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[llvm-objcopy][ELF] Implement --only-keep-debug
--only-keep-debug produces a debug file as the output that only preserves contents of sections useful for debugging purposes (the binutils implementation preserves SHT_NOTE and non-SHF_ALLOC sections), by changing their section types to SHT_NOBITS and rewritting file offsets. See https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html The intended use case is: ``` llvm-objcopy --only-keep-debug a a.dbg llvm-objcopy --strip-debug a b llvm-objcopy --add-gnu-debuglink=a.dbg b ``` The current layout algorithm is incapable of deleting contents and shrinking segments, so it is not suitable for implementing the functionality. This patch adds a new algorithm which assigns sh_offset to sections first, then modifies p_offset/p_filesz of program headers. It bears a resemblance to lld/ELF/Writer.cpp. Reviewed By: jhenderson, jakehehrlich Differential Revision: https://reviews.llvm.org/D67137
This commit is contained in:
Fangrui Song
parent
c05a8dafbb
commit
dd2cb2d821
@ -67,6 +67,15 @@ multiple file formats.
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Print a summary of command line options.
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.. option:: --only-keep-debug
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Produce a debug file as the output that only preserves contents of sections
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useful for debugging purposes.
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For ELF objects, this removes the contents of `SHF_ALLOC` sections that are not
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`SHT_NOTE` by making them `SHT_NOBITS` and shrinking the program headers where
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possible.
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.. option:: --only-section <section>, -j
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Remove all sections from the output, except for sections named ``<section>``.
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@ -177,11 +186,6 @@ The following options are implemented only for COFF objects. If used with other
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objects, :program:`llvm-objcopy` will either emit an error or silently ignore
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them.
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.. option:: --only-keep-debug
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Remove the contents of non-debug sections from the output, but keep the section
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headers.
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ELF-SPECIFIC OPTIONS
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--------------------
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@ -1,22 +0,0 @@
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# NOTE: This test is only intended to be valid as long as --only-keep-debug is
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# implemented as a NOP. This test should fail when that changes and you
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# will need to update this test.
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# RUN: yaml2obj %s > %t
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# RUN: llvm-objcopy %t %t2
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# RUN: llvm-objcopy --only-keep-debug %t %t3
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# RUN: cmp %t2 %t3
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# RUN: llvm-strip --only-keep-debug --no-strip-all %t -o %t4
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# RUN: cmp %t2 %t4
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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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Sections:
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- Name: .text
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Type: SHT_PROGBITS
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Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
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Content: "DEADBEEF"
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224
test/tools/llvm-objcopy/ELF/only-keep-debug.test
Normal file
224
test/tools/llvm-objcopy/ELF/only-keep-debug.test
Normal file
@ -0,0 +1,224 @@
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# RUN: yaml2obj --docnum=1 %s -o %t1
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# RUN: llvm-objcopy --only-keep-debug %t1 %t1.dbg
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# RUN: llvm-readelf -S -l -x .note1 -x .note2 -x .debug_abbrev -x .debug_frame -x .debug_info %t1.dbg | FileCheck %s
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## Check that SHT_NOTE and .debug* are kept, but others are changed to SHT_NOBITS.
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## SHT_NOBITS sections do not occupy space in the output.
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# CHECK: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# CHECK: [ 1] .note1 NOTE 0000000000000400 000400 000001 00 A 0 0 1024
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# CHECK-NEXT: [ 2] .note2 NOTE 0000000000000401 000401 000001 00 A 0 0 0
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# CHECK-NEXT: [ 3] .text NOBITS 0000000000000402 000402 000001 00 AX 0 0 0
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# CHECK-NEXT: [ 4] .tdata NOBITS 0000000000001480 000480 000007 00 WAT 0 0 128
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# CHECK-NEXT: [ 5] .tbss NOBITS 0000000000001487 000480 000005 00 WAT 0 0 0
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# CHECK-NEXT: [ 6] .bss NOBITS 00000000000014a0 000480 00003f 00 WA 0 0 32
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## objcopy sets sh_offset to 0x402. We don't do this to keep sh_offset non-decreasing.
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# CHECK-NEXT: [ 7] .debug_abbrev PROGBITS 0000000000000000 000480 000001 00 0 0 0
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# CHECK-NEXT: [ 8] .debug_frame PROGBITS 0000000000000000 000488 000001 00 0 0 8
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# CHECK-NEXT: [ 9] .debug_info PROGBITS 0000000000000000 000489 000001 00 0 0 0
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# CHECK-NEXT: [10] .strtab STRTAB 0000000000000000 00048a 000001 00 0 0 1
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# CHECK-NEXT: [11] .shstrtab STRTAB 0000000000000000 00048b 000060 00 0 0 1
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# CHECK: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
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# CHECK-NEXT: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000402 0x000403 R E 0x1000
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# CHECK-NEXT: LOAD 0x000480 0x0000000000001480 0x0000000000000000 0x000000 0x00005f RW 0x1000
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# CHECK-NEXT: TLS 0x000480 0x0000000000001480 0x0000000000000000 0x000000 0x00000c RW 0x80
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# CHECK-NEXT: NOTE 0x000400 0x0000000000000400 0x0000000000000000 0x000002 0x000002 0x400
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## Contents of SHT_NOTE and .debug* are kept.
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# CHECK: Hex dump of section '.note1':
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# CHECK-NEXT: 0x00000400 01
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# CHECK: Hex dump of section '.note2':
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# CHECK-NEXT: 0x00000401 02
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# CHECK: Hex dump of section '.debug_abbrev':
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# CHECK-NEXT: 0x00000000 03
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# CHECK: Hex dump of section '.debug_frame':
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# CHECK-NEXT: 0x00000000 04
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# CHECK: Hex dump of section '.debug_info':
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# CHECK-NEXT: 0x00000000 05
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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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Sections:
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- Name: .note1
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Type: SHT_NOTE
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Flags: [ SHF_ALLOC ]
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Address: 0x400
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AddressAlign: 0x400
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Content: 01
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- Name: .note2
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Type: SHT_NOTE
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Flags: [ SHF_ALLOC ]
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Address: 0x401
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Content: 02
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- Name: .text
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Type: SHT_PROGBITS
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Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
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Address: 0x402
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Content: c3
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- Name: .tdata
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Type: SHT_PROGBITS
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Flags: [ SHF_ALLOC, SHF_WRITE, SHF_TLS ]
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Address: 0x1480 # Ensure Address=0x1000+Offset
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AddressAlign: 0x80
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# An arbitrary non-zero Size tests that .tdata does not occupy space
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# and we can rewrite p_filesz of PT_TLS.
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Size: 7
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- Name: .tbss
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Type: SHT_NOBITS
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Flags: [ SHF_ALLOC, SHF_WRITE, SHF_TLS ]
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Address: 0x1487 # Ensure Address=0x1000+Offset
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Size: 5
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- Name: .bss
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Type: SHT_NOBITS
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Flags: [ SHF_ALLOC, SHF_WRITE ]
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Address: 0x14a0 # Ensure Address=0x1000+Offset
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AddressAlign: 0x20
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# An arbitrary non-zero Size tests that .bss does not occupy space.
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Size: 63
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- Name: .debug_abbrev
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Type: SHT_PROGBITS
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Content: 03
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- Name: .debug_frame
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Type: SHT_PROGBITS
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# AddressAlign tests the file offset assignment leaves a gap.
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AddressAlign: 0x8
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Content: 04
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- Name: .debug_info
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Type: SHT_PROGBITS
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Content: 05
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ProgramHeaders:
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- Type: PT_LOAD
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Flags: [ PF_R, PF_X ]
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Offset: 0
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Align: 0x1000
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Sections:
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- Section: .note1
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- Section: .note2
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- Section: .text
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- Type: PT_LOAD
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Flags: [ PF_R, PF_W ]
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VAddr: 0x1480 # Ensure Offset=VAddr (mod Align) if Offset changes
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Align: 0x1000
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Sections:
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- Section: .tdata
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- Section: .bss
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- Type: PT_TLS
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Flags: [ PF_R, PF_W ]
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VAddr: 0x1480 # Ensure Offset=VAddr (mod Align) if Offset changes
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Sections:
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- Section: .tdata
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- Section: .tbss
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- Type: PT_NOTE
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VAddr: 0x400
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Sections:
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- Section: .note1
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- Section: .note2
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...
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# RUN: yaml2obj --docnum=2 %s -o %t2
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# RUN: llvm-objcopy --only-keep-debug %t2 %t2.dbg
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# RUN: llvm-readelf -S -l %t2.dbg | FileCheck --check-prefix=CHECK2 %s
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## Only the tail of a segment can be trimmed. .text still occupies space because
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## it is followed by .note which is not SHT_NOBITS.
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# CHECK2: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# CHECK2: [ 1] .text NOBITS 0000000000000200 000200 000001 00 AX 0 0 512
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# CHECK2-NEXT: [ 2] .note NOTE 0000000000000201 000201 000001 00 A 0 0 0
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# CHECK2-NEXT: [ 3] .debug_info PROGBITS 0000000000000000 000220 000001 00 0 0 32
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# CHECK2-NEXT: [ 4] .strtab STRTAB 0000000000000000 000221 000001 00 0 0 1
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# CHECK2-NEXT: [ 5] .shstrtab STRTAB 0000000000000000 000222 00002b 00 0 0 1
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## Check that p_offset or p_filesz of empty segments or PT_PHDR are not modified.
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# CHECK2: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
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# CHECK2-NEXT: PHDR 0x000040 0x0000000000000040 0x0000000000000000 0x0000a8 0x0000a8 R 0x8
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# CHECK2-NEXT: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000202 0x000202 R E 0x1000
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# CHECK2-NEXT: LOAD 0x000202 0x0000000000000202 0x0000000000000000 0x00000e 0x00000e RW 0x1
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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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Sections:
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- Name: .text
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Type: SHT_PROGBITS
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Flags: [ SHF_ALLOC, SHF_EXECINSTR ]
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Address: 0x200
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AddressAlign: 0x200
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Content: c3
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- Name: .note
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Type: SHT_NOTE
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Flags: [ SHF_ALLOC ]
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Address: 0x201
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Content: 01
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- Name: .debug_info
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Type: SHT_PROGBITS
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AddressAlign: 0x20
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Content: 02
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ProgramHeaders:
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- Type: PT_PHDR
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Flags: [ PF_R ]
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Offset: 0x40
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VAddr: 0x40
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# 3 * sizeof(Elf64_Phdr) = 0xa8
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FileSize: 0xa8
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MemSize: 0xa8
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Align: 8
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- Type: PT_LOAD
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Flags: [ PF_R, PF_X ]
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Offset: 0
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Align: 4096
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Sections:
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- Section: .text
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- Section: .note
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- Type: PT_LOAD
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Flags: [ PF_R, PF_W ]
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Offset: 0x202
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VAddr: 0x202
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FileSize: 14
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MemSize: 14
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...
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## If .symtab or .strtab has the SHF_ALLOC flag, it will be changed to SHT_NOBITS.
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# RUN: yaml2obj --docnum=3 %s -o %t3
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# RUN: llvm-objcopy --only-keep-debug %t3 %t3.dbg
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# RUN: llvm-readelf -S -l %t3.dbg | FileCheck --check-prefix=CHECK3 %s
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# CHECK3: [Nr] Name Type Address Off Size ES Flg Lk Inf Al
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# CHECK3: [ 1] .dynsym NOBITS 0000000000000000 000040 000018 18 A 2 1 1024
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# CHECK3-NEXT: [ 2] .dynstr NOBITS 0000000000000000 000040 000001 00 A 0 0 0
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# CHECK3-NEXT: [ 3] .symtab NOBITS 0000000000000000 000040 000018 18 A 4 1 0
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# CHECK3-NEXT: [ 4] .strtab NOBITS 0000000000000000 000040 000001 00 A 0 0 0
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# CHECK3-NEXT: [ 5] .shstrtab STRTAB 0000000000000000 000040 00002b 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_DYN
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Machine: EM_X86_64
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Sections:
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- Name: .dynsym
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Type: SHT_DYNSYM
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Flags: [ SHF_ALLOC ]
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Link: .dynstr
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AddressAlign: 0x400
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- Name: .dynstr
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Type: SHT_STRTAB
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Flags: [ SHF_ALLOC ]
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- Name: .symtab
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Type: SHT_STRTAB
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Flags: [ SHF_ALLOC ]
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Link: .strtab
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- Name: .strtab
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Type: SHT_STRTAB
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Flags: [ SHF_ALLOC ]
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DynamicSymbols: []
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Symbols: []
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@ -86,8 +86,9 @@ def keep_file_symbols : Flag<["--"], "keep-file-symbols">,
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def only_keep_debug
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: Flag<["--"], "only-keep-debug">,
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HelpText<"Clear sections that would not be stripped by --strip-debug. "
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"Currently only implemented for COFF.">;
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HelpText<
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"Produce a debug file as the output that only preserves contents of "
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"sections useful for debugging purposes">;
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def discard_locals : Flag<["--"], "discard-locals">,
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HelpText<"Remove compiler-generated local symbols, (e.g. "
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@ -136,17 +136,17 @@ static std::unique_ptr<Writer> createELFWriter(const CopyConfig &Config,
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// Depending on the initial ELFT and OutputFormat we need a different Writer.
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switch (OutputElfType) {
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case ELFT_ELF32LE:
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return std::make_unique<ELFWriter<ELF32LE>>(Obj, Buf,
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!Config.StripSections);
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return std::make_unique<ELFWriter<ELF32LE>>(Obj, Buf, !Config.StripSections,
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Config.OnlyKeepDebug);
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case ELFT_ELF64LE:
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return std::make_unique<ELFWriter<ELF64LE>>(Obj, Buf,
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!Config.StripSections);
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return std::make_unique<ELFWriter<ELF64LE>>(Obj, Buf, !Config.StripSections,
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Config.OnlyKeepDebug);
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case ELFT_ELF32BE:
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return std::make_unique<ELFWriter<ELF32BE>>(Obj, Buf,
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!Config.StripSections);
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return std::make_unique<ELFWriter<ELF32BE>>(Obj, Buf, !Config.StripSections,
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Config.OnlyKeepDebug);
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case ELFT_ELF64BE:
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return std::make_unique<ELFWriter<ELF64BE>>(Obj, Buf,
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!Config.StripSections);
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return std::make_unique<ELFWriter<ELF64BE>>(Obj, Buf, !Config.StripSections,
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Config.OnlyKeepDebug);
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}
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llvm_unreachable("Invalid output format");
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}
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@ -694,6 +694,11 @@ static Error handleArgs(const CopyConfig &Config, Object &Obj,
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}
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}
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if (Config.OnlyKeepDebug)
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for (auto &Sec : Obj.sections())
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if (Sec.Flags & SHF_ALLOC && Sec.Type != SHT_NOTE)
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Sec.Type = SHT_NOBITS;
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for (const auto &Flag : Config.AddSection) {
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std::pair<StringRef, StringRef> SecPair = Flag.split("=");
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StringRef SecName = SecPair.first;
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@ -1797,10 +1797,9 @@ template <class ELFT> void ELFWriter<ELFT>::writeSectionData() {
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template <class ELFT> void ELFWriter<ELFT>::writeSegmentData() {
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for (Segment &Seg : Obj.segments()) {
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uint8_t *B = Buf.getBufferStart() + Seg.Offset;
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assert(Seg.FileSize == Seg.getContents().size() &&
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"Segment size must match contents size");
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std::memcpy(B, Seg.getContents().data(), Seg.FileSize);
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size_t Size = std::min<size_t>(Seg.FileSize, Seg.getContents().size());
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std::memcpy(Buf.getBufferStart() + Seg.Offset, Seg.getContents().data(),
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Size);
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}
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// Iterate over removed sections and overwrite their old data with zeroes.
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@ -1815,8 +1814,10 @@ template <class ELFT> void ELFWriter<ELFT>::writeSegmentData() {
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}
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template <class ELFT>
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ELFWriter<ELFT>::ELFWriter(Object &Obj, Buffer &Buf, bool WSH)
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: Writer(Obj, Buf), WriteSectionHeaders(WSH && Obj.HadShdrs) {}
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ELFWriter<ELFT>::ELFWriter(Object &Obj, Buffer &Buf, bool WSH,
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bool OnlyKeepDebug)
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: Writer(Obj, Buf), WriteSectionHeaders(WSH && Obj.HadShdrs),
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OnlyKeepDebug(OnlyKeepDebug) {}
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Error Object::removeSections(bool AllowBrokenLinks,
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std::function<bool(const SectionBase &)> ToRemove) {
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@ -1957,6 +1958,78 @@ static uint64_t layoutSections(Range Sections, uint64_t Offset) {
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return Offset;
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}
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// Rewrite sh_offset after some sections are changed to SHT_NOBITS and thus
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// occupy no space in the file.
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static uint64_t layoutSectionsForOnlyKeepDebug(Object &Obj, uint64_t Off) {
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uint32_t Index = 1;
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for (auto &Sec : Obj.sections()) {
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Sec.Index = Index++;
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auto *FirstSec = Sec.ParentSegment && Sec.ParentSegment->Type == PT_LOAD
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? Sec.ParentSegment->firstSection()
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: nullptr;
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// The first section in a PT_LOAD has to have congruent offset and address
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// modulo the alignment, which usually equals the maximum page size.
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if (FirstSec && FirstSec == &Sec)
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Off = alignTo(Off, Sec.ParentSegment->Align, Sec.Addr);
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// sh_offset is not significant for SHT_NOBITS sections, but the congruence
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// rule must be followed if it is the first section in a PT_LOAD. Do not
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// advance Off.
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if (Sec.Type == SHT_NOBITS) {
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Sec.Offset = Off;
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continue;
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}
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if (!FirstSec) {
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// FirstSec being nullptr generally means that Sec does not have the
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// SHF_ALLOC flag.
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Off = Sec.Align ? alignTo(Off, Sec.Align) : Off;
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} else if (FirstSec != &Sec) {
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// The offset is relative to the first section in the PT_LOAD segment. Use
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// sh_offset for non-SHF_ALLOC sections.
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Off = Sec.OriginalOffset - FirstSec->OriginalOffset + FirstSec->Offset;
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}
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Sec.Offset = Off;
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Off += Sec.Size;
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}
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return Off;
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}
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// Rewrite p_offset and p_filesz of non-empty non-PT_PHDR segments after
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// sh_offset values have been updated.
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static uint64_t layoutSegmentsForOnlyKeepDebug(std::vector<Segment *> &Segments,
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uint64_t HdrEnd) {
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uint64_t MaxOffset = 0;
|
||||
for (Segment *Seg : Segments) {
|
||||
const SectionBase *Sec = Seg->firstSection();
|
||||
if (Seg->Type == PT_PHDR || !Sec)
|
||||
continue;
|
||||
|
||||
uint64_t Offset = Sec->Offset;
|
||||
uint64_t FileSize = 0;
|
||||
for (const SectionBase *Sec : Seg->Sections) {
|
||||
uint64_t Size = Sec->Type == SHT_NOBITS ? 0 : Sec->Size;
|
||||
if (Sec->Offset + Size > Offset)
|
||||
FileSize = std::max(FileSize, Sec->Offset + Size - Offset);
|
||||
}
|
||||
|
||||
// If the segment includes EHDR and program headers, don't make it smaller
|
||||
// than the headers.
|
||||
if (Seg->Offset < HdrEnd && HdrEnd <= Seg->Offset + Seg->FileSize) {
|
||||
FileSize += Offset - Seg->Offset;
|
||||
Offset = Seg->Offset;
|
||||
FileSize = std::max(FileSize, HdrEnd - Offset);
|
||||
}
|
||||
|
||||
Seg->Offset = Offset;
|
||||
Seg->FileSize = FileSize;
|
||||
MaxOffset = std::max(MaxOffset, Offset + FileSize);
|
||||
}
|
||||
return MaxOffset;
|
||||
}
|
||||
|
||||
template <class ELFT> void ELFWriter<ELFT>::initEhdrSegment() {
|
||||
Segment &ElfHdr = Obj.ElfHdrSegment;
|
||||
ElfHdr.Type = PT_PHDR;
|
||||
@ -1977,12 +2050,24 @@ template <class ELFT> void ELFWriter<ELFT>::assignOffsets() {
|
||||
OrderedSegments.push_back(&Obj.ElfHdrSegment);
|
||||
OrderedSegments.push_back(&Obj.ProgramHdrSegment);
|
||||
orderSegments(OrderedSegments);
|
||||
// Offset is used as the start offset of the first segment to be laid out.
|
||||
// Since the ELF Header (ElfHdrSegment) must be at the start of the file,
|
||||
// we start at offset 0.
|
||||
uint64_t Offset = 0;
|
||||
Offset = layoutSegments(OrderedSegments, Offset);
|
||||
Offset = layoutSections(Obj.sections(), Offset);
|
||||
|
||||
uint64_t Offset;
|
||||
if (OnlyKeepDebug) {
|
||||
// For --only-keep-debug, the sections that did not preserve contents were
|
||||
// changed to SHT_NOBITS. We now rewrite sh_offset fields of sections, and
|
||||
// then rewrite p_offset/p_filesz of program headers.
|
||||
uint64_t HdrEnd =
|
||||
sizeof(Elf_Ehdr) + llvm::size(Obj.segments()) * sizeof(Elf_Phdr);
|
||||
Offset = layoutSectionsForOnlyKeepDebug(Obj, HdrEnd);
|
||||
Offset = std::max(Offset,
|
||||
layoutSegmentsForOnlyKeepDebug(OrderedSegments, HdrEnd));
|
||||
} else {
|
||||
// Offset is used as the start offset of the first segment to be laid out.
|
||||
// Since the ELF Header (ElfHdrSegment) must be at the start of the file,
|
||||
// we start at offset 0.
|
||||
Offset = layoutSegments(OrderedSegments, 0);
|
||||
Offset = layoutSections(Obj.sections(), Offset);
|
||||
}
|
||||
// If we need to write the section header table out then we need to align the
|
||||
// Offset so that SHOffset is valid.
|
||||
if (WriteSectionHeaders)
|
||||
|
||||
@ -342,9 +342,13 @@ public:
|
||||
virtual ~ELFWriter() {}
|
||||
bool WriteSectionHeaders;
|
||||
|
||||
// For --only-keep-debug, select an alternative section/segment layout
|
||||
// algorithm.
|
||||
bool OnlyKeepDebug;
|
||||
|
||||
Error finalize() override;
|
||||
Error write() override;
|
||||
ELFWriter(Object &Obj, Buffer &Buf, bool WSH);
|
||||
ELFWriter(Object &Obj, Buffer &Buf, bool WSH, bool OnlyKeepDebug);
|
||||
};
|
||||
|
||||
class BinaryWriter : public Writer {
|
||||
@ -435,8 +439,6 @@ private:
|
||||
}
|
||||
};
|
||||
|
||||
std::set<const SectionBase *, SectionCompare> Sections;
|
||||
|
||||
public:
|
||||
uint32_t Type;
|
||||
uint32_t Flags;
|
||||
@ -451,6 +453,7 @@ public:
|
||||
uint64_t OriginalOffset;
|
||||
Segment *ParentSegment = nullptr;
|
||||
ArrayRef<uint8_t> Contents;
|
||||
std::set<const SectionBase *, SectionCompare> Sections;
|
||||
|
||||
explicit Segment(ArrayRef<uint8_t> Data) : Contents(Data) {}
|
||||
Segment() {}
|
||||
|
||||
Loading…
Reference in New Issue
Block a user