1b21067cf2
Previously when objcopy generated section headers, it padded the LEB that encodes the section size out to 5 bytes, matching the behavior of clang. This is correct, but results in a binary that differs from the input. This can sometimes have undesirable consequences (e.g. breaking source maps). This change makes the object reader remember the size of the LEB encoding in the section header, so that llvm-objcopy can reproduce it exactly. For sections not read from an object file (e.g. that llvm-objcopy is adding itself), pad to 5 bytes. Reviewed By: jhenderson Differential Revision: https://reviews.llvm.org/D155535
83 lines
2.8 KiB
C++
83 lines
2.8 KiB
C++
//===- WasmWriter.cpp -----------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "WasmWriter.h"
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#include "llvm/BinaryFormat/Wasm.h"
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#include "llvm/Support/Endian.h"
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#include "llvm/Support/Errc.h"
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#include "llvm/Support/LEB128.h"
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#include "llvm/Support/raw_ostream.h"
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namespace llvm {
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namespace objcopy {
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namespace wasm {
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using namespace object;
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using namespace llvm::wasm;
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Writer::SectionHeader Writer::createSectionHeader(const Section &S,
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size_t &SectionSize) {
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SectionHeader Header;
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raw_svector_ostream OS(Header);
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OS << S.SectionType;
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bool HasName = S.SectionType == WASM_SEC_CUSTOM;
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SectionSize = S.Contents.size();
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if (HasName)
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SectionSize += getULEB128Size(S.Name.size()) + S.Name.size();
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// If we read this section from an object file, use its original size for the
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// padding of the LEB value to avoid changing the file size. Otherwise, pad
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// out to 5 bytes to make it predictable, and match the behavior of clang.
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unsigned HeaderSecSizeEncodingLen =
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S.HeaderSecSizeEncodingLen ? *S.HeaderSecSizeEncodingLen : 5;
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encodeULEB128(SectionSize, OS, HeaderSecSizeEncodingLen);
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if (HasName) {
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encodeULEB128(S.Name.size(), OS);
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OS << S.Name;
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}
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// Total section size is the content size plus 1 for the section type and
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// the LEB-encoded size.
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SectionSize = SectionSize + 1 + HeaderSecSizeEncodingLen;
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return Header;
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}
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size_t Writer::finalize() {
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size_t ObjectSize = sizeof(WasmMagic) + sizeof(WasmVersion);
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SectionHeaders.reserve(Obj.Sections.size());
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// Finalize the headers of each section so we know the total size.
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for (const Section &S : Obj.Sections) {
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size_t SectionSize;
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SectionHeaders.push_back(createSectionHeader(S, SectionSize));
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ObjectSize += SectionSize;
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}
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return ObjectSize;
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}
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Error Writer::write() {
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size_t TotalSize = finalize();
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Out.reserveExtraSpace(TotalSize);
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// Write the header.
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Out.write(Obj.Header.Magic.data(), Obj.Header.Magic.size());
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uint32_t Version;
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support::endian::write32le(&Version, Obj.Header.Version);
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Out.write(reinterpret_cast<const char *>(&Version), sizeof(Version));
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// Write each section.
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for (size_t I = 0, S = SectionHeaders.size(); I < S; ++I) {
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Out.write(SectionHeaders[I].data(), SectionHeaders[I].size());
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Out.write(reinterpret_cast<const char *>(Obj.Sections[I].Contents.data()),
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Obj.Sections[I].Contents.size());
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}
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return Error::success();
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}
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} // end namespace wasm
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} // end namespace objcopy
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} // end namespace llvm
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