On Fri, 27 Aug 2021 at 11:03, Andy Gibbs via cfe-users < cfe-users@lists.llvm.org> wrote:
> Hi there, > > I'm hitting a rather difficult problem. I have to compile with RTTI data > structures generated because, even though I am not using dynamic_cast or > typeid in my application code, I am linking and using a library that does > use dynamic_cast. Therefore my code will crash if I compile with -fno-rtti. > > The problem is, then, that the size of my code is greatly increased, and > also (which is more important) critical information is being leached into > the resulting application binary by the RTTI type name string that is > generated. Unlike normal symbols, these cannot be stripped from the > executable. > > Therefore I would like to make a change to the clang compiler to either > replace all the type name strings with a single "?" string (this would be > best) or doing something like a rot-x encryption on the complete string (I > would rather not do this since these strings are literally hundreds of > characters long given that the types are complex template types). > > My suggestion would be that I would attempt to add a -fno-rtti-names > parameter. If this is of interest to the general clang community I would > be happy to submit a patch for consideration, but at the very least I need > something for my own purposes. > > This brings me to my request. I would be very grateful if someone here > might be able to direct me into the right place for making such a change. > > Looking at the source code there is a > ItaniumRTTIBuilder::BuildTypeInfo(...) function in > CodeGen/ItaniumCXXABI.cpp (see > https://github.com/llvm/llvm-project/blob/fe177a1773e4f88dde1aa37d34a0d3f8cb582f14/clang/lib/CodeGen/ItaniumCXXABI.cpp#L3730). > In there, the first thing it does is lay down a field for the mangled type > name. My guess is that it should be possible to substitute the line > > llvm::GlobalVariable *TypeName = GetAddrOfTypeName(Ty, Linkage); > > with something that generates a static string "?" and returns the address > of that. Then it will build the table pointing at this string, I am > guessing. > > Is this a feasible approach or will this break loads of things elsewhere > in the compiler and/or c++ runtime? I am not interested in a run-time > ability to get the mangled (or otherwise) name of the class, so if > replacing this string has no effect on, for example, the correction > function of dynamic_cast or typeid and only means that std::type_info::name > returns a bogus value, then I'm happy with that. > The address and (sometimes) contents of the _ZTS type_info name are used for type_info equality comparisons. The implementation of dynamic_cast internally uses type_info comparisons to find the destination type within the source type's type_info tree. So changing the contents of the string to be non-unique may lead to problems, especially if the ABI rule in question results in the use of strcmp. You could perhaps instead consider replacing the string contents with something like a hash of the mangled name of the type (though be aware that the ABI library will want to interpret it as a nul-terminated byte string).
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