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George Rimar
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[yaml2obj] - Add a support for defining null sections in YAMLs.
ELF spec shows (Figure 4-10: Section Header Table Entry:Index 0, http://www.sco.com/developers/gabi/latest/ch4.sheader.html) that section header at index 0 (null section) can have sh_size and sh_link fields set to non-zero values. It says (https://docs.oracle.com/cd/E19683-01/817-3677/6mj8mbtc9/index.html): "If the number of sections is greater than or equal to SHN_LORESERVE (0xff00), this member has the value zero and the actual number of section header table entries is contained in the sh_size field of the section header at index 0. Otherwise, the sh_size member of the initial entry contains 0." and: "If the section name string table section index is greater than or equal to SHN_LORESERVE (0xff00), this member has the value SHN_XINDEX (0xffff) and the actual index of the section name string table section is contained in the sh_link field of the section header at index 0. Otherwise, the sh_link member of the initial entry contains 0." At this moment it is not possible to create custom section headers at index 0 using yaml2obj. This patch implements this. Differential revision: https://reviews.llvm.org/D64913 llvm-svn: 366794
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‎llvm/test/tools/yaml2obj/elf-custom-null-section.yaml

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## In this test we check that can redefine the null section in the YAML.
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## Test the default output first.
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# RUN: yaml2obj --docnum=1 %s -o %t1
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# RUN: llvm-readelf --sections %t1 | FileCheck %s --check-prefix=DEFAULT
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# DEFAULT: Section Headers:
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# DEFAULT-NEXT: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# DEFAULT-NEXT: [ 0] NULL 0000000000000000 000000 000000 00 0 0 0
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# DEFAULT-NEXT: [ 1] .symtab SYMTAB 0000000000000000 000140 000018 18 2 1 8
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# DEFAULT-NEXT: [ 2] .strtab STRTAB 0000000000000000 000158 000001 00 0 0 1
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# DEFAULT-NEXT: [ 3] .shstrtab STRTAB 0000000000000000 000159 00001b 00 0 0 1
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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## Now define a SHT_NULL section with fields all zeroed.
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## In this case it is equal to the section created by default.
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# RUN: yaml2obj --docnum=2 %s -o %t2
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# RUN: llvm-readelf --sections %t2 | FileCheck %s --check-prefix=DEFAULT
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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Sections:
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- Type: SHT_NULL
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Name: ''
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Flags: [ ]
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AddressAlign: 0x0
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Size: 0x0
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EntSize: 0x0
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Link: 0
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Info: 0
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Address: 0x0
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## Check we are still able to describe other sections too.
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# RUN: yaml2obj --docnum=3 %s -o %t3
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# RUN: llvm-readelf --sections %t3 | FileCheck %s --check-prefix=OTHER-SECTION
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# OTHER-SECTION: Section Headers:
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# OTHER-SECTION-NEXT: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# OTHER-SECTION-NEXT: [ 0] NULL 0000000000000000 000000 000000 00 0 0 0
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# OTHER-SECTION-NEXT: [ 1] foo PROGBITS 0000000000000000 000180 000000 00 0 0 0
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# OTHER-SECTION-NEXT: [ 2] .symtab SYMTAB 0000000000000000 000180 000018 18 3 1 8
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# OTHER-SECTION-NEXT: [ 3] .strtab STRTAB 0000000000000000 000198 000001 00 0 0 1
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# OTHER-SECTION-NEXT: [ 4] .shstrtab STRTAB 0000000000000000 000199 00001f 00 0 0 1
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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Sections:
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- Type: SHT_NULL
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Name: ''
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Flags: [ ]
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AddressAlign: 0x0
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Size: 0x0
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EntSize: 0x0
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Link: 0
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- Type: SHT_PROGBITS
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Name: 'foo'
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## Check we can redefine sh_size and sh_link fields of the SHT_NULL section.
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# RUN: yaml2obj --docnum=4 %s -o %t4
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# RUN: llvm-readelf --sections %t4 | FileCheck %s --check-prefix=REDEFINE
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# REDEFINE: Section Headers:
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# REDEFINE-NEXT: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# REDEFINE-NEXT: [ 0] NULL 0000000000000000 000000 000123 00 1 0 0
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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Sections:
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- Type: SHT_NULL
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Link: .foo
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Size: 0x123
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- Type: SHT_PROGBITS
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Name: .foo
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## The same as above, but using a number as a Link value.
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# RUN: yaml2obj --docnum=5 %s -o %t5
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# RUN: llvm-readelf --sections %t5 | FileCheck %s --check-prefix=REDEFINE
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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Sections:
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- Type: SHT_NULL
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Link: 1
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Size: 0x123
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- Type: SHT_PROGBITS
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Name: .foo
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## Check we report an error if null section sh_link field refers to an unknown section.
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# RUN: not yaml2obj --docnum=6 %s -o %t6 2>&1 | FileCheck %s --check-prefix=CASE4
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# CASE4: error: Unknown section referenced: '.foo' at YAML section ''.
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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Sections:
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- Type: SHT_NULL
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Link: .foo
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## Check that null section fields are set to zero, if they are unspecified.
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# RUN: yaml2obj --docnum=7 %s -o %t7
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# RUN: llvm-readelf --sections %t7 | FileCheck %s --check-prefix=DEFAULT
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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Sections:
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- Type: SHT_NULL
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## Check we do not crash if we have more than one SHT_NULL section.
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# RUN: yaml2obj --docnum=8 %s -o %t8
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# RUN: llvm-readelf --sections %t8 | FileCheck %s --check-prefix=MULTIPLE
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# MULTIPLE: Section Headers:
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# MULTIPLE-NEXT: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# MULTIPLE-NEXT: [ 0] NULL 0000000000000000 000000 000000 00 0 0 0
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# MULTIPLE-NEXT: [ 1] NULL 0000000000000123 000180 000020 10 A 1 2 0
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_REL
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Machine: EM_X86_64
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Sections:
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- Type: SHT_NULL
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- Type: SHT_NULL
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Flags: [ SHF_ALLOC ]
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Size: 0x20
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EntSize: 0x10
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Link: 1
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Info: 2
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Address: 0x123

‎llvm/tools/yaml2obj/yaml2elf.cpp

+23-5
Original file line numberDiff line numberDiff line change
@@ -192,10 +192,11 @@ ELFState<ELFT>::ELFState(ELFYAML::Object &D) : Doc(D) {
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if (!D->Name.empty())
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DocSections.insert(D->Name);
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// Insert SHT_NULL section implicitly.
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Doc.Sections.insert(
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Doc.Sections.begin(),
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llvm::make_unique<ELFYAML::Section>(
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// Insert SHT_NULL section implicitly when it is not defined in YAML.
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if (Doc.Sections.empty() || Doc.Sections.front()->Type != ELF::SHT_NULL)
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Doc.Sections.insert(
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Doc.Sections.begin(),
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llvm::make_unique<ELFYAML::Section>(
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ELFYAML::Section::SectionKind::RawContent, /*IsImplicit=*/true));
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std::vector<StringRef> ImplicitSections = {".symtab", ".strtab", ".shstrtab"};
@@ -325,10 +326,27 @@ bool ELFState<ELFT>::initSectionHeaders(ELFState<ELFT> &State,
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// valid SHN_UNDEF entry since SHT_NULL == 0.
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SHeaders.resize(Doc.Sections.size());
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for (size_t I = 1; I < Doc.Sections.size(); ++I) {
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for (size_t I = 0; I < Doc.Sections.size(); ++I) {
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Elf_Shdr &SHeader = SHeaders[I];
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ELFYAML::Section *Sec = Doc.Sections[I].get();
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if (I == 0) {
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if (Sec->IsImplicit)
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continue;
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if (auto S = dyn_cast<ELFYAML::RawContentSection>(Sec))
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if (S->Size)
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SHeader.sh_size = *S->Size;
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if (!Sec->Link.empty()) {
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unsigned Index;
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if (!convertSectionIndex(SN2I, Sec->Name, Sec->Link, Index))
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return false;
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SHeader.sh_link = Index;
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}
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continue;
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}
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// We have a few sections like string or symbol tables that are usually
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// added implicitly to the end. However, if they are explicitly specified
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// in the YAML, we need to write them here. This ensures the file offset

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