[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

(cherry picked from commit ec786906f5feb4dceba1b5338927079e63e78095)

1 files changed, 7 insertions, 6 deletions

diff --git a/llvm/tools/llvm-objcopy/ELF/Object.cpp b/llvm/tools/llvm-objcopy/ELF/Object.cpp
index ad53c75663e..98f72959ae5 100644
--- a/llvm/tools/llvm-objcopy/ELF/Object.cpp
+++ b/llvm/tools/llvm-objcopy/ELF/Object.cpp
@@ -2254,25 +2254,26 @@ Error BinaryWriter::finalize() {
   OrderedSegments.erase(End, std::end(OrderedSegments));
 
   // Compute the section LMA based on its sh_offset and the containing segment's
-  // p_offset and p_paddr. Also compute the minimum LMA of all sections as
-  // MinAddr. In the output, the contents between address 0 and MinAddr will be
-  // skipped.
+  // p_offset and p_paddr. Also compute the minimum LMA of all non-empty
+  // sections as MinAddr. In the output, the contents between address 0 and
+  // MinAddr will be skipped.
   uint64_t MinAddr = UINT64_MAX;
   for (SectionBase &Sec : Obj.allocSections()) {
     if (Sec.ParentSegment != nullptr)
       Sec.Addr =
           Sec.Offset - Sec.ParentSegment->Offset + Sec.ParentSegment->PAddr;
-    MinAddr = std::min(MinAddr, Sec.Addr);
+    if (Sec.Size > 0)
+      MinAddr = std::min(MinAddr, Sec.Addr);
   }
 
   // Now that every section has been laid out we just need to compute the total
   // file size. This might not be the same as the offset returned by
   // layoutSections, because we want to truncate the last segment to the end of
-  // its last section, to match GNU objcopy's behaviour.
+  // its last non-empty section, to match GNU objcopy's behaviour.
   TotalSize = 0;
   for (SectionBase &Sec : Obj.allocSections()) {
     Sec.Offset = Sec.Addr - MinAddr;
-    if (Sec.Type != SHT_NOBITS)
+    if (Sec.Type != SHT_NOBITS && Sec.Size > 0)
       TotalSize = std::max(TotalSize, Sec.Offset + Sec.Size);
   }