Problem linking in some dataflow sanitizer functions?

Clang Frontend
1

Hello, I'm using the dataflow sanitizer in clang 3.6 for the first
time, I'm I'm running into some linker errors. I was wondering if
someone could point out my mistake?

I downloaded (today) from git the current llvm, clang, and compiler-rt
code. It built without any problem.

Using that build of clang/llvm, I'm able to run the example code shown
here: http://clang.llvm.org/docs/DataFlowSanitizer.html#example

However, I have a linking problem when I try to extend the example.
When I try to call the function "dfsan_get_label_info", the linker
can't find it. This is a bit of a surprise, because it does appear to
be defined in my LLVM build:

cconvey@spaceman /tmp $ nm
/home/cconvey/dw/clang-llvm-git/build/bin/../lib/clang/3.6.0/lib/linux/libclang_rt.dfsan-x86_64.a

grep dfsan_get_label_info

0000000000000990 T _Z20dfsan_get_label_infot
cconvey@spaceman /tmp $ c++filt _Z20dfsan_get_label_infot
dfsan_get_label_info(unsigned short)

And I'm reasonably sure I'm linking in that .a file:

cconvey@spaceman /tmp $
/home/cconvey/dw/clang-llvm-git/build/bin/clang -fsanitize=dataflow
foo.c -v
clang version 3.6.0 (http://llvm.org/git/clang.git
48d2f2367001a51a3253cc34aea4a91c92fd45fb)
(http://llvm.org/git/llvm.git
ad017096fc8eec5afd256f572d8d0b9d5d8aa09f)
...
End of search list.
"/usr/bin/ld" -pie --eh-frame-hdr -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -o a.out
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/Scrt1.o
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/4.8/crtbeginS.o
-L/usr/lib/gcc/x86_64-linux-gnu/4.8
-L/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu
-L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/4.8/../../..
-L/home/cconvey/dw/clang-llvm-git/build/bin/../lib -L/lib -L/usr/lib
-whole-archive /home/cconvey/dw/clang-llvm-git/build/bin/../lib/clang/3.6.0/lib/linux/libclang_rt.dfsan-x86_64.a
-no-whole-archive
--dynamic-list=/home/cconvey/dw/clang-llvm-git/build/bin/../lib/clang/3.6.0/lib/linux/libclang_rt.dfsan-x86_64.a.syms
/tmp/foo-5310a2.o --no-as-needed -lpthread -lrt -lm -ldl -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s
--no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.8/crtendS.o
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crtn.o
/tmp/foo-5310a2.o: In function `dfs$execute_tree':
foo.c:(.text+0xa89): undefined reference to `dfsan_get_label_info'
/tmp/foo-5310a2.o: In function `dfsw$dfsan_get_label_info':
foo.c:(.text.dfsw$dfsan_get_label_info[dfsw$dfsan_get_label_info]+0xb):
undefined reference to `dfsan_get_label_info'
clang-3.6: error: linker command failed with exit code 1 (use -v to
see invocation)
cconvey@spaceman /tmp $

(I'm also having linkage problems when my code uses C++ iostreams, but
I figured I'd work through this issue first.)

2

Hi Christian,

There was a bug that prevented clients from using the dfsan_get_label_info
function. That bug is now fixed in compiler-rt r225692.

Peter

3

Thanks Peter. That seems to have done the trick for dfsan_get_label_info.

I'm still having trouble with C++ STL functions, though. Any idea why
I might be getting the following?

I have this simple test program, foo2.c:

#include <iostream>

using namespace std;

int main(void) {
    cout << "Hellow, world!" << endl;
    return 0;
}

When I build it without the dataflow sanitizer, things work fine:
/home/cconvey/dw/clang-llvm-git/build/bin/clang++ -std=c++11 foo2.cpp

However, when I enable the sanitizer, there are a lot of unmet
dependencies. Any idea if I'm doing something wrong on the command
line?
cconvey@spaceman /tmp $
/home/cconvey/dw/clang-llvm-git/build/bin/clang++ -std=c++11
-fsanitize=dataflow foo2.cpp
/tmp/foo2-8b9f0f.o: In function `main':
foo2.cpp:(.text+0xb): undefined reference to
`dfs$_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_'
foo2.cpp:(.text+0x44): undefined reference to
`dfs$_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc'
foo2.cpp:(.text+0x6a): undefined reference to `dfs$_ZNSolsEPFRSoS_E'
/tmp/foo2-8b9f0f.o: In function `dfs$__cxx_global_var_init':
foo2.cpp:(.text.startup+0x1d): undefined reference to
`dfs$_ZNSt8ios_base4InitC1Ev'
foo2.cpp:(.text.startup+0x24): undefined reference to
`dfs$_ZNSt8ios_base4InitD1Ev'
clang-3.6: error: linker command failed with exit code 1 (use -v to
see invocation)

Thanks,
Christian

4

Hi Christian,

If you wish to compile C++ programs with DFSan, you will first need to
compile the C++ standard library with DFSan. Unfortunately, the way to do
this is quite painful at the moment, and needs to be simplified and documented
better. This way worked for me:

1. Check out the libcxx and libcxxabi projects into the LLVM projects directory.

2. Use cmake to configure a separate build directory in which to build libcxx
   and libcxxabi with DFSan:

    # assuming that your LLVM source directory is ~/src/llvm and build directory is ~/src/llvm-build
    mkdir ~/src/llvm-build-dfsan
    cd ~/src/llvm-build-dfsan
    CC=~/src/llvm-build/bin/clang CXX=~/src/llvm-build/bin/clang++ cmake -G Ninja ../llvm -DCMAKE_C_FLAGS=-fsanitize=dataflow -DCMAKE_CXX_FLAGS=-fsanitize=dataflow -DLIBCXXABI_ENABLE_SHARED=NO -DLIBCXX_ENABLE_SHARED=NO -DLLVM_FORCE_USE_OLD_TOOLCHAIN=YES -DLIBCXX_CXX_ABI=libcxxabi

3. Build only libc++ and libc++abi in the just-configured build directory:
    ninja cxx cxxabi

3. To compile your program, pass the following flags to clang++:

   compile flags: -stdlib=libc++ -fsanitize=dataflow -I$HOME/src/llvm-build-dfsan/include/c++/v1
   link flags: -stdlib=libc++ -fsanitize=dataflow -L$HOME/src/llvm-build-dfsan/lib -Wl,--start-group,-lc++abi

I was able to compile and run your hello world program this way. Please let
me know if it works for you.

Thanks,
Peter

5

Hi Peter,

Sorry for the slow reply. I tried to following your instructions
precisely (since I think that was one of your main points), and I ran
into trouble on the "ninja" invocation:

cconvey@spaceman ~/dw/clang-llvm-git/llvm-build-dfsan $ ninja cxx cxxabi
ninja: error: unknown target 'cxx', did you mean 'lli'?

Please let me know if you'd like me to try something else.

Cheers,
Christian

6

Hi Christian,

Are you sure that you have checked out libcxx and libcxxabi into llvm/projects
(i.e. alongside compiler-rt)? Once you have done so, it may be necessary
to run CMake again to pick up the new projects (the safest thing to do is
probably to delete the llvm-build-dfsan directory and redo step 2).

Peter