llvm-project/llvm/docs/PDB/DbiStream.rst

=====================================
The PDB DBI (Debug Info) Stream
=====================================

.. contents::
   :local:

.. _dbi_intro:

Introduction
============

The PDB DBI Stream (Index 3) is one of the largest and most important streams
in a PDB file.  It contains information about how the program was compiled,
(e.g. compilation flags, etc), the compilands (e.g. object files) that
were used to link together the program, the source files which were used
to build the program, as well as references to other streams that contain more
detailed information about each compiland, such as the CodeView symbol records
contained within each compiland and the source and line information for
functions and other symbols within each compiland.


.. _dbi_header:

Stream Header
=============
At offset 0 of the DBI Stream is a header with the following layout:


.. code-block:: c++

  struct DbiStreamHeader {
    int32_t VersionSignature;
    uint32_t VersionHeader;
    uint32_t Age;
    uint16_t GlobalStreamIndex;
    uint16_t BuildNumber;
    uint16_t PublicStreamIndex;
    uint16_t PdbDllVersion;
    uint16_t SymRecordStream;
    uint16_t PdbDllRbld;
    int32_t ModInfoSize;
    int32_t SectionContributionSize;
    int32_t SectionMapSize;
    int32_t SourceInfoSize;
    int32_t TypeServerMapSize;
    uint32_t MFCTypeServerIndex;
    int32_t OptionalDbgHeaderSize;
    int32_t ECSubstreamSize;
    uint16_t Flags;
    uint16_t Machine;
    uint32_t Padding;
  };

- **VersionSignature** - Unknown meaning.  Appears to always be ``-1``.

- **VersionHeader** - A value from the following enum.

.. code-block:: c++

  enum class DbiStreamVersion : uint32_t {
    VC41 = 930803,
    V50 = 19960307,
    V60 = 19970606,
    V70 = 19990903,
    V110 = 20091201
  };

Similar to the :doc:`PDB Stream <PdbStream>`, this value always appears to be
``V70``, and it is not clear what the other values are for.

- **Age** - The number of times the PDB has been written.  Equal to the same
  field from the :ref:`PDB Stream header <pdb_stream_header>`.

- **GlobalStreamIndex** - The index of the :doc:`Global Symbol Stream <GlobalStream>`,
  which contains CodeView symbol records for all global symbols.  Actual records
  are stored in the symbol record stream, and are referenced from this stream.

- **BuildNumber** - A bitfield containing values representing the major and minor
  version number of the toolchain (e.g. 12.0 for MSVC 2013) used to build the
  program, with the following layout:

.. code-block:: c++

  uint16_t MinorVersion : 8;
  uint16_t MajorVersion : 7;
  uint16_t NewVersionFormat : 1;

For the purposes of LLVM, we assume ``NewVersionFormat`` to be always ``true``.
If it is ``false``, the layout above does not apply and the reader should consult
the `Microsoft Source Code <https://github.com/Microsoft/microsoft-pdb>`__ for
further guidance.

- **PublicStreamIndex** - The index of the :doc:`Public Symbol Stream <PublicStream>`,
  which contains CodeView symbol records for all public symbols.  Actual records
  are stored in the symbol record stream, and are referenced from this stream.

- **PdbDllVersion** - The version number of ``mspdbXXXX.dll`` used to produce this
  PDB.  Note this obviously does not apply for LLVM as LLVM does not use ``mspdb.dll``.

- **SymRecordStream** - The stream containing all CodeView symbol records used
  by the program.  This is used for deduplication, so that many different
  compilands can refer to the same symbols without having to include the full record
  content inside of each module stream.

- **PdbDllRbld** - Unknown

- **MFCTypeServerIndex** - The index of the MFC type server in the
  :ref:`dbi_type_server_map_substream`.

- **Flags** - A bitfield with the following layout, containing various
  information about how the program was built:

.. code-block:: c++

  uint16_t WasIncrementallyLinked : 1;
  uint16_t ArePrivateSymbolsStripped : 1;
  uint16_t HasConflictingTypes : 1;
  uint16_t Reserved : 13;

The only one of these that is not self-explanatory is ``HasConflictingTypes``.
Although undocumented, ``link.exe`` contains a hidden flag ``/DEBUG:CTYPES``.
If it is passed to ``link.exe``, this field will be set.  Otherwise it will
not be set.  It is unclear what this flag does, although it seems to have
subtle implications on the algorithm used to look up type records.

- **Machine** - A value from the `CV_CPU_TYPE_e <https://msdn.microsoft.com/en-us/library/b2fc64ek.aspx>`__
  enumeration.  Common values are ``0x8664`` (x86-64) and ``0x14C`` (x86).

Immediately after the fixed-size DBI Stream header are ``7`` variable-length
`substreams`.  The following ``7`` fields of the DBI Stream header specify the
number of bytes of the corresponding substream.  Each substream's contents will
be described in detail :ref:`below <dbi_substreams>`.  The length of the entire
DBI Stream should equal ``64`` (the length of the header above) plus the value
of each of the following ``7`` fields.

- **ModInfoSize** - The length of the :ref:`dbi_mod_info_substream`.

- **SectionContributionSize** - The length of the :ref:`dbi_sec_contr_substream`.

- **SectionMapSize** - The length of the :ref:`dbi_section_map_substream`.

- **SourceInfoSize** - The length of the :ref:`dbi_file_info_substream`.

- **TypeServerMapSize** - The length of the :ref:`dbi_type_server_map_substream`.

- **OptionalDbgHeaderSize** - The length of the :ref:`dbi_optional_dbg_stream`.

- **ECSubstreamSize** - The length of the :ref:`dbi_ec_substream`.

.. _dbi_substreams:

Substreams
==========

.. _dbi_mod_info_substream:

Module Info Substream
^^^^^^^^^^^^^^^^^^^^^

Begins at offset ``0`` immediately after the :ref:`header <dbi_header>`.  The
module info substream is an array of variable-length records, each one
describing a single module (e.g. object file) linked into the program.  Each
record in the array has the format:

.. code-block:: c++

  struct ModInfo {
    uint32_t Unused1;
    struct SectionContribEntry {
      uint16_t Section;
      char Padding1[2];
      int32_t Offset;
      int32_t Size;
      uint32_t Characteristics;
      uint16_t ModuleIndex;
      char Padding2[2];
      uint32_t DataCrc;
      uint32_t RelocCrc;
    } SectionContr;
    uint16_t Flags;
    uint16_t ModuleSymStream;
    uint32_t SymByteSize;
    uint32_t C11ByteSize;
    uint32_t C13ByteSize;
    uint16_t SourceFileCount;
    char Padding[2];
    uint32_t Unused2;
    uint32_t SourceFileNameIndex;
    uint32_t PdbFilePathNameIndex;
    char ModuleName[];
    char ObjFileName[];
  };

- **SectionContr** - Describes the properties of the section in the final binary
  which contain the code and data from this module.

  ``SectionContr.Characteristics`` corresponds to the ``Characteristics`` field
  of the `IMAGE_SECTION_HEADER <https://msdn.microsoft.com/en-us/library/windows/desktop/ms680341(v=vs.85).aspx>`__
  structure.


- **Flags** - A bitfield with the following format:

.. code-block:: c++

  // ``true`` if this ModInfo has been written since reading the PDB.  This is
  // likely used to support incremental linking, so that the linker can decide
  // if it needs to commit changes to disk.
  uint16_t Dirty : 1;
  // ``true`` if EC information is present for this module. EC is presumed to
  // stand for "Edit & Continue", which LLVM does not support.  So this flag
  // will always be be false.
  uint16_t EC : 1;
  uint16_t Unused : 6;
  // Type Server Index for this module.  This is assumed to be related to /Zi,
  // but as LLVM treats /Zi as /Z7, this field will always be invalid for LLVM
  // generated PDBs.
  uint16_t TSM : 8;


- **ModuleSymStream** - The index of the stream that contains symbol information
  for this module.  This includes CodeView symbol information as well as source
  and line information.  If this field is -1, then no additional debug info will
  be present for this module (for example, this is what happens when you strip
  private symbols from a PDB).

- **SymByteSize** - The number of bytes of data from the stream identified by
  ``ModuleSymStream`` that represent CodeView symbol records.

- **C11ByteSize** - The number of bytes of data from the stream identified by
  ``ModuleSymStream`` that represent C11-style CodeView line information.

- **C13ByteSize** - The number of bytes of data from the stream identified by
  ``ModuleSymStream`` that represent C13-style CodeView line information.  At
  most one of ``C11ByteSize`` and ``C13ByteSize`` will be non-zero.  Modern PDBs
  always use C13 instead of C11.

- **SourceFileCount** - The number of source files that contributed to this
  module during compilation.

- **SourceFileNameIndex** - The offset in the names buffer of the primary
  translation unit used to build this module.  All PDB files observed to date
  always have this value equal to 0.

- **PdbFilePathNameIndex** - The offset in the names buffer of the PDB file
  containing this module's symbol information.  This has only been observed
  to be non-zero for the special ``* Linker *`` module.

- **ModuleName** - The module name.  This is usually either a full path to an
  object file (either directly passed to ``link.exe`` or from an archive) or
  a string of the form ``Import:<dll name>``.

- **ObjFileName** - The object file name.  In the case of an module that is
  linked directly passed to ``link.exe``, this is the same as **ModuleName**.
  In the case of a module that comes from an archive, this is usually the full
  path to the archive.

.. _dbi_sec_contr_substream:

Section Contribution Substream
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Begins at offset ``0`` immediately after the :ref:`dbi_mod_info_substream` ends,
and consumes ``Header->SectionContributionSize`` bytes.  This substream begins
with a single ``uint32_t`` which will be one of the following values:

.. code-block:: c++

  enum class SectionContrSubstreamVersion : uint32_t {
    Ver60 = 0xeffe0000 + 19970605,
    V2 = 0xeffe0000 + 20140516
  };

``Ver60`` is the only value which has been observed in a PDB so far.  Following
this is an array of fixed-length structures.  If the version is ``Ver60``,
it is an array of ``SectionContribEntry`` structures (this is the nested structure
from the ``ModInfo`` type.  If the version is ``V2``, it is an array of
``SectionContribEntry2`` structures, defined as follows:

.. code-block:: c++

  struct SectionContribEntry2 {
    SectionContribEntry SC;
    uint32_t ISectCoff;
  };

The purpose of the second field is not well understood.  The name implies that
is the index of the COFF section, but this also describes the existing field
``SectionContribEntry::Section``.


.. _dbi_section_map_substream:

Section Map Substream
^^^^^^^^^^^^^^^^^^^^^
Begins at offset ``0`` immediately after the :ref:`dbi_sec_contr_substream` ends,
and consumes ``Header->SectionMapSize`` bytes.  This substream begins with an ``4``
byte header followed by an array of fixed-length records.  The header and records
have the following layout:

.. code-block:: c++

  struct SectionMapHeader {
    uint16_t Count;    // Number of segment descriptors
    uint16_t LogCount; // Number of logical segment descriptors
  };

  struct SectionMapEntry {
    uint16_t Flags;         // See the SectionMapEntryFlags enum below.
    uint16_t Ovl;           // Logical overlay number
    uint16_t Group;         // Group index into descriptor array.
    uint16_t Frame;
    uint16_t SectionName;   // Byte index of segment / group name in string table, or 0xFFFF.
    uint16_t ClassName;     // Byte index of class in string table, or 0xFFFF.
    uint32_t Offset;        // Byte offset of the logical segment within physical segment.  If group is set in flags, this is the offset of the group.
    uint32_t SectionLength; // Byte count of the segment or group.
  };

  enum class SectionMapEntryFlags : uint16_t {
    Read = 1 << 0,              // Segment is readable.
    Write = 1 << 1,             // Segment is writable.
    Execute = 1 << 2,           // Segment is executable.
    AddressIs32Bit = 1 << 3,    // Descriptor describes a 32-bit linear address.
    IsSelector = 1 << 8,        // Frame represents a selector.
    IsAbsoluteAddress = 1 << 9, // Frame represents an absolute address.
    IsGroup = 1 << 10           // If set, descriptor represents a group.
  };

Many of these fields are not well understood, so will not be discussed further.

.. _dbi_file_info_substream:

File Info Substream
^^^^^^^^^^^^^^^^^^^
Begins at offset ``0`` immediately after the :ref:`dbi_section_map_substream` ends,
and consumes ``Header->SourceInfoSize`` bytes.  This substream defines the mapping
from module to the source files that contribute to that module.  Since multiple
modules can use the same source file (for example, a header file), this substream
uses a string table to store each unique file name only once, and then have each
module use offsets into the string table rather than embedding the string's value
directly.  The format of this substream is as follows:

.. code-block:: c++

  struct FileInfoSubstream {
    uint16_t NumModules;
    uint16_t NumSourceFiles;

    uint16_t ModIndices[NumModules];
    uint16_t ModFileCounts[NumModules];
    uint32_t FileNameOffsets[NumSourceFiles];
    char NamesBuffer[][NumSourceFiles];
  };

**NumModules** - The number of modules for which source file information is
contained within this substream.  Should match the corresponding value from the
ref:`dbi_header`.

**NumSourceFiles**: In theory this is supposed to contain the number of source
files for which this substream contains information.  But that would present a
problem in that the width of this field being ``16``-bits would prevent one from
having more than 64K source files in a program.  In early versions of the file
format, this seems to have been the case.  In order to support more than this, this
field of the is simply ignored, and computed dynamically by summing up the values of
the ``ModFileCounts`` array (discussed below).  In short, this value should be
ignored.

**ModIndices** - This array is present, but does not appear to be useful.

**ModFileCountArray** - An array of ``NumModules`` integers, each one containing
the number of source files which contribute to the module at the specified index.
While each individual module is limited to 64K contributing source files, the
union of all modules' source files may be greater than 64K.  The real number of
source files is thus computed by summing this array.  Note that summing this array
does not give the number of `unique` source files, only the total number of source
file contributions to modules.

**FileNameOffsets** - An array of **NumSourceFiles** integers (where **NumSourceFiles**
here refers to the 32-bit value obtained from summing **ModFileCountArray**), where
each integer is an offset into **NamesBuffer** pointing to a null terminated string.

**NamesBuffer** - An array of null terminated strings containing the actual source
file names.

.. _dbi_type_server_map_substream:

Type Server Map Substream
^^^^^^^^^^^^^^^^^^^^^^^^^
Begins at offset ``0`` immediately after the :ref:`dbi_file_info_substream`
ends, and consumes ``Header->TypeServerMapSize`` bytes.  Neither the purpose
nor the layout of this substream is understood, although it is assumed to
related somehow to the usage of ``/Zi`` and ``mspdbsrv.exe``.  This substream
will not be discussed further.

.. _dbi_ec_substream:

EC Substream
^^^^^^^^^^^^
Begins at offset ``0`` immediately after the
:ref:`dbi_type_server_map_substream` ends, and consumes
``Header->ECSubstreamSize`` bytes.  This is presumed to be related to Edit &
Continue support in MSVC.  LLVM does not support Edit & Continue, so this
stream will not be discussed further.

.. _dbi_optional_dbg_stream:

Optional Debug Header Stream
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Begins at offset ``0`` immediately after the :ref:`dbi_ec_substream` ends, and
consumes ``Header->OptionalDbgHeaderSize`` bytes.  This field is an array of
stream indices (e.g. ``uint16_t``'s), each of which identifies a stream
index in the larger MSF file which contains some additional debug information.
Each position of this array has a special meaning, allowing one to determine
what kind of debug information is at the referenced stream.  ``11`` indices
are currently understood, although it's possible there may be more.  The
layout of each stream generally corresponds exactly to a particular type
of debug data directory from the PE/COFF file.  The format of these fields
can be found in the `Microsoft PE/COFF Specification <https://www.microsoft.com/en-us/download/details.aspx?id=19509>`__.
If any of these fields is -1, it means the corresponding type of debug info is
not present in the PDB.

**FPO Data** - ``DbgStreamArray[0]``.  The data in the referenced stream is an
array of ``FPO_DATA`` structures.  This contains the relocated contents of
any ``.debug$F`` section from any of the linker inputs.

**Exception Data** - ``DbgStreamArray[1]``.  The data in the referenced stream
is a debug data directory of type ``IMAGE_DEBUG_TYPE_EXCEPTION``.

**Fixup Data** - ``DbgStreamArray[2]``.  The data in the referenced stream is a
debug data directory of type ``IMAGE_DEBUG_TYPE_FIXUP``.

**Omap To Src Data** - ``DbgStreamArray[3]``.  The data in the referenced stream
is a debug data directory of type ``IMAGE_DEBUG_TYPE_OMAP_TO_SRC``.  This
is used for mapping addresses between instrumented and uninstrumented code.

**Omap From Src Data** - ``DbgStreamArray[4]``.  The data in the referenced stream
is a debug data directory of type ``IMAGE_DEBUG_TYPE_OMAP_FROM_SRC``.  This
is used for mapping addresses between instrumented and uninstrumented code.

**Section Header Data** - ``DbgStreamArray[5]``.  A dump of all section headers from
the original executable.

**Token / RID Map** - ``DbgStreamArray[6]``.  The layout of this stream is not
understood, but it is assumed to be a mapping from ``CLR Token`` to
``CLR Record ID``.  Refer to `ECMA 335 <http://www.ecma-international.org/publications/standards/Ecma-335.htm>`__
for more information.

**Xdata** - ``DbgStreamArray[7]``.  A copy of the ``.xdata`` section from the
executable.

**Pdata** - ``DbgStreamArray[8]``. This is assumed to be a copy of the ``.pdata``
section from the executable, but that would make it identical to
``DbgStreamArray[1]``.  The difference between these two indices is not well
understood.

**New FPO Data** - ``DbgStreamArray[9]``.  The data in the referenced stream is a
debug data directory of type ``IMAGE_DEBUG_TYPE_FPO``.  Note that this is different
from ``DbgStreamArray[0]`` in that ``.debug$F`` sections are only emitted by MASM.
Thus, it is possible for both to appear in the same PDB if both MASM object files
and cl object files are linked into the same program.

**Original Section Header Data** - ``DbgStreamArray[10]``.  Similar to
``DbgStreamArray[5]``, but contains the section headers before any binary translation
has been performed.  This can be used in conjunction with ``DebugStreamArray[3]``
and ``DbgStreamArray[4]`` to map instrumented and uninstrumented addresses.