Feb 6, 2014 at 12:21 AM, Keno Fischer
<kfischer at college.harvard.edu> wrote:
> Looks like when you materialize the stores, you should check the size of the
> the store and emit an appropriate amount of stores to the origin shadow (or
> just a memset intrinsic?).
Yes. There is an old bug for that on the code.google.com site, but it
has _never_ caused problems. I guess partial reads at a non-zero
offset are pretty uncommon, and also LLVM IR tends to use smaller
stores (larger ones are transformed to memset/memcpy and we handle
those correctly). Should be fixed anyway.
Note that __msan_print_shadow can lie about origin a bit if the
address is not 4-byte aligned (it would print origins for aligned
locations instead).
>
>
> On Wed, Feb 5, 2014 at 2:13 PM, Keno Fischer <kfischer at college.harvard.edu>
> wrote:
>>
>> The @entry stuff is just a gdb artifact. I've been tracking this back a
>> little further, and it seems there's at least an origin propagation problem:
>>
>> (gdb) list
>> 281 goto value_to_pointer_error;
>> 282 }
>> 283 else {
>> 284 if (jl_is_cpointer_type(jvt) && jl_tparam0(jvt) == jt) {
>> 285 void *ptr = jl_unbox_voidpointer(v);
>> 286 assert(__msan_test_shadow(&ptr,sizeof(void**)) == -1);
>> 287 return (void*)ptr;
>> 288 }
>> 289 }
>> 290
>> (gdb) p __msan_print_shadow(v,16)
>> 00 00 00 00 00 00 00 00 ff ff ff ff ff ff ff ff
>> o: 40000e23 o: 40000e23 o: 40000e23 o: 40000e23
>> $21 = void
>> (gdb) p __msan_print_shadow(&ptr,8)
>> ff ff ff ff ff ff ff ff
>> o: 40000e23 o: 80007614
>> $22 = void
>>
>> Notice the origin of the lower bits is off. Any idea as to why?
>>
>>
>> On Mon, Feb 3, 2014 at 2:52 AM, Evgeniy Stepanov
>> <eugeni.stepanov at gmail.com> wrote:
>>>
>>> The code for ccall looks right. Sounds like you have a very small
>>> range of instructions where an uninitialized value appear. You could
>>> try debugging at asm level. Shadow for b should be passed at offset 0
>>> in __msan_param_tls.
>>>
>>> MSan could propagate shadow through arithmetic and even some logic
>>> operations (like select). It could be that b is clean on function
>>> entry, but then something uninitialized gets mixed in.
>>>
>>> Also, what is this @entry stuff? It looks like bitvector_any1 has 6
>>> arguments, but your ccall snippet only sets up tls for 3.
>>>
>>>
>>> On Sun, Feb 2, 2014 at 10:39 PM, Keno Fischer
>>> <kfischer at college.harvard.edu> wrote:
>>> > I don't think we're doing anything unusual. Here's the relevant snippet
>>> > from
>>> > the generated IR for ccall (after instrumentation):
>>> >
>>> > ccall: ; preds = %pass6,
>>> > %113
>>> > %114 = phi i8* [ %110, %113 ], [ %108, %pass6 ]
>>> > %115 = inttoptr i64 %17 to i64*
>>> > %116 = bitcast i8* %69 to i32*, !dbg !7
>>> > %117 = bitcast i8* %114 to i32 (i32*, i64, i64)*, !dbg !7
>>> > store i64 %_msret13, i64* getelementptr inbounds ([1000 x i64]*
>>> > @__msan_param_tls, i64 0, i64 0), align 8, !dbg !7
>>> > store i32 %70, i32* getelementptr inbounds ([1000 x i32]*
>>> > @__msan_param_origin_tls, i64 0, i64 0), align 4, !dbg !7
>>> > store i64 0, i64* inttoptr (i64 add (i64 ptrtoint ([1000 x i64]*
>>> > @__msan_param_tls to i64), i64 8) to i64*), align 8, !dbg !7
>>> > store i32 0, i32* inttoptr (i64 add (i64 ptrtoint ([1000 x i32]*
>>> > @__msan_param_origin_tls to i64), i64 8) to i32*), align 4, !dbg !7
>>> > store i64 0, i64* inttoptr (i64 add (i64 ptrtoint ([1000 x i64]*
>>> > @__msan_param_tls to i64), i64 16) to i64*), align 8, !dbg !7
>>> > store i32 0, i32* inttoptr (i64 add (i64 ptrtoint ([1000 x i32]*
>>> > @__msan_param_origin_tls to i64), i64 16) to i32*), align 4, !dbg !7
>>> > store i32 0, i32* bitcast ([8 x i64]* @__msan_retval_tls to i32*),
>>> > align
>>> > 8, !dbg !7
>>> > %118 = call i32 %117(i32* %116, i64 %87, i64 %107), !dbg !7,
>>> > !julia_type
>>> > !9
>>> >
>>> >
>>> >
>>> > On Sun, Feb 2, 2014 at 6:18 AM, Evgeniy Stepanov
>>> > <eugeni.stepanov at gmail.com>
>>> > wrote:
>>> >>
>>> >> How is ccall() implemented? If it manually sets up a stack frame, then
>>> >> it also needs to store argument shadow values in paramtls.
>>> >>
>>> >> I don't think there is an overflow, unless you have a _lot_ of
>>> >> arguments in a function call.
>>> >>
>>> >> On Sun, Feb 2, 2014 at 9:26 AM, Keno Fischer
>>> >> <kfischer at college.harvard.edu> wrote:
>>> >> > 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
>>> >> >>>> >>
>>> >> >>>
>>> >> >>>
>>> >> >>
>>> >> >
>>> >
>>> >
>>
>>
>
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