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[llvm-objcopy] -O binary: skip empty sections

Authored by MaskRay on Apr 30 2020, 10:15 PM.

Description

[llvm-objcopy] -O binary: skip empty sections

After SHF_ALLOC sections are ordered by LMA:

  • If initial sections are empty, GNU objcopy skips their contents while we emit leading zeros. (binary-paddr.test %t4)
  • If trailing sections are empty, GNU objcopy skips their contents while we emit trailing zeros. (binary-paddr.test %t5)

This patch matches GNU objcopy's behavior. Linkers don't keep p_memsz
PT_LOAD segments. Such empty sections would not have a containing
PT_LOAD and Section::ParentSegment might be null if linkers fail to
optimize the file offsets (lld D79254).

In particular, without D79254, the arm Linux kernel's multi_v5_defconfig
depends on this behavior: in vmlinux, an empty .text_itcm is mapped at
a very high address (0xfffe0000) but the kernel does not expect
objcopy -O binary to create a very large arch/arm/boot/Image
(0xfffe0000-0xc0000000 ~= 1GiB). See https://bugs.llvm.org/show_bug.cgi?id=45632

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D79229

Details

Committed
MaskRayMay 1 2020, 2:25 PM
Reviewer
jhenderson
Differential Revision
D79229: [llvm-objcopy] -O binary: skip empty sections
Parents
rG7585ba208e67: [clang][OpenMP] Fix mangling of linear parameters.
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