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[cdac] Synthesize a valid ECMA-335 image for read/write metadata inst…
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…ead of providing a separate metadata reader (dotnet#106164)

* Synthesize a valid ECMA-335 image for read/write metadata instead of providing a separate metadata reader

* Remove copies

* Add a comment

* Create EcmaMetadata contract for handling all metadata retrieval and remove metadata handling completely from the Loader contract.

* Add EcmaMetadata contract to CoreCLR's list of implemented contracts

---------

Co-authored-by: Elinor Fung <[email protected]>
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@jkoritzinsky @elinor-fung
jkoritzinsky and elinor-fung authored Aug 14, 2024
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340 changes: 340 additions & 0 deletions docs/design/datacontracts/EcmaMetadata.md
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# Contract EcmaMetadata

This contract provides methods to get a view of the ECMA-335 metadata for a given module.

## APIs of contract

```csharp
TargetSpan GetReadOnlyMetadataAddress(ModuleHandle handle);
System.Reflection.Metadata.MetadataReader? GetMetadata(ModuleHandle handle);
```

Types from other contracts:

| Type | Contract |
|------|----------|
| ModuleHandle | [Loader](./Loader.md#apis-of-contract) |

## Version 1


Data descriptors used:
| Data Descriptor Name | Field | Meaning |
| --- | --- | --- |
| `Module` | `Base` | Pointer to start of PE file in memory |
| `Module` | `DynamicMetadata` | Pointer to saved metadata for reflection emit modules |
| `Module` | `FieldDefToDescMap` | Mapping table |
| `DynamicMetadata` | `Size` | Size of the dynamic metadata blob (as a 32bit uint) |
| `DynamicMetadata` | `Data` | Start of dynamic metadata data array |


```csharp
using System.IO;
using System.Reflection.Metadata;
using System.Runtime.InteropServices;

TargetSpan GetReadOnlyMetadataAddress(ModuleHandle handle)
{
TargetPointer baseAddress = Target.ReadPointer(handle.Address + /* Module::Base offset */);
if (baseAddress == TargetPointer.Null)
{
return default;
}

// Read CLR header per https://learn.microsoft.com/windows/win32/debug/pe-format
ulong clrHeaderRVA = ...

// Read Metadata per ECMA-335 II.25.3.3 CLI Header
ulong metadataDirectoryAddress = baseAddress + clrHeaderRva + /* offset to Metadata */
int rva = Target.Read<int>(metadataDirectoryAddress);
ulong size = Target.Read<int>(metadataDirectoryAddress + sizeof(int));
return new(baseAddress + rva, size);
}

MetadataReader? GetMetadata(ModuleHandle handle)
{
AvailableMetadataType type = GetAvailableMetadataType(handle);

switch (type)
{
case AvailableMetadataType.None:
return null;
case AvailableMetadataType.ReadOnly:
{
TargetSpan address = GetReadOnlyMetadataAddress(handle);
byte[] data = new byte[address.Size];
_target.ReadBuffer(address.Address, data);
return MetadataReaderProvider.FromMetadataImage(ImmutableCollectionsMarshal.AsImmutableArray(data)).GetMetadataReader();
}
case AvailableMetadataType.ReadWriteSavedCopy:
{
TargetSpan address = GetReadWriteSavedMetadataAddress(handle);
byte[] data = new byte[address.Size];
_target.ReadBuffer(address.Address, data);
return MetadataReaderProvider.FromMetadataImage(ImmutableCollectionsMarshal.AsImmutableArray(data)).GetMetadataReader();
}
case AvailableMetadataType.ReadWrite:
{
var targetEcmaMetadata = GetReadWriteMetadata(handle);

// From the multiple different target spans, we need to build a single
// contiguous ECMA-335 metadata blob.
BlobBuilder builder = new BlobBuilder();
builder.WriteUInt32(0x424A5342);

// major version
builder.WriteUInt16(1);

// minor version
builder.WriteUInt16(1);

// reserved
builder.WriteUInt32(0);

string version = targetEcmaMetadata.Schema.MetadataVersion;
builder.WriteInt32(AlignUp(version.Length, 4));
Write4ByteAlignedString(builder, version);

// reserved
builder.WriteUInt16(0);

// number of streams
ushort numStreams = 5; // #Strings, #US, #Blob, #GUID, #~ (metadata)
if (targetEcmaMetadata.Schema.VariableSizedColumnsAreAll4BytesLong)
{
// We direct MetadataReader to use 4-byte encoding for all variable-sized columns
// by providing the marker stream for a "minimal delta" image.
numStreams++;
}
builder.WriteUInt16(numStreams);

// Write Stream headers
if (targetEcmaMetadata.Schema.VariableSizedColumnsAreAll4BytesLong)
{
// Write the #JTD stream to indicate that all variable-sized columns are 4 bytes long.
WriteStreamHeader(builder, "#JTD", 0).WriteInt32(builder.Count);
}

BlobWriter stringsOffset = WriteStreamHeader(builder, "#Strings", (int)AlignUp(targetEcmaMetadata.StringHeap.Size, 4ul));
BlobWriter blobOffset = WriteStreamHeader(builder, "#Blob", (int)targetEcmaMetadata.BlobHeap.Size);
BlobWriter guidOffset = WriteStreamHeader(builder, "#GUID", (int)targetEcmaMetadata.GuidHeap.Size);
BlobWriter userStringOffset = WriteStreamHeader(builder, "#US", (int)targetEcmaMetadata.UserStringHeap.Size);

// We'll use the "uncompressed" tables stream name as the runtime may have created the *Ptr tables
// that are only present in the uncompressed tables stream.
BlobWriter tablesOffset = WriteStreamHeader(builder, "#-", 0);

// Write the heap-style Streams
stringsOffset.WriteInt32(builder.Count);
WriteTargetSpan(builder, targetEcmaMetadata.StringHeap);
for (ulong i = targetEcmaMetadata.StringHeap.Size; i < AlignUp(targetEcmaMetadata.StringHeap.Size, 4ul); i++)
{
builder.WriteByte(0);
}

blobOffset.WriteInt32(builder.Count);
WriteTargetSpan(builder, targetEcmaMetadata.BlobHeap);

guidOffset.WriteInt32(builder.Count);
WriteTargetSpan(builder, targetEcmaMetadata.GuidHeap);

userStringOffset.WriteInt32(builder.Count);
WriteTargetSpan(builder, targetEcmaMetadata.UserStringHeap);

// Write tables stream
tablesOffset.WriteInt32(builder.Count);

// Write tables stream header
builder.WriteInt32(0); // reserved
builder.WriteByte(2); // major version
builder.WriteByte(0); // minor version
uint heapSizes =
(targetEcmaMetadata.Schema.LargeStringHeap ? 1u << 0 : 0) |
(targetEcmaMetadata.Schema.LargeBlobHeap ? 1u << 1 : 0) |
(targetEcmaMetadata.Schema.LargeGuidHeap ? 1u << 2 : 0);

builder.WriteByte((byte)heapSizes);
builder.WriteByte(1); // reserved
ulong validTables = 0;
for (int i = 0; i < targetEcmaMetadata.Schema.RowCount.Length; i++)
{
if (targetEcmaMetadata.Schema.RowCount[i] != 0)
{
validTables |= 1ul << i;
}
}

ulong sortedTables = 0;
for (int i = 0; i < targetEcmaMetadata.Schema.IsSorted.Length; i++)
{
if (targetEcmaMetadata.Schema.IsSorted[i])
{
sortedTables |= 1ul << i;
}
}

builder.WriteUInt64(validTables);
builder.WriteUInt64(sortedTables);

foreach (int rowCount in targetEcmaMetadata.Schema.RowCount)
{
if (rowCount > 0)
{
builder.WriteInt32(rowCount);
}
}

// Write the tables
foreach (TargetSpan span in targetEcmaMetadata.Tables)
{
WriteTargetSpan(builder, span);
}

MemoryStream metadataStream = new MemoryStream();
builder.WriteContentTo(metadataStream);
return MetadataReaderProvider.FromMetadataStream(metadataStream).GetMetadataReader();

void WriteTargetSpan(BlobBuilder builder, TargetSpan span)
{
Blob blob = builder.ReserveBytes(checked((int)span.Size));
_target.ReadBuffer(span.Address, blob.GetBytes().AsSpan());
}

static BlobWriter WriteStreamHeader(BlobBuilder builder, string name, int size)
{
BlobWriter offset = new(builder.ReserveBytes(4));
builder.WriteInt32(size);
Write4ByteAlignedString(builder, name);
return offset;
}

static void Write4ByteAlignedString(BlobBuilder builder, string value)
{
int bufferStart = builder.Count;
builder.WriteUTF8(value);
builder.WriteByte(0);
int stringEnd = builder.Count;
for (int i = stringEnd; i < bufferStart + AlignUp(value.Length, 4); i++)
{
builder.WriteByte(0);
}
}
}
}
}
```

### Helper Methods

``` csharp
using System;
using System.Numerics;

struct EcmaMetadataSchema
{
public EcmaMetadataSchema(string metadataVersion, bool largeStringHeap, bool largeBlobHeap, bool largeGuidHeap, int[] rowCount, bool[] isSorted, bool variableSizedColumnsAre4BytesLong)
{
MetadataVersion = metadataVersion;
LargeStringHeap = largeStringHeap;
LargeBlobHeap = largeBlobHeap;
LargeGuidHeap = largeGuidHeap;

_rowCount = rowCount;
_isSorted = isSorted;

VariableSizedColumnsAreAll4BytesLong = variableSizedColumnsAre4BytesLong;
}

public readonly string MetadataVersion;

public readonly bool LargeStringHeap;
public readonly bool LargeBlobHeap;
public readonly bool LargeGuidHeap;

// Table data, these structures hold MetadataTable.Count entries
private readonly int[] _rowCount;
public readonly ReadOnlySpan<int> RowCount => _rowCount;

private readonly bool[] _isSorted;
public readonly ReadOnlySpan<bool> IsSorted => _isSorted;

// In certain scenarios the size of the tables is forced to be the maximum size
// Otherwise the size of columns should be computed based on RowSize/the various heap flags
public readonly bool VariableSizedColumnsAreAll4BytesLong;
}

class TargetEcmaMetadata
{
public TargetEcmaMetadata(EcmaMetadataSchema schema,
TargetSpan[] tables,
TargetSpan stringHeap,
TargetSpan userStringHeap,
TargetSpan blobHeap,
TargetSpan guidHeap)
{
Schema = schema;
_tables = tables;
StringHeap = stringHeap;
UserStringHeap = userStringHeap;
BlobHeap = blobHeap;
GuidHeap = guidHeap;
}

public EcmaMetadataSchema Schema { get; init; }

private TargetSpan[] _tables;
public ReadOnlySpan<TargetSpan> Tables => _tables;
public TargetSpan StringHeap { get; init; }
public TargetSpan UserStringHeap { get; init; }
public TargetSpan BlobHeap { get; init; }
public TargetSpan GuidHeap { get; init; }
}

[Flags]
enum AvailableMetadataType
{
None = 0,
ReadOnly = 1,
ReadWriteSavedCopy = 2,
ReadWrite = 4
}

AvailableMetadataType GetAvailableMetadataType(ModuleHandle handle)
{
Data.Module module = new Data.Module(Target, handle.Address);

AvailableMetadataType flags = AvailableMetadataType.None;

TargetPointer dynamicMetadata = Target.ReadPointer(handle.Address + /* Module::DynamicMetadata offset */);

if (dynamicMetadata != TargetPointer.Null)
flags |= AvailableMetadataType.ReadWriteSavedCopy;
else
flags |= AvailableMetadataType.ReadOnly;

return flags;
}

TargetSpan GetReadWriteSavedMetadataAddress(ModuleHandle handle)
{
Data.Module module = new Data.Module(Target, handle.Address);
TargetPointer dynamicMetadata = Target.ReadPointer(handle.Address + /* Module::DynamicMetadata offset */);

ulong size = Target.Read<uint>(handle.Address + /* DynamicMetadata::Size offset */);
TargetPointer result = handle.Address + /* DynamicMetadata::Data offset */;
return new(result, size);
}

TargetEcmaMetadata GetReadWriteMetadata(ModuleHandle handle)
{
// [cdac] TODO.
}

T AlignUp<T>(T input, T alignment)
where T : IBinaryInteger<T>
{
return input + (alignment - T.One) & ~(alignment - T.One);
}
```
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