[PATCH] Ensure __va_list_tag has default visibility

Alp Toker alp at nuanti.com
Wed Jul 9 01:29:27 PDT 2014

On 09/07/2014 09:55, Stephan Bergmann wrote:
> ping
> On 06/25/2014 05:45 PM, Stephan Bergmann wrote:
>> I stumbled across this on Linux with -fvisibility=hidden when
>> -fsanitize=function reported a false positive for an indirect call to a
>> function with a va_list (aka __builtin_va_list, aka __va_list_tag
>> (*)[1]) parameter.  Because __va_list_tag was considered hidden, the
>> RTTI for the function's type was not exported from the two shared
>> libraries involved, so the equality check failed.
>> This would apparently also need to be fixed in the other
>> Create*BuiltinVaListDecl variants, but I'm not sure (a) whether the
>> added const_cast is really deemed appropriate here (though I guess so,
>> given the large number of existing similar const_casts across
>> ASTContext.cpp), and (b) what the preferred way would be to break that
>> long line. ;)

Hi Stephan,

I wrote the buildImplicitRecord() utility, and in principle the 
attribute could be added centrally to that function -- if these are the 
semantics we want for most built-in types, as you suggest, it makes 
sense to centralize the decision.

Another option would be to calculate the special linkage in 
computeLVForDecl() instead of using an attribute, but it doesn't really 
matter either way I think as the result is the same.

The bigger question is whether (1) default visibility is correct for all 
built-in types currently being generated by buildImplicitRecord() and 
(2) whether this is a valid choice for all targets we support including 
MSVC drop-in compatibility mode.

To help answer question (1), the affected built-in types would be:

lib/AST/ASTContext.cpp:    Float128StubDecl = 
lib/AST/ASTContext.cpp:    CFConstantStringTypeDecl = 
lib/AST/ASTContext.cpp:    RecordDecl *ObjCSuperTypeDecl = 
lib/AST/ASTContext.cpp:  RD = buildImplicitRecord("__block_descriptor");
lib/AST/ASTContext.cpp:  RD = 
lib/AST/ASTContext.cpp:  RecordDecl *VaListTagDecl = 
lib/AST/ASTContext.cpp:  VaListTagDecl = 
lib/AST/ASTContext.cpp:  VaListTagDecl = 
lib/AST/ASTContext.cpp:  RecordDecl *VaListDecl = 
lib/AST/ASTContext.cpp:  VaListTagDecl = 
lib/CodeGen/CodeGenModule.cpp:    RecordDecl *D = 
lib/CodeGen/CodeGenModule.cpp:    RecordDecl *D = 
PushOnScopeChains(Context.buildImplicitRecord("type_info", TTK_Class),

I'll need help answering those questions from linkage/visibility folks 
on the list before going forward with a change like this.


>> Stephan
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