[llvm] r187827 - Change the linkage of these global values to 'internal'.

[Kostya Serebryany]

Bill, Alex,

This change makes the asan-private globals to stay in the symbol
table, which costs code size:

% cat a.cc
int x, y, z;
int main() { }
% clang -fsanitize=address  a.cc && nm a.out | grep asan_gen
000000000046f178 r __asan_gen_
000000000046724b r __asan_gen_1
000000000046d48a r __asan_gen_2
000000000046f17d r __asan_gen_3

E.g. for chrome's content_shell the size increase is 1.5% (6Mb), which is huge.

Is this CL a proper fix, or just a workaround for a linker bug on Mac?
If this is due to Mac-specific linker bug, can we have InternalLinkage
on Mac and PrivateLinkage on Linux?
Anyway, Alex, if this is not something we can fix quickly please file
a bug -- we need to get the binary size back.

--kcc


On Thu, Aug 8, 2013 at 3:19 PM, Alexander Potapenko <glider at google.com> wrote:
> Looks like your commit has fixed
> https://code.google.com/p/address-sanitizer/issues/detail?id=171. Yay!
>
> On Wed, Aug 7, 2013 at 2:52 AM, Bill Wendling <isanbard at gmail.com> wrote:
>> Author: void
>> Date: Tue Aug  6 17:52:42 2013
>> New Revision: 187827
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=187827&view=rev
>> Log:
>> Change the linkage of these global values to 'internal'.
>>
>> The globals being generated here were given the 'private' linkage type. However,
>> this caused them to end up in different sections with the wrong prefix. E.g.,
>> they would be in the __TEXT,__const section with an 'L' prefix instead of an 'l'
>> (lowercase ell) prefix.
>>
>> The problem is that the linker will eat a literal label with 'L'. If a weak
>> symbol is then placed into the __TEXT,__const section near that literal, then it
>> cannot distinguish between the literal and the weak symbol.
>>
>> Part of the problems here was introduced because the address sanitizer converted
>> some C strings into constant initializers with trailing nuls. (Thus putting them
>> in the __const section with the wrong prefix.) The others were variables that
>> the address sanitizer created but simply had the wrong linkage type.
>>
>> Modified:
>>     llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp
>>     llvm/trunk/test/Instrumentation/AddressSanitizer/do-not-instrument-internal-globals.ll
>>
>> Modified: llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp?rev=187827&r1=187826&r2=187827&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp (original)
>> +++ llvm/trunk/lib/Transforms/Instrumentation/AddressSanitizer.cpp Tue Aug  6 17:52:42 2013
>> @@ -547,11 +547,11 @@ static size_t TypeSizeToSizeIndex(uint32
>>    return Res;
>>  }
>>
>> -// Create a constant for Str so that we can pass it to the run-time lib.
>> +// \brief Create a constant for Str so that we can pass it to the run-time lib.
>>  static GlobalVariable *createPrivateGlobalForString(Module &M, StringRef Str) {
>>    Constant *StrConst = ConstantDataArray::getString(M.getContext(), Str);
>>    GlobalVariable *GV = new GlobalVariable(M, StrConst->getType(), true,
>> -                            GlobalValue::PrivateLinkage, StrConst,
>> +                            GlobalValue::InternalLinkage, StrConst,
>>                              kAsanGenPrefix);
>>    GV->setUnnamedAddr(true);  // Ok to merge these.
>>    GV->setAlignment(1);  // Strings may not be merged w/o setting align 1.
>> @@ -961,8 +961,11 @@ bool AddressSanitizerModule::runOnModule
>>      GlobalVariable *Name = createPrivateGlobalForString(M, G->getName());
>>
>>      // Create a new global variable with enough space for a redzone.
>> +    GlobalValue::LinkageTypes Linkage = G->getLinkage();
>> +    if (G->isConstant() && Linkage == GlobalValue::PrivateLinkage)
>> +      Linkage = GlobalValue::InternalLinkage;
>>      GlobalVariable *NewGlobal = new GlobalVariable(
>> -        M, NewTy, G->isConstant(), G->getLinkage(),
>> +        M, NewTy, G->isConstant(), Linkage,
>>          NewInitializer, "", G, G->getThreadLocalMode());
>>      NewGlobal->copyAttributesFrom(G);
>>      NewGlobal->setAlignment(MinRZ);
>> @@ -995,7 +998,7 @@ bool AddressSanitizerModule::runOnModule
>>
>>    ArrayType *ArrayOfGlobalStructTy = ArrayType::get(GlobalStructTy, n);
>>    GlobalVariable *AllGlobals = new GlobalVariable(
>> -      M, ArrayOfGlobalStructTy, false, GlobalVariable::PrivateLinkage,
>> +      M, ArrayOfGlobalStructTy, false, GlobalVariable::InternalLinkage,
>>        ConstantArray::get(ArrayOfGlobalStructTy, Initializers), "");
>>
>>    // Create calls for poisoning before initializers run and unpoisoning after.
>>
>> Modified: llvm/trunk/test/Instrumentation/AddressSanitizer/do-not-instrument-internal-globals.ll
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Instrumentation/AddressSanitizer/do-not-instrument-internal-globals.ll?rev=187827&r1=187826&r2=187827&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Instrumentation/AddressSanitizer/do-not-instrument-internal-globals.ll (original)
>> +++ llvm/trunk/test/Instrumentation/AddressSanitizer/do-not-instrument-internal-globals.ll Tue Aug  6 17:52:42 2013
>> @@ -16,5 +16,5 @@ declare void @_Z3fooPi(i32*)
>>  ; We create one global string constant for the stack frame above.
>>  ; It should have unnamed_addr and align 1.
>>  ; Make sure we don't create any other global constants.
>> -; CHECK: = private unnamed_addr constant{{.*}}align 1
>> -; CHECK-NOT: = private unnamed_addr constant
>> +; CHECK: = internal unnamed_addr constant{{.*}}align 1
>> +; CHECK-NOT: = internal unnamed_addr constant
>>
>>
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>
>
>
> --
> Alexander Potapenko
> Software Engineer
> Google Moscow
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