ObjectFilePECOFF: Create a "container" section spanning the entire module image

Summary:
This is coming from the discussion in D55356 (the most interesting part
happened on the mailing list, so it isn't reflected on the review page).

In short the issue is that lldb assumes that all bytes of a module image
in memory will be backed by a "section". This isn't the case for PECOFF
files because the initial bytes of the module image will contain the
file header, which does not correspond to any normal section in the
file. In particular, this means it is not possible to implement
GetBaseAddress function for PECOFF files, because that's supposed point
to the first byte of that header.

If my (limited) understanding of how PECOFF files work is correct, then
the OS is expecded to load the entire module into one continuous chunk
of memory. The address of that chunk (+/- ASLR) is given by the "image
base" field in the COFF header, and it's size by "image size". All of
the COFF sections are then loaded into this range.

If that's true, then we can model this behavior in lldb by creating a
"container" section to represent the entire module image, and then place
other sections inside that. This would make be consistent with how MachO
and ELF files are modelled (except that those can have multiple
top-level containers as they can be loaded into multiple discontinuous
chunks of memory).

This change required a small number of fixups in the PDB plugins, which
assumed a certain order of sections within the object file (which
obivously changes now). I fix this by changing the lookup code to use
section IDs (which are unchanged) instead of indexes. This has the nice
benefit of removing spurious -1s in the plugins as the section IDs in
the pdbs match the 1-based section IDs in the COFF plugin.

Besides making the implementation of GetBaseAddress possible, this also
improves the lookup of addresses in the gaps between the object file
sections, which will now be correctly resolved as belonging to the
object file.

Reviewers: zturner, amccarth, stella.stamenova, clayborg, lemo

Reviewed By: clayborg, lemo

Subscribers: JDevlieghere, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D56537

llvm-svn: 353916

4 files changed, 23 insertions, 31 deletions

diff --git a/lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp b/lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp
index ddc59aea62e..0b5da9e06a3 100644
--- a/lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp
+++ b/lldb/source/Plugins/ObjectFile/PECOFF/ObjectFilePECOFF.cpp
@@ -706,6 +706,15 @@ void ObjectFilePECOFF::CreateSections(SectionList &unified_section_list) {
   ModuleSP module_sp(GetModule());
   if (module_sp) {
     std::lock_guard<std::recursive_mutex> guard(module_sp->GetMutex());
+
+    SectionSP image_sp = std::make_shared<Section>(
+        module_sp, this, ~user_id_t(0), ConstString(), eSectionTypeContainer,
+        m_coff_header_opt.image_base, m_coff_header_opt.image_size,
+        /*file_offset*/ 0, /*file_size*/ 0, m_coff_header_opt.sect_alignment,
+        /*flags*/ 0);
+    m_sections_up->AddSection(image_sp);
+    unified_section_list.AddSection(image_sp);
+
     const uint32_t nsects = m_sect_headers.size();
     ModuleSP module_sp(GetModule());
     for (uint32_t idx = 0; idx < nsects; ++idx) {
@@ -808,20 +817,16 @@ void ObjectFilePECOFF::CreateSections(SectionList &unified_section_list) {
           section_type = eSectionTypeData;
       }
 
-      // Use a segment ID of the segment index shifted left by 8 so they
-      // never conflict with any of the sections.
       SectionSP section_sp(new Section(
-          module_sp, // Module to which this section belongs
-          this,      // Object file to which this section belongs
-          idx + 1, // Section ID is the 1 based segment index shifted right by
-                   // 8 bits as not to collide with any of the 256 section IDs
-                   // that are possible
+          image_sp,        // Parent section
+          module_sp,       // Module to which this section belongs
+          this,            // Object file to which this section belongs
+          idx + 1,         // Section ID is the 1 based section index.
           const_sect_name, // Name of this section
-          section_type,    // This section is a container of other sections.
-          m_coff_header_opt.image_base +
-              m_sect_headers[idx].vmaddr, // File VM address == addresses as
-                                          // they are found in the object file
-          m_sect_headers[idx].vmsize,     // VM size in bytes of this section
+          section_type,
+          m_sect_headers[idx].vmaddr, // File VM address == addresses as
+                                      // they are found in the object file
+          m_sect_headers[idx].vmsize, // VM size in bytes of this section
           m_sect_headers[idx]
               .offset, // Offset to the data for this section in the file
           m_sect_headers[idx]
@@ -829,10 +834,7 @@ void ObjectFilePECOFF::CreateSections(SectionList &unified_section_list) {
           m_coff_header_opt.sect_alignment, // Section alignment
           m_sect_headers[idx].flags));      // Flags for this section
 
-      // section_sp->SetIsEncrypted (segment_is_encrypted);
-
-      unified_section_list.AddSection(section_sp);
-      m_sections_up->AddSection(section_sp);
+      image_sp->GetChildren().AddSection(std::move(section_sp));
     }
   }
 }
diff --git a/lldb/source/Plugins/SymbolFile/NativePDB/DWARFLocationExpression.cpp b/lldb/source/Plugins/SymbolFile/NativePDB/DWARFLocationExpression.cpp
index 2b9a56e1ba8..5db7de6f7c7 100644
--- a/lldb/source/Plugins/SymbolFile/NativePDB/DWARFLocationExpression.cpp
+++ b/lldb/source/Plugins/SymbolFile/NativePDB/DWARFLocationExpression.cpp
@@ -207,13 +207,7 @@ DWARFExpression lldb_private::npdb::MakeGlobalLocationExpression(
         SectionList *section_list = module->GetSectionList();
         assert(section_list);
 
-        // Section indices in PDB are 1-based, but in DWARF they are 0-based, so
-        // we need to subtract 1.
-        uint32_t section_idx = section - 1;
-        if (section_idx >= section_list->GetSize())
-          return false;
-
-        auto section_ptr = section_list->GetSectionAtIndex(section_idx);
+        auto section_ptr = section_list->FindSectionByID(section);
         if (!section_ptr)
           return false;
 
diff --git a/lldb/source/Plugins/SymbolFile/PDB/PDBLocationToDWARFExpression.cpp b/lldb/source/Plugins/SymbolFile/PDB/PDBLocationToDWARFExpression.cpp
index 0026aaeee98..c7cf8b235bf 100644
--- a/lldb/source/Plugins/SymbolFile/PDB/PDBLocationToDWARFExpression.cpp
+++ b/lldb/source/Plugins/SymbolFile/PDB/PDBLocationToDWARFExpression.cpp
@@ -90,11 +90,9 @@ DWARFExpression ConvertPDBLocationToDWARFExpression(
     if (!section_list)
       return DWARFExpression(nullptr);
 
-    uint32_t section_idx = symbol.getAddressSection() - 1;
-    if (section_idx >= section_list->GetSize())
-      return DWARFExpression(nullptr);
+    uint32_t section_id = symbol.getAddressSection();
 
-    auto section = section_list->GetSectionAtIndex(section_idx);
+    auto section = section_list->FindSectionByID(section_id);
     if (!section)
       return DWARFExpression(nullptr);
 
diff --git a/lldb/source/Plugins/SymbolFile/PDB/SymbolFilePDB.cpp b/lldb/source/Plugins/SymbolFile/PDB/SymbolFilePDB.cpp
index 368b15680b3..8d71b1309b1 100644
--- a/lldb/source/Plugins/SymbolFile/PDB/SymbolFilePDB.cpp
+++ b/lldb/source/Plugins/SymbolFile/PDB/SymbolFilePDB.cpp
@@ -1349,11 +1349,9 @@ void SymbolFilePDB::AddSymbols(lldb_private::Symtab &symtab) {
     return;
 
   while (auto pub_symbol = results->getNext()) {
-    auto section_idx = pub_symbol->getAddressSection() - 1;
-    if (section_idx >= section_list->GetSize())
-      continue;
+    auto section_id = pub_symbol->getAddressSection();
 
-    auto section = section_list->GetSectionAtIndex(section_idx);
+    auto section = section_list->FindSectionByID(section_id);
     if (!section)
       continue;