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[llvm-objcopy] Fix file offsets when PT_INTERP/PT_LOAD offsets are equal (#80562)
(#79887) When the offset of a PT_INTERP segment equals the offset of a PT_LOAD segment, we consider that the parent of the PT_LOAD segment is the PT_INTERP segment. In `layoutSegments`, we place both segments to be after the current `Offset`, ignoring the PT_LOAD alignment. This scenario is possible with fixed section addresses, but doesn't happen with default linker layouts (.interp precedes other sections and is part of a PT_LOAD segment containing the ELF header and program headers). ``` % cat a.s .globl _start; _start: ret .rodata; .byte 0 .tdata; .balign 4096; .byte 0 % clang -fuse-ld=lld a.s -o a -nostdlib -no-pie -z separate-loadable-segments -Wl,-Ttext=0x201000,--section-start=.interp=0x202000,--section-start=.rodata=0x202020,-z,nognustack % llvm-objcopy a a2 % llvm-readelf -l a2 # incorrect offset(PT_LOAD) Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align PHDR 0x000040 0x0000000000200040 0x0000000000200040 0x0001c0 0x0001c0 R 0x8 INTERP 0x001001 0x0000000000202000 0x0000000000202000 0x00001c 0x00001c R 0x1 [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2] LOAD 0x000000 0x0000000000200000 0x0000000000200000 0x000200 0x000200 R 0x1000 LOAD 0x001000 0x0000000000201000 0x0000000000201000 0x000001 0x000001 R E 0x1000 //// incorrect offset LOAD 0x001001 0x0000000000202000 0x0000000000202000 0x000021 0x000021 R 0x1000 LOAD 0x002000 0x0000000000203000 0x0000000000203000 0x000001 0x001000 RW 0x1000 TLS 0x002000 0x0000000000203000 0x0000000000203000 0x000001 0x000001 R 0x1000 GNU_RELRO 0x002000 0x0000000000203000 0x0000000000203000 0x000001 0x001000 R 0x1000 ``` The same issue occurs for PT_TLS/PT_GNU_RELRO if we PT_TLS's alignment is smaller and we place the PT_LOAD after PT_TLS/PT_GNU_RELRO segments (not linker default, but possible with a `PHDRS` linker script command). Fix #79887: when two segments have the same offset, order the one with a larger alignment first. In the previous case, the PT_LOAD segment will go before the PT_INTERP segment. In case of equal alignments, it doesn't matter which segment is treated as the parent segment.
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‎llvm/lib/ObjCopy/ELF/ELFObject.cpp

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@@ -1234,6 +1234,12 @@ static bool compareSegmentsByOffset(const Segment *A, const Segment *B) {
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return true;
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if (A->OriginalOffset > B->OriginalOffset)
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return false;
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// If alignments are different, the one with a smaller alignment cannot be the
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// parent; otherwise, layoutSegments will not respect the larger alignment
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// requirement. This rule ensures that PT_LOAD/PT_INTERP/PT_GNU_RELRO/PT_TLS
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// segments at the same offset will be aligned correctly.
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if (A->Align != B->Align)
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return A->Align > B->Align;
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return A->Index < B->Index;
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}
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‎llvm/test/tools/llvm-objcopy/ELF/non-load-at-load-start.test

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## When the offset of a non-PT_LOAD segment (e.g. PT_INTERP) equals the offset
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## of a PT_LOAD segment, set the parent of the segment with a smaller alignment
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## to the segment with a larger alignment, ensuring that the offset is correctly
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## aligned.
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# RUN: yaml2obj %s -o %t
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# RUN: llvm-objcopy %t %t2
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# RUN: llvm-readelf -Sl %t2 | FileCheck %s
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# CHECK: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# CHECK-NEXT: [ 0] NULL 0000000000000000 000000 000000 00 0 0 0
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# CHECK-NEXT: [ 1] .text PROGBITS 0000000000201000 001000 000001 00 0 0 4
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# CHECK-NEXT: [ 2] .interp PROGBITS 0000000000202000 002000 00001c 00 0 0 1
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# CHECK-NEXT: [ 3] .rodata PROGBITS 0000000000202020 002020 000001 00 0 0 1
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# CHECK-NEXT: [ 4] .tdata PROGBITS 0000000000203000 003000 000001 00 0 0 4096
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# CHECK-NEXT: [ 5] .relro_padding NOBITS 0000000000203001 003001 000fff 00 0 0 1
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# CHECK-NEXT: [ 6] .strtab STRTAB 0000000000000000 003001 000001 00 0 0 1
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# CHECK-NEXT: [ 7] .shstrtab STRTAB 0000000000000000 003002 00003f 00 0 0 1
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# CHECK: Program Headers:
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# CHECK-NEXT: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
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# CHECK-NEXT: PHDR 0x000040 0x0000000000200040 0x0000000000200040 0x0001c0 0x0001c0 0x8
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# CHECK-NEXT: INTERP 0x002000 0x0000000000202000 0x0000000000202000 0x00001c 0x00001c 0x1
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# CHECK-NEXT: [Requesting program interpreter: ]
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# CHECK-NEXT: LOAD 0x000000 0x0000000000200000 0x0000000000200000 0x000200 0x000200 0x1000
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# CHECK-NEXT: LOAD 0x001000 0x0000000000201000 0x0000000000201000 0x000001 0x000001 0x1000
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# CHECK-NEXT: LOAD 0x002000 0x0000000000202000 0x0000000000202000 0x000021 0x000021 0x1000
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# CHECK-NEXT: TLS 0x003000 0x0000000000203000 0x0000000000203000 0x000001 0x001000 0x1000
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# CHECK-NEXT: GNU_RELRO 0x003000 0x0000000000203000 0x0000000000203000 0x000001 0x001000 0x1000
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# CHECK-NEXT: LOAD 0x003000 0x0000000000203000 0x0000000000203000 0x000001 0x001000 0x1000
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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_EXEC
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Machine: EM_X86_64
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ProgramHeaders:
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- Type: PT_PHDR
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VAddr: 0x200040
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Align: 0x8
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Offset: 0x40
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FileSize: 0x1c0
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MemSize: 0x1c0
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- Type: PT_INTERP
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FirstSec: .interp
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LastSec: .interp
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## The address equals the address of its containing PT_LOAD.
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VAddr: 0x202000
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Offset: 0x2000
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- Type: PT_LOAD
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VAddr: 0x200000
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Align: 0x1000
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Offset: 0x0
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FileSize: 0x200
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MemSize: 0x200
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- Type: PT_LOAD
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FirstSec: .text
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LastSec: .text
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VAddr: 0x201000
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Align: 0x1000
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Offset: 0x1000
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- Type: PT_LOAD
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FirstSec: .interp
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LastSec: .rodata
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VAddr: 0x202000
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Align: 0x1000
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Offset: 0x2000
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## Intentionally place PT_TLS/PT_GNU_RELRO before PT_LOAD to test that we
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## correctly set parent segments.
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- Type: PT_TLS
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FirstSec: .tdata
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LastSec: .relro_padding
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VAddr: 0x203000
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Align: 0x1000
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Offset: 0x3000
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- Type: PT_GNU_RELRO
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FirstSec: .tdata
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LastSec: .relro_padding
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VAddr: 0x203000
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Align: 0x1000
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Offset: 0x3000
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- Type: PT_LOAD
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FirstSec: .tdata
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LastSec: .relro_padding
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VAddr: 0x203000
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Align: 0x1000
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Offset: 0x3000
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Sections:
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- Name: .text
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Type: SHT_PROGBITS
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Address: 0x201000
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AddressAlign: 0x4
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Offset: 0x1000
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Content: C3
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- Name: .interp
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Type: SHT_PROGBITS
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Address: 0x202000
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AddressAlign: 0x1
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Offset: 0x2000
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Size: 0x1C
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- Name: .rodata
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Type: SHT_PROGBITS
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Address: 0x202020
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AddressAlign: 0x1
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Offset: 0x2020
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Size: 1
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- Name: .tdata
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Type: SHT_PROGBITS
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Address: 0x203000
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AddressAlign: 0x1000
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Offset: 0x3000
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Size: 1
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- Name: .relro_padding
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Type: SHT_NOBITS
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Address: 0x203001
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AddressAlign: 0x1
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Size: 0xFFF

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