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Add more comments about copy relocations. 

llvm-svn: 295632
This commit is contained in:
Rui Ueyama 2017-02-20 02:22:56 +00:00
parent c184b671d9
commit 3fbf02ddd8
1 changed files with 19 additions and 4 deletions
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23
lld/ELF/Relocations.cpp
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@ -461,6 +461,21 @@ static std::vector<SharedSymbol<ELFT> *> getSymbolsAt(SharedSymbol<ELFT> *SS) {
// dynamic linker, and the relocation contains not only symbol name but
// various other informtion about the symbol. So, such attributes become a
// part of the ABI.
//
// Note for application developers: I can give you a piece of advice if
// you are writing a shared library. You probably should export only
// functions from your library. You shouldn't export variables.
//
// As an example what can happen when you export variables without knowing
// the semantics of copy relocations, assume that you have an exported
// variable of type T. It is an ABI-breaking change to add new members at
// end of T even though doing that doesn't change the layout of the
// existing members. That's because the space for the new members are not
// reserved in .bss unless you recompile the main program. That means they
// are likely to overlap with other data that happens to be laid out next
// to the variable in .bss. This kind of issue is sometimes very hard to
// debug. What's a solution? Instead of exporting a varaible V from a DSO,
// define an accessor getV().
template <class ELFT> static void addCopyRelSymbol(SharedSymbol<ELFT> *SS) {
typedef typename ELFT::uint uintX_t;
@ -482,10 +497,10 @@ template <class ELFT> static void addCopyRelSymbol(SharedSymbol<ELFT> *SS) {
// Look through the DSO's dynamic symbol table for aliases and create a
// dynamic symbol for each one. This causes the copy relocation to correctly
// interpose any aliases.
for (SharedSymbol<ELFT> *Alias : getSymbolsAt(SS)) {
Alias->NeedsCopy = true;
Alias->Section = ISec;
Alias->symbol()->IsUsedInRegularObj = true;
for (SharedSymbol<ELFT> *Sym : getSymbolsAt(SS)) {
Sym->NeedsCopy = true;
Sym->Section = ISec;
Sym->symbol()->IsUsedInRegularObj = true;
}
In<ELFT>::RelaDyn->addReloc({Target->CopyRel, ISec, 0, false, SS, 0});