[cDAC] DAC like entry point

[max-charlamb]

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[jkoritzinsky]

Can we use System.Reflection.PortableExecutable.PEReader to get to the export directory here?

Something like the following:

using Stream stream = new DataTargetStream(_dataTarget, _baseAddress);
using PEReader reader = new(stream);

var exportsDirectory = reader.PEHeaders.PEHeader.ExportTableDirectory;

if (!reader.PEHeaders.TryGetDirectoryOffset(exportsDirectory, out var offset))
{
    return false;
}

stream.Seek(offset, SeekOrigin.Begin);
_exportDir = new IMAGE_EXPORT_DIRECTORY(new BinaryReader(stream));

return true;

This would let you get out of most of the PE decoding.

[max-charlamb]

I saw the PEReader and based some of the code off of it.

My concern is that the PEReader does not have the signature checking semantics I was looking for.

Because the DAC only targets a single platform, it invoked the exact reader for the executable format.

As far as I can tell, using the ICLRDataTarget, we don't know which platform we are targeting. Unless I'm overlooking something, my plan is to run the PEDecoder, ELFDecoder, and MACHODecoder on each target which will be verified using the known signatures.

It looks like the PEDecoder assumes that the PE is a COFF file if the PE DOS magic isn't correct. In this case, I'd want to bail.

runtime/src/libraries/System.Reflection.Metadata/src/System/Reflection/PortableExecutable/PEHeaders.cs

Line 249 in 1d1bf92

private static void SkipDosHeader(ref PEBinaryReader reader, out bool isCOFFOnly)

[max-charlamb]

Looking at it again, if the file parses as COFFOnly, the PEHeader won't be available.

I can check if it's valid using that.

[max-charlamb]

Unfortunately, the PEReader is not safe to use on non-PE/COFF files. Even if the signatures fail to parse, the PEHeaders.cs will attempt to use the garbage COFFHeader to read an arbitrary number of sections potentially causing stream overrun.

runtime/src/libraries/System.Reflection.Metadata/src/System/Reflection/PortableExecutable/PEHeaders.cs

Lines 292 to 308 in 4dde471

	private ImmutableArray<SectionHeader> ReadSectionHeaders(ref PEBinaryReader reader)
	{
	int numberOfSections = _coffHeader.NumberOfSections;
	if (numberOfSections < 0)
	{
	throw new BadImageFormatException(SR.InvalidNumberOfSections);
	}
	var builder = ImmutableArray.CreateBuilder<SectionHeader>(numberOfSections);
	for (int i = 0; i < numberOfSections; i++)
	{
	builder.Add(new SectionHeader(ref reader));
	}
	return builder.MoveToImmutable();
	}

[jkoritzinsky]

Dang... Yeah that's not particularly helpful in this case.

[teo-tsirpanis]

Opened #112830 to add this fail-fast capability to PEReader; this is not the first time it was desired. In the meantime you can do a minimal signature validation yourself and create the PEReader afterwards:

using DataTargetStream stream = new(_dataTarget, _baseAddress);
using BinaryReader reader = new(stream, Encoding.UTF8, leaveOpen: true);

ushort dosMagic = reader.ReadUInt16();
if (dosMagic != 0x5A4D) // "MZ"
    return false;

// PE Header offset is at 0x3C in DOS header
reader.BaseStream.Seek(0x3C, SeekOrigin.Begin);
uint peSigOffset = reader.ReadUInt32();

// Read PE signature
reader.BaseStream.Seek(peSigOffset, SeekOrigin.Begin);
uint peSig = reader.ReadUInt32();
if (peSig != 0x00004550) // "PE00"
    return false;

using PEReader reader = new(stream);

var exportsDirectory = reader.PEHeaders.PEHeader!.ExportTableDirectory;

if (!reader.PEHeaders.TryGetDirectoryOffset(exportsDirectory, out var offset))
{
    return false;
}

stream.Seek(offset, SeekOrigin.Begin);
_exportDir = new IMAGE_EXPORT_DIRECTORY(reader);

return true;

[teo-tsirpanis]

You should also override the Read overload that takes a span.