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If a symbol needs both JUMP_SLOT and GLOB_DAT relocations, there is a minor linker optimization to keep just GLOB_DAT. This optimization is only implemented by GNU ld's x86 port and mold. https://maskray.me/blog/2021-08-29-all-about-global-offset-table#combining-.got-and-.got.plt With the optimizing, the PLT entry is placed in .plt.got and the associated GOTPLT entry is placed in .got (ld.bfd -z now) or .got.plt (ld.bfd -z lazy). The relocation is in .rel[a].dyn. This patch synthesizes `symbol@plt` labels for these .plt.got entries. Example: ``` cat > a.s <<e .globl _start; _start: mov combined0@gotpcrel(%rip), %rax; mov combined1@gotpcrel(%rip), %rax call combined0@plt; call combined1@plt call foo0@plt; call foo1@plt e cat > b.s <<e .globl foo0, foo1, combined0, combined1 foo0: foo1: combined0: combined1: e gcc -fuse-ld=bfd -shared b.s -o b.so gcc -fuse-ld=bfd -pie -nostdlib a.s b.so -o a ``` ``` Disassembly of section .plt: 0000000000001000 <.plt>: 1000: ff 35 ea 1f 00 00 pushq 0x1fea(%rip) # 0x2ff0 <_GLOBAL_OFFSET_TABLE_+0x8> 1006: ff 25 ec 1f 00 00 jmpq *0x1fec(%rip) # 0x2ff8 <_GLOBAL_OFFSET_TABLE_+0x10> 100c: 0f 1f 40 00 nopl (%rax) 0000000000001010 <foo1@plt>: 1010: ff 25 ea 1f 00 00 jmpq *0x1fea(%rip) # 0x3000 <_GLOBAL_OFFSET_TABLE_+0x18> 1016: 68 00 00 00 00 pushq $0x0 101b: e9 e0 ff ff ff jmp 0x1000 <.plt> 0000000000001020 <foo0@plt>: 1020: ff 25 e2 1f 00 00 jmpq *0x1fe2(%rip) # 0x3008 <_GLOBAL_OFFSET_TABLE_+0x20> 1026: 68 01 00 00 00 pushq $0x1 102b: e9 d0 ff ff ff jmp 0x1000 <.plt> Disassembly of section .plt.got: 0000000000001030 <combined0@plt>: 1030: ff 25 a2 1f 00 00 jmpq *0x1fa2(%rip) # 0x2fd8 <foo1+0x2fd8> 1036: 66 90 nop 0000000000001038 <combined1@plt>: 1038: ff 25 a2 1f 00 00 jmpq *0x1fa2(%rip) # 0x2fe0 <foo1+0x2fe0> 103e: 66 90 nop ``` For x86-32, with -z now, if we remove `foo0` and `foo1`, the absence of regular PLT will cause GNU ld to omit .got.plt, and our code cannot synthesize @plt labels. This is an extreme corner case that almost never happens in practice (to trigger the case, ensure every PLT symbol has been taken address). To fix it, we can get the `_GLOBAL_OFFSET_TABLE_` symbol value, but the complexity is not worth it. Close https://github.com/llvm/llvm-project/issues/62537 Reviewed By: bd1976llvm Differential Revision: https://reviews.llvm.org/D149817
234 lines
7.3 KiB
Plaintext
234 lines
7.3 KiB
Plaintext
## If a symbol needing a PLT entry also needs a GLOB_DAT relocation, GNU ld's
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## x86 port places the PLT entry in .plt.got, relocated by a GLOB_DAT. The
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## JUMP_SLOT relocation is unused in this case.
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## Test that we synthesize @plt symbols for such PLT entries.
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# RUN: yaml2obj --docnum=1 %s -o %t.x86-64
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# RUN: llvm-objdump -d %t.x86-64 | FileCheck %s --check-prefix=64
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# RUN: yaml2obj --docnum=2 %s -o %t.x86-32
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# RUN: llvm-objdump -d %t.x86-32 | FileCheck %s --check-prefix=32
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# 64: Disassembly of section .plt:
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# 64-EMPTY:
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# 64-NEXT: <.plt>:
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# 64: <foo1@plt>:
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# 64: <foo0@plt>:
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# 64: Disassembly of section .plt.got:
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# 64-EMPTY:
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# 64-NEXT: <combined0@plt>:
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# 64: <combined1@plt>:
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# 64: <_start>:
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# 64-NEXT: movq {{.*}}(%rip), %rax
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# 64-NEXT: movq {{.*}}(%rip), %rax
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# 64-NEXT: callq {{.*}} <combined0@plt>
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# 64-NEXT: callq {{.*}} <combined1@plt>
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# 64-NEXT: callq {{.*}} <foo0@plt>
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# 64-NEXT: callq {{.*}} <foo1@plt>
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# 32: Disassembly of section .plt:
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# 32-EMPTY:
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# 32-NEXT: <.plt>:
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# 32: <foo1@plt>:
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# 32: <foo0@plt>:
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# 32: Disassembly of section .plt.got:
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# 32-EMPTY:
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# 32-NEXT: <combined0@plt>:
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# 32: <combined1@plt>:
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# 32: <_start>:
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# 32-NEXT: movl -0x8(%eax), %eax
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# 32-NEXT: movl -0x4(%eax), %eax
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# 32-NEXT: calll {{.*}} <combined0@plt>
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# 32-NEXT: calll {{.*}} <combined1@plt>
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# 32-NEXT: calll {{.*}} <foo0@plt>
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# 32-NEXT: calll {{.*}} <foo1@plt>
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## %t.x86-64 is linked with ld.bfd -pie -z now a.o b.so and
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## doesn't have .got.plt.
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## %t.x86-32 is linked with ld.bfd -pie -z lazy a.o b.so and
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## has .got.plt.
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## a.s
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## .globl _start; _start:
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## mov combined0@gotpcrel(%rip), %rax # movl combined0@GOT(%eax), %eax for x86-32
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## mov combined1@gotpcrel(%rip), %rax # movl combined1@GOT(%eax), %eax for x86-32
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## call combined0@plt
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## call combined1@plt
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## call foo0@plt
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## call foo1@plt
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## b.s
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## .globl foo0, foo1, combined0, combined1
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## foo0: foo1: combined0: combined1:
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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_DYN
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Machine: EM_X86_64
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Entry: 0x1040
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Sections:
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- Name: .rela.dyn
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Type: SHT_RELA
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Flags: [ SHF_ALLOC ]
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Address: 0x340
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Link: .dynsym
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AddressAlign: 0x8
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Relocations:
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- Offset: 0x2FF0
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Symbol: combined0
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- Offset: 0x2FF8
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Symbol: combined1
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Type: R_X86_64_GLOB_DAT
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- Name: .rela.plt
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Type: SHT_RELA
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Link: .dynsym
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Info: .got
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Relocations:
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- Offset: 0x2FE0
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Symbol: foo1
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Type: R_X86_64_JUMP_SLOT
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- Offset: 0x2FE8
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Symbol: foo0
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Type: R_X86_64_JUMP_SLOT
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- Name: .plt
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Type: SHT_PROGBITS
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Content: FF35CA1F0000FF25CC1F00000F1F4000FF25CA1F00006800000000E9E0FFFFFFFF25C21F00006801000000E9D0FFFFFF
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- Name: .plt.got
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Type: SHT_PROGBITS
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Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
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Address: 0x1030
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EntSize: 0x8
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Content: FF25BA1F00006690FF25BA1F00006690
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EntSize: 0x8
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Content: '682E000000000000000000000000000000000000000000001610000000000000261000000000000000000000000000000000000000000000'
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Symbols:
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DynamicSymbols:
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- Name: foo1
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Binding: STB_GLOBAL
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- Name: foo0
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Binding: STB_GLOBAL
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- Name: combined0
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Binding: STB_GLOBAL
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- Name: combined1
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Binding: STB_GLOBAL
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...
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--- !ELF
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FileHeader:
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Class: ELFCLASS32
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Data: ELFDATA2LSB
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Type: ET_DYN
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Machine: EM_386
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Entry: 0x1040
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Sections:
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- Name: .rel.dyn
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Type: SHT_REL
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Flags: [ SHF_ALLOC ]
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Address: 0x218
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Link: .dynsym
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AddressAlign: 0x4
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Relocations:
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- Offset: 0x2FEC
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Symbol: combined0
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Type: R_386_GLOB_DAT
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- Offset: 0x2FF0
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Symbol: combined1
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Type: R_386_GLOB_DAT
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- Name: .rel.plt
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Type: SHT_REL
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Flags: [ SHF_ALLOC, SHF_INFO_LINK ]
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Address: 0x228
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Link: .dynsym
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AddressAlign: 0x4
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Info: .got.plt
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Relocations:
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- Offset: 0x3000
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Symbol: foo1
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Type: R_386_JUMP_SLOT
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- Offset: 0x3004
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Symbol: foo0
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Type: R_386_JUMP_SLOT
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- Name: .plt
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Type: SHT_PROGBITS
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Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
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Address: 0x1000
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AddressAlign: 0x10
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Offset: 0x1000
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Content: FFB304000000FFA30800000000000000FFA30C0000006800000000E9E0FFFFFFFFA3100000006808000000E9D0FFFFFF
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- Name: .plt.got
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Type: SHT_PROGBITS
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Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
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Content: FFA3F8FFFFFF6690FFA3FCFFFFFF6690
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Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
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Address: 0x1040
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Content: 8B80F8FFFFFF8B80FCFFFFFFE8DFFFFFFFE8E2FFFFFFE8C5FFFFFFE8B0FFFFFF
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Type: SHT_PROGBITS
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Flags: [ SHF_WRITE, SHF_ALLOC ]
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Address: 0x2FEC
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EntSize: 0x4
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Content: '0000000000000000'
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- Name: .got.plt
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Type: SHT_PROGBITS
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Flags: [ SHF_WRITE, SHF_ALLOC ]
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Address: 0x2FF4
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AddressAlign: 0x4
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EntSize: 0x4
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Content: 442F000000000000000000001610000026100000
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Symbols:
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- Name: _GLOBAL_OFFSET_TABLE_
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Type: STT_OBJECT
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Section: .got.plt
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Value: 0x2FF4
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- Name: _start
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Section: .text
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Binding: STB_GLOBAL
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Value: 0x1040
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DynamicSymbols:
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- Name: combined0
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Binding: STB_GLOBAL
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- Name: foo1
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Binding: STB_GLOBAL
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- Name: foo0
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Binding: STB_GLOBAL
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- Name: combined1
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Binding: STB_GLOBAL
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...
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