diff options
| -rw-r--r-- | lld/ELF/EhFrame.cpp | 1 | ||||
| -rw-r--r-- | lld/ELF/Relocations.cpp | 26 |
2 files changed, 27 insertions, 0 deletions
diff --git a/lld/ELF/EhFrame.cpp b/lld/ELF/EhFrame.cpp index 2428473d901..5f037dffb4e 100644 --- a/lld/ELF/EhFrame.cpp +++ b/lld/ELF/EhFrame.cpp @@ -62,6 +62,7 @@ template <class ELFT> size_t elf::readEhRecordSize(InputSectionBase<ELFT> *S, size_t Off) { return EhReader<ELFT>(S, S->Data.slice(Off)).readEhRecordSize(); } + // .eh_frame section is a sequence of records. Each record starts with // a 4 byte length field. This function reads the length. template <class ELFT> size_t EhReader<ELFT>::readEhRecordSize() { diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp index d8a48b1f07d..c700db1f779 100644 --- a/lld/ELF/Relocations.cpp +++ b/lld/ELF/Relocations.cpp @@ -435,6 +435,32 @@ static std::vector<SharedSymbol<ELFT> *> getSymbolsAt(SharedSymbol<ELFT> *SS) { } // Reserve space in .bss or .bss.rel.ro for copy relocation. +// +// The copy relocation is pretty much a hack. If you use a copy relocation +// in your program, not only the symbol name but the symbol's size, RW/RO +// bit and alignment become part of the ABI. In addition to that, if the +// symbol has aliases, the aliases become part of the ABI. That's subtle, +// but if you violate that implicit ABI, that can cause very counter- +// intuitive consequences. +// +// So, what is the copy relocation? It's for linking non-position +// independent code to DSOs. In an ideal world, all references to data +// exported by DSOs should go indirectly through GOT. But if object files +// are compiled as non-PIC, all data references are direct. There is no +// way for the linker to transform the code to use GOT, as machine +// instructions are already set in stone in object files. This is where +// the copy relocation takes a role. +// +// A copy relocation instructs the dynamic linker to copy data from a DSO +// to a specified address (which is usually in .bss) at load-time. If the +// static linker (that's us) finds a direct data reference to a DSO +// symbol, it creates a copy relocation, so that the symbol can be +// resolved as if it were in .bss rather than in a DSO. +// +// As you can see in this function, we create a copy relocation for the +// 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. template <class ELFT> static void addCopyRelSymbol(SharedSymbol<ELFT> *SS) { typedef typename ELFT::uint uintX_t; |
