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Improve FAT chain loop detection
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Improve loop detection time, but limit memory usage and HashSet
operations with binary exponential backoff.

Fixes: #242
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@jeremy-visionaid
jeremy-visionaid committed Nov 20, 2024
1 parent 6d27a94 commit 7a80c56
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Showing 5 changed files with 31 additions and 74 deletions.
Binary file added OpenMcdf.Tests/FatChainLoop_v3.cfs
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74 changes: 1 addition & 73 deletions OpenMcdf.Tests/OpenMcdf.Tests.csproj
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Expand Up @@ -36,79 +36,7 @@
</ItemGroup>

<ItemGroup>
<None Update="MultipleStorage.cfs">
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8 changes: 8 additions & 0 deletions OpenMcdf.Tests/StorageTests.cs
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Expand Up @@ -37,6 +37,14 @@ public void OpenStorage(string fileName)
Assert.AreEqual("MyStorage", storage.EntryInfo.Name);
}

[TestMethod]
[DataRow("FatChainLoop_v3.cfs")]
public void FatChainLoop(string fileName)
{
using var rootStorage = RootStorage.OpenRead(fileName);
Assert.ThrowsException<FileFormatException>(() => rootStorage.OpenStorage("Anything"));
}

[TestMethod]
[DataRow(Version.V3, 0)]
[DataRow(Version.V3, 1)]
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17 changes: 16 additions & 1 deletion OpenMcdf/FatChainEnumerator.cs
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@@ -1,6 +1,7 @@
using System.Collections;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Numerics;

namespace OpenMcdf;

Expand All @@ -11,6 +12,7 @@ internal sealed class FatChainEnumerator : IEnumerator<uint>
{
private readonly Fat fat;
private readonly FatEnumerator fatEnumerator;
readonly HashSet<uint> visited = new();
private uint startId;
private bool start = true;
private uint index = uint.MaxValue;
Expand Down Expand Up @@ -63,6 +65,7 @@ public bool MoveNext()
index = 0;
current = startId;
start = false;
visited.Add(current);
return true;
}

Expand All @@ -83,9 +86,20 @@ public bool MoveNext()
// If the index is greater than the maximum, then the chain must contain a loop
index = uint.MaxValue;
current = uint.MaxValue;
throw new FileFormatException("FAT sector chain is corrupt.");
throw new FileFormatException("FAT chain index is greater than the sector count.");
}

if (visited.Contains(value))
{
// If the sector has already been visited, then the chain must contain a loop
index = uint.MaxValue;
current = uint.MaxValue;
throw new FileFormatException("FAT chain contains a loop.");
}

if (BitOperations.IsPow2(index))
visited.Add(value);

current = value;
return true;
}
Expand Down Expand Up @@ -232,6 +246,7 @@ public void Reset(uint startSectorId)
start = true;
index = uint.MaxValue;
current = uint.MaxValue;
visited.Clear();
}

[ExcludeFromCodeCoverage]
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6 changes: 6 additions & 0 deletions OpenMcdf/System/BitOperations.cs
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@@ -0,0 +1,6 @@
namespace System.Numerics;

internal static class BitOperations
{
public static bool IsPow2(uint value) => (value & (value - 1)) == 0 && value != 0;
}

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