[cfe-commits] r169328 - in /cfe/trunk: lib/Driver/Tools.cpp test/Driver/asan-ld.c

[Kostya Serebryany]

On Wed, Dec 5, 2012 at 12:36 PM, Chandler Carruth <chandlerc at gmail.com>wrote:

> On Wed, Dec 5, 2012 at 12:13 AM, Kostya Serebryany <kcc at google.com> wrote:
>
>> +asan/tsan/msan folks
>>
>> That's not as simple as this, I afraid.
>> In *san there are two different kinds of interceptors.
>> 1. foo from libc/libstc++ is completely replaced with foo from asan.
>> Example: malloc, operator new
>> 2. foo from libc/libstc++ is wrapped, such that when a user calls foo it
>> calls the asan copy, but then the asan function calls the original foo.
>> Example: pthread_create, __cxa_throw
>>
>> This change allows to use -static-libstdc++ with asan w/o link failures,
>> but asan is still broken for -static-libstdc++ in a subtle way (since the
>> __cxa_throw is not intercepted, asan will have rare false positives in
>> presence of exceptions).
>>
>
> Can you describe the rare false positive? What would be needed to fully
> fix this?
>

__cxa_throw is unfriendly to asan as it never returns from the current
function.
If there is poisoned stack it will neve get unpoisoned after throw. Some
one else later will step on the poisoned stack and get a false positive.
This is why we
  1. Intercept __cxa_throw and call __asan_handle_no_return
(asan/asan_interceptors.cc)
  2. We also call __asan_handle_no_return before any noreturn call when we
instrument it.

Today if you use -static-libstdc++ the asan's __cxa_throw interceptor is
not called.
So, if a throw happens in a code that we don't instrument, we will not
call __asan_handle_no_return
at all. Booom.




>
>
>>  Tsan also intercepts __cxa_guard in the similar way, so it will have
>> false positives on function scope statics.
>>
>> Do we really care that much about linking asan with static libstdc++?
>>
>
> In my opinion, yes. We already have too much that is special about linking
> with asan....
>

Well, yes. E.g. it does not support fully static linking at all, and hardly
ever will.

--kcc



>
>
>>
>> --kcc
>>
>>
>> On Wed, Dec 5, 2012 at 2:54 AM, Chandler Carruth <chandlerc at gmail.com>wrote:
>>
>>> Author: chandlerc
>>> Date: Tue Dec  4 16:54:37 2012
>>> New Revision: 169328
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=169328&view=rev
>>> Log:
>>> Add -whole-archive around the ASan runtime archive in the link command.
>>>
>>> This ensures that even though it comes first, we pick up its .o files.
>>> Note that if we can use this (or something similar / equivalent) on
>>> other platforms, we could potentially remove
>>> ReplaceOperatorsNewAndDelete from the ASan runtimes.
>>>
>>> We should probably do something similar for TSan and MSan as well.
>>>
>>> Modified:
>>>     cfe/trunk/lib/Driver/Tools.cpp
>>>     cfe/trunk/test/Driver/asan-ld.c
>>>
>>> Modified: cfe/trunk/lib/Driver/Tools.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Driver/Tools.cpp?rev=169328&r1=169327&r2=169328&view=diff
>>>
>>> ==============================================================================
>>> --- cfe/trunk/lib/Driver/Tools.cpp (original)
>>> +++ cfe/trunk/lib/Driver/Tools.cpp Tue Dec  4 16:54:37 2012
>>> @@ -1524,8 +1524,14 @@
>>>        // The ASan runtime needs to come before -lstdc++ (or -lc++,
>>> libstdc++.a,
>>>        // etc.) so that the linker picks ASan's versions of the global
>>> 'operator
>>>        // new' and 'operator delete' symbols. We take the extreme (but
>>> simple)
>>> -      // strategy of inserting it at the front of the link command.
>>> -      CmdArgs.insert(CmdArgs.begin(), Args.MakeArgString(LibAsan));
>>> +      // strategy of inserting it at the front of the link command. It
>>> also
>>> +      // needs to be forced to end up in the executable, so wrap it in
>>> +      // whole-archive.
>>> +      SmallVector<const char*, 3> PrefixArgs;
>>> +      PrefixArgs.push_back("-whole-archive");
>>> +      PrefixArgs.push_back(Args.MakeArgString(LibAsan));
>>> +      PrefixArgs.push_back("-no-whole-archive");
>>> +      CmdArgs.insert(CmdArgs.begin(), PrefixArgs.begin(),
>>> PrefixArgs.end());
>>>        CmdArgs.push_back("-lpthread");
>>>        CmdArgs.push_back("-ldl");
>>>        CmdArgs.push_back("-export-dynamic");
>>>
>>> Modified: cfe/trunk/test/Driver/asan-ld.c
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Driver/asan-ld.c?rev=169328&r1=169327&r2=169328&view=diff
>>>
>>> ==============================================================================
>>> --- cfe/trunk/test/Driver/asan-ld.c (original)
>>> +++ cfe/trunk/test/Driver/asan-ld.c Tue Dec  4 16:54:37 2012
>>> @@ -19,7 +19,7 @@
>>>  //
>>>  // CHECK-LINUX-CXX: "{{.*}}ld{{(.exe)?}}"
>>>  // CHECK-LINUX-CXX-NOT: "-lc"
>>> -// CHECK-LINUX-CXX: libclang_rt.asan-i386.a"
>>> +// CHECK-LINUX-CXX: "-whole-archive" "{{.*}}libclang_rt.asan-i386.a"
>>> "-no-whole-archive"
>>>  // CHECK-LINUX-CXX: "-lpthread"
>>>  // CHECK-LINUX-CXX: "-ldl"
>>>  // CHECK-LINUX-CXX: "-export-dynamic"
>>> @@ -32,7 +32,7 @@
>>>  //
>>>  // CHECK-LINUX-CXX-STATIC: "{{.*}}ld{{(.exe)?}}"
>>>  // CHECK-LINUX-CXX-STATIC-NOT: stdc++
>>> -// CHECK-LINUX-CXX-STATIC: libclang_rt.asan-i386.a"
>>> +// CHECK-LINUX-CXX-STATIC: "-whole-archive"
>>> "{{.*}}libclang_rt.asan-i386.a" "-no-whole-archive"
>>>  // CHECK-LINUX-CXX-STATIC: stdc++
>>>
>>>  // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
>>>
>>>
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>>> cfe-commits at cs.uiuc.edu
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>>>
>>
>>
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