Reland "[llvm-objcopy] Add support for --strip-sections to remove all section headers leaving only program headers and loadable segment data"
ubsan caught an issue I made where I was converting a null pointer to a
reference.
elf utils implements a particularly extreme form of stripping that I'd
like to support. eu-strip has an option called "strip-sections" that
removes all section headers and leaves only program headers and the
segment data. I have implemented this option partly as a test but mainly
because in Fuchsia we would like to use this option to minimize the size
of our executables. The other strip options that are on my list include
--strip-all and --strip-debug. This is a preliminary implementation that
I'd like to start using in Fuchsia builds if possible. This change
implements such a stripping option for llvm-objcopy
Differential Revision: https://reviews.llvm.org/D38335
llvm-svn: 315484
diff --git a/llvm/tools/llvm-objcopy/Object.cpp b/llvm/tools/llvm-objcopy/Object.cpp
index 2bd3aa1..f9acf00 100644
--- a/llvm/tools/llvm-objcopy/Object.cpp
+++ b/llvm/tools/llvm-objcopy/Object.cpp
@@ -575,14 +575,20 @@
Ehdr.e_version = Version;
Ehdr.e_entry = Entry;
Ehdr.e_phoff = sizeof(Elf_Ehdr);
- Ehdr.e_shoff = SHOffset;
Ehdr.e_flags = Flags;
Ehdr.e_ehsize = sizeof(Elf_Ehdr);
Ehdr.e_phentsize = sizeof(Elf_Phdr);
Ehdr.e_phnum = Segments.size();
Ehdr.e_shentsize = sizeof(Elf_Shdr);
- Ehdr.e_shnum = Sections.size() + 1;
- Ehdr.e_shstrndx = SectionNames->Index;
+ if (WriteSectionHeaders) {
+ Ehdr.e_shoff = SHOffset;
+ Ehdr.e_shnum = Sections.size() + 1;
+ Ehdr.e_shstrndx = SectionNames->Index;
+ } else {
+ Ehdr.e_shoff = 0;
+ Ehdr.e_shnum = 0;
+ Ehdr.e_shstrndx = 0;
+ }
}
template <class ELFT>
@@ -626,19 +632,19 @@
std::begin(Sections), std::end(Sections), [=](const SecPtr &Sec) {
if (ToRemove(*Sec))
return false;
- if (auto RelSec = dyn_cast<RelocationSectionBase>(Sec.get()))
- return !ToRemove(*RelSec->getSection());
+ if (auto RelSec = dyn_cast<RelocationSectionBase>(Sec.get())) {
+ if (auto ToRelSec = RelSec->getSection())
+ return !ToRemove(*ToRelSec);
+ }
return true;
});
if (SymbolTable != nullptr && ToRemove(*SymbolTable))
SymbolTable = nullptr;
if (ToRemove(*SectionNames)) {
- // Right now llvm-objcopy always outputs section headers. This will not
- // always be the case. Eventully the section header table will become
- // optional and if no section header is output then there dosn't need to be
- // a section header string table.
- error("Cannot remove " + SectionNames->Name +
- " because it is the section header string table.");
+ if (WriteSectionHeaders)
+ error("Cannot remove " + SectionNames->Name +
+ " because it is the section header string table.");
+ SectionNames = nullptr;
}
// Now make sure there are no remaining references to the sections that will
// be removed. Sometimes it is impossible to remove a reference so we emit
@@ -732,29 +738,34 @@
}
}
- Offset = alignTo(Offset, sizeof(typename ELFT::Addr));
+ if (this->WriteSectionHeaders) {
+ Offset = alignTo(Offset, sizeof(typename ELFT::Addr));
+ }
this->SHOffset = Offset;
}
template <class ELFT> size_t ELFObject<ELFT>::totalSize() const {
// We already have the section header offset so we can calculate the total
// size by just adding up the size of each section header.
+ auto NullSectionSize = this->WriteSectionHeaders ? sizeof(Elf_Shdr) : 0;
return this->SHOffset + this->Sections.size() * sizeof(Elf_Shdr) +
- sizeof(Elf_Shdr);
+ NullSectionSize;
}
template <class ELFT> void ELFObject<ELFT>::write(FileOutputBuffer &Out) const {
this->writeHeader(Out);
this->writeProgramHeaders(Out);
this->writeSectionData(Out);
- this->writeSectionHeaders(Out);
+ if (this->WriteSectionHeaders)
+ this->writeSectionHeaders(Out);
}
template <class ELFT> void ELFObject<ELFT>::finalize() {
// Make sure we add the names of all the sections.
- for (const auto &Section : this->Sections) {
- this->SectionNames->addString(Section->Name);
- }
+ if (this->SectionNames != nullptr)
+ for (const auto &Section : this->Sections) {
+ this->SectionNames->addString(Section->Name);
+ }
// Make sure we add the names of all the symbols.
if (this->SymbolTable != nullptr)
this->SymbolTable->addSymbolNames();
@@ -763,14 +774,16 @@
assignOffsets();
// Finalize SectionNames first so that we can assign name indexes.
- this->SectionNames->finalize();
+ if (this->SectionNames != nullptr)
+ this->SectionNames->finalize();
// Finally now that all offsets and indexes have been set we can finalize any
// remaining issues.
uint64_t Offset = this->SHOffset + sizeof(Elf_Shdr);
for (auto &Section : this->Sections) {
Section->HeaderOffset = Offset;
Offset += sizeof(Elf_Shdr);
- Section->NameIndex = this->SectionNames->findIndex(Section->Name);
+ if (this->WriteSectionHeaders)
+ Section->NameIndex = this->SectionNames->findIndex(Section->Name);
Section->finalize();
}
}