PEReader does not throw BadImageFormatException for some invalid PE files

[FiniteReality]

Description

Creating a PEReader for an invalid file type, such as an ELF can sometimes lead to a situation where the PEReader thinks the file is valid and it will return garbage data, causing errors to occur downstream.

Attached is a zipfile containing an ELF which reproduces the issue, trimmed using dd to reduce its filesize while still reproducing the issue.

Here's the code I'm running:

using System.Reflection.PortableExecutable;

var reader = new PEReader(File.OpenRead("path/to/invalid.so"));

Console.WriteLine(reader.HasMetadata);

Running the same code on a different ELF file throws a BadImageFormatException, as expected.

Configuration

.NET SDK/Runtime info

$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   5.0.102
 Commit:    71365b4d42

Runtime Environment:
 OS Name:     debian
 OS Version:  
 OS Platform: Linux
 RID:         debian-x64
 Base Path:   /usr/share/dotnet/sdk/5.0.102/

Host (useful for support):
  Version: 5.0.2
  Commit:  cb5f173b96

.NET SDKs installed:
  3.1.405 [/usr/share/dotnet/sdk]
  5.0.102 [/usr/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 3.1.11 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.2 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 3.1.11 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.2 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

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