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8d7a4db2d7
The information about the named stream map and feature codes was not present. This patch adds it. llvm-svn: 357788
155 lines
6.8 KiB
ReStructuredText
155 lines
6.8 KiB
ReStructuredText
========================================
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The PDB Info Stream (aka the PDB Stream)
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========================================
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.. contents::
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:local:
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.. _pdb_stream_header:
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Stream Header
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=============
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At offset 0 of the PDB Stream is a header with the following layout:
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.. code-block:: c++
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struct PdbStreamHeader {
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ulittle32_t Version;
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ulittle32_t Signature;
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ulittle32_t Age;
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Guid UniqueId;
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};
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- **Version** - A Value from the following enum:
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.. code-block:: c++
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enum class PdbStreamVersion : uint32_t {
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VC2 = 19941610,
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VC4 = 19950623,
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VC41 = 19950814,
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VC50 = 19960307,
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VC98 = 19970604,
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VC70Dep = 19990604,
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VC70 = 20000404,
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VC80 = 20030901,
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VC110 = 20091201,
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VC140 = 20140508,
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};
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While the meaning of this field appears to be obvious, in practice we have
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never observed a value other than ``VC70``, even with modern versions of
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the toolchain, and it is unclear why the other values exist. It is assumed
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that certain aspects of the PDB stream's layout, and perhaps even that of
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the other streams, will change if the value is something other than ``VC70``.
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- **Signature** - A 32-bit time-stamp generated with a call to ``time()`` at
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the time the PDB file is written. Note that due to the inherent uniqueness
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problems of using a timestamp with 1-second granularity, this field does not
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really serve its intended purpose, and as such is typically ignored in favor
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of the ``Guid`` field, described below.
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- **Age** - The number of times the PDB file has been written. This can be used
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along with ``Guid`` to match the PDB to its corresponding executable.
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- **Guid** - A 128-bit identifier guaranteed to be unique across space and time.
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In general, this can be thought of as the result of calling the Win32 API
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`UuidCreate <https://msdn.microsoft.com/en-us/library/windows/desktop/aa379205(v=vs.85).aspx>`__,
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although LLVM cannot rely on that, as it must work on non-Windows platforms.
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.. _pdb_named_stream_map:
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Named Stream Map
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================
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Following the header is a serialized hash table whose key type is a string, and
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whose value type is an integer. The existence of a mapping ``X -> Y`` means
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that the stream with the name ``X`` has stream index ``Y`` in the underlying MSF
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file. Note that not all streams are named (for example, the
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:doc:`TPI Stream <TpiStream>` has a fixed index and as such there is no need to
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look up its index by name). In practice, there are usually only a small number
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of named streams and these are enumerated in the table of streams in :doc:`index`.
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A corollary of this is if a stream does have a name (and as such is in the named
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stream map) then consulting the Named Stream Map is likely to be the only way to
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discover the stream's MSF stream index. Several important streams (such as the
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global string table, which is called ``/names``) can only be located this way, and
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so it is important to both produce and consume this correctly as tools will not
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function correctly without it.
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.. important::
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Some streams are located by fixed indices (e.g TPI Stream has index 2), but
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other streams are located by fixed names (e.g. the string table is called
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``/names``) and can only be located by consulting the Named Stream Map.
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The on-disk layout of the Named Stream Map consists of 2 components. The first is
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a buffer of string data prefixed by a 32-bit length. The second is a serialized
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hash table whose key and value types are both ``uint32_t``. The key is the offset
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of a null-terminated string in the string data buffer specifying the name of the
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stream, and the value is the MSF stream index of the stream with said name.
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Note that although the key is an integer, the hash function used to find the right
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bucket hashes the string at the corresponding offset in the string data buffer.
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The on-disk layout of the serialized hash table is described at :doc:`HashTable`.
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Note that the entire Named Stream Map is not length-prefixed, so the only way to
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get to the data following it is to de-serialize it in its entirety.
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.. _pdb_stream_features:
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PDB Feature Codes
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=================
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Following the Named Stream Map, and consuming all remaining bytes of the PDB
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Stream is a list of values from the following enumeration:
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.. code-block:: c++
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enum class PdbRaw_FeatureSig : uint32_t {
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VC110 = 20091201,
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VC140 = 20140508,
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NoTypeMerge = 0x4D544F4E,
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MinimalDebugInfo = 0x494E494D,
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};
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The meaning of these values is summarized by the following table:
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+------------------+-------------------------------------------------+
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| Flag | Meaning |
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+==================+=================================================+
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| VC110 | - No other features flags are present |
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| | - PDB contains an :doc:`IPI Stream <TpiStream>` |
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+------------------+-------------------------------------------------+
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| VC140 | - Other feature flags may be present |
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| | - PDB contains an :doc:`IPI Stream <TpiStream>` |
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+------------------+-------------------------------------------------+
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| NoTypeMerge | - Presumably duplicate types can appear in the |
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| | TPI Stream, although it's unclear why this |
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| | might happen. |
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+------------------+-------------------------------------------------+
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| MinimalDebugInfo | - Program was linked with /DEBUG:FASTLINK |
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| | - There is no TPI / IPI stream, all type info |
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| | is contained in the original object files. |
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+------------------+-------------------------------------------------+
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Matching a PDB to its executable
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================================
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The linker is responsible for writing both the PDB and the final executable, and
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as a result is the only entity capable of writing the information necessary to
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match the PDB to the executable.
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In order to accomplish this, the linker generates a guid for the PDB (or
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re-uses the existing guid if it is linking incrementally) and increments the Age
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field.
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The executable is a PE/COFF file, and part of a PE/COFF file is the presence of
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number of "directories". For our purposes here, we are interested in the "debug
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directory". The exact format of a debug directory is described by the
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`IMAGE_DEBUG_DIRECTORY structure <https://msdn.microsoft.com/en-us/library/windows/desktop/ms680307(v=vs.85).aspx>`__.
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For this particular case, the linker emits a debug directory of type
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``IMAGE_DEBUG_TYPE_CODEVIEW``. The format of this record is defined in
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``llvm/DebugInfo/CodeView/CVDebugRecord.h``, but it suffices to say here only
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that it includes the same ``Guid`` and ``Age`` fields. At runtime, a
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debugger or tool can scan the COFF executable image for the presence of
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a debug directory of the correct type and verify that the Guid and Age match.
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