[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

llvm/test/tools/llvm-objcopy/ELF/binary-paddr.test

@@ -48,7 +48,7 @@ ProgramHeaders:
 # RUN: od -A x -t x2 %t2.out | FileCheck %s --check-prefix=CHECK2 --ignore-case
 # RUN: wc -c %t2.out | FileCheck %s --check-prefix=SIZE2
 
-## The computed LMA of .data is 0x4000. The minimum LMA of all sections is 0x1000.
+## The computed LMA of .data is 0x4000. The minimum LMA of all non-empty sections is 0x1000.
 ## The content of .data will be written at 0x4000-0x1000 = 0x3000.
 # CHECK2:      000000 c3c3 c3c3 0000 0000 0000 0000 0000 0000
 # CHECK2-NEXT: 000010 0000 0000 0000 0000 0000 0000 0000 0000
@@ -93,7 +93,7 @@ ProgramHeaders:
 # RUN: od -A x -t x2 %t3.out | FileCheck %s --check-prefix=CHECK3 --ignore-case
 # RUN: wc -c %t3.out | FileCheck %s --check-prefix=SIZE3
 
-## The minimum LMA of all sections is 0x1000.
+## The minimum LMA of all non-empty sections is 0x1000.
 ## The content of .data will be written at 0x3000-0x1000 = 0x2000.
 # CHECK3:      000000 c3c3 c3c3 0000 0000 0000 0000 0000 0000
 # CHECK3-NEXT: 000010 0000 0000 0000 0000 0000 0000 0000 0000
@@ -129,3 +129,58 @@ ProgramHeaders:
     VAddr: 0x3000
     Sections:
       - Section: .data
+
+## The first section (.text) is empty. Test that we skip its LMA until the first
+## non-empty section, otherwise we would leave a large number of leading zeroes.
+# RUN: yaml2obj --docnum=4 %s -o %t4
+# RUN: llvm-objcopy -O binary %t4 %t4.out
+# RUN: od -A x -t x2 %t4.out | FileCheck %s --check-prefix=SKIPEMPTY
+
+# SKIPEMPTY:      000000 3232
+# SKIPEMPTY-NEXT: 000002
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_X86_64
+Sections:
+  - Name:         .text
+    Type:         SHT_PROGBITS
+    Flags:        [ SHF_ALLOC, SHF_EXECINSTR ]
+    Address:      0x1000
+    AddressAlign: 0x1000
+  - Name:         gap
+    Type:         Fill
+    Size:         0x1000
+  - Name:         .data
+    Type:         SHT_PROGBITS
+    Flags:        [ SHF_ALLOC, SHF_WRITE ]
+    Content:      "3232"
+
+## The last section (.data) is empty. Test that we stop dumping after the last
+## non-empty section, otherwise we would leave a large number of trailing zeroes.
+# RUN: yaml2obj --docnum=5 %s -o %t5
+# RUN: llvm-objcopy -O binary %t5 %t5.out
+# RUN: od -A x -t x2 %t5.out | FileCheck %s --check-prefix=SKIPEMPTY
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_X86_64
+Sections:
+  - Name:         .text
+    Type:         SHT_PROGBITS
+    Flags:        [ SHF_ALLOC, SHF_EXECINSTR ]
+    Address:      0x1000
+    AddressAlign: 0x1000
+    Content:      "3232"
+  - Name:         gap
+    Type:         Fill
+    Size:         0xffd
+  - Name:         .data
+    Type:         SHT_PROGBITS
+    Flags:        [ SHF_ALLOC, SHF_WRITE ]

llvm/tools/llvm-objcopy/ELF/Object.cpp

@@ -2217,25 +2217,26 @@ Error BinaryWriter::write() {
 
 Error BinaryWriter::finalize() {
   // 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);
   }