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Fix ResolveLinkTarget for intermediate symlinks #132042

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Fix ResolveLinkTarget for intermediate symlinks #132042

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101 changes: 83 additions & 18 deletions src/libraries/System.Private.CoreLib/src/System/IO/FileSystem.Unix.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -697,26 +697,15 @@ internal static void CreateHardLink(string path, string pathToTarget)
}
else
{
string? current = linkTarget;
int visitCount = 1;

while (current != null)
{
if (visitCount > MaxFollowedLinks)
{
sb.Dispose();
// We went over the limit and couldn't reach the final target
throw new IOException(SR.Format(SR.IO_TooManySymbolicLinkLevels, linkPath));
}

GetLinkTargetFullPath(ref sb, current);
current = Interop.Sys.ReadLink(sb.AsSpan());
visitCount++;
}
sb.Dispose();
linkTarget = ResolveFinalTarget(linkPath, linkTarget);
}

Debug.Assert(sb.Length > 0);
linkTarget = sb.ToString(); // ToString disposes
if (!returnFinalTarget)
{
Debug.Assert(sb.Length > 0);
linkTarget = sb.ToString(); // ToString disposes
}

return isDirectory ?
new DirectoryInfo(linkTarget) :
Expand All @@ -738,6 +727,82 @@ static void GetLinkTargetFullPath(ref ValueStringBuilder sb, ReadOnlySpan<char>
}
sb.Append(linkTarget);
}

static string ResolveFinalTarget(string linkPath, string linkTarget)
{
int visitCount = 1;
string currentPath = Path.GetFullPath(linkPath);
string[] pendingComponents = new string[8];
int pendingComponentCount = 0;

// Resolve relative targets one component at a time because an intermediate component may itself be a link.
FollowLinkTarget(linkTarget);

while (pendingComponentCount > 0)
{
string component = pendingComponents[--pendingComponentCount];
pendingComponents[pendingComponentCount] = null!;

if (component == ".")
{
continue;
}

if (component == "..")
{
currentPath = Path.GetDirectoryName(currentPath) ?? currentPath;
continue;
}

currentPath = Path.Join(currentPath, component);
string? nestedTarget = Interop.Sys.ReadLink(currentPath);
if (nestedTarget != null)
{
ThrowIfTooManyLinks();
FollowLinkTarget(nestedTarget);
}
}

return currentPath;

void FollowLinkTarget(string target)
{
// Preserve the lexical path of absolute targets while following a link at that exact path.
while (!PathInternal.IsPartiallyQualified(target))
{
currentPath = target;
string? nextTarget = Interop.Sys.ReadLink(currentPath);
if (nextTarget == null)
{
return;
}

ThrowIfTooManyLinks();
target = nextTarget;
}

currentPath = Path.GetDirectoryName(currentPath)!;
string[] components = target.Split(PathInternal.DirectorySeparatorChar, StringSplitOptions.RemoveEmptyEntries);
if (pendingComponentCount + components.Length > pendingComponents.Length)
{
Array.Resize(ref pendingComponents, Math.Max(pendingComponentCount + components.Length, pendingComponents.Length * 2));
}

for (int i = components.Length - 1; i >= 0; i--)
{
pendingComponents[pendingComponentCount++] = components[i];
}
}

void ThrowIfTooManyLinks()
{
visitCount++;
if (visitCount > MaxFollowedLinks)
{
throw new IOException(SR.Format(SR.IO_TooManySymbolicLinkLevels, linkPath));
}
}
}
}
}
}
45 changes: 45 additions & 0 deletions ...ntime/tests/System.IO.FileSystem.Tests/Base/SymbolicLinks/BaseSymbolicLinks.FileSystem.cs
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Expand Up @@ -262,6 +262,51 @@ public void ResolveLinkTarget_ReturnFinalTarget_Relative_WithRedundantSegments()
filePath: filePath);
}

[Theory]
[InlineData(false)]
[InlineData(true)]
[PlatformSpecific(TestPlatforms.AnyUnix)]
public void ResolveLinkTarget_ReturnFinalTarget_RelativeIntermediateDirectoryLink(bool targetExists)
{
string rootPath = GetRandomDirPath();
Directory.CreateDirectory(rootPath);

const string versionDirectoryName = "..2025_08_14_07_17_19.1395403829";
string versionDirectoryPath = Path.Join(rootPath, versionDirectoryName);
Directory.CreateDirectory(versionDirectoryPath);

string targetName = IsDirectoryTest ? "target-directory" : "target-file";
string targetPath = Path.Join(versionDirectoryPath, targetName);
if (targetExists)
{
CreateFileOrDirectory(targetPath);
}

Directory.CreateSymbolicLink(Path.Join(rootPath, "..data"), versionDirectoryName);

string linkPath = Path.Join(rootPath, "link");
CreateSymbolicLink(linkPath, Path.Join("..data", targetName));

FileSystemInfo targetInfo = ResolveLinkTarget(linkPath, returnFinalTarget: true);
Assert.Equal(targetPath, targetInfo.FullName);
Assert.Equal(targetExists, targetInfo.Exists);
}

[Fact]
[PlatformSpecific(TestPlatforms.AnyUnix)]
public void ResolveLinkTarget_ReturnFinalTarget_IntermediateDirectoryLinkCycle_Throws()
{
string rootPath = GetRandomDirPath();
Directory.CreateDirectory(rootPath);

Directory.CreateSymbolicLink(Path.Join(rootPath, "cycle"), "cycle");

string linkPath = Path.Join(rootPath, "link");
CreateSymbolicLink(linkPath, Path.Join("cycle", "target"));

Assert.Throws<IOException>(() => ResolveLinkTarget(linkPath, returnFinalTarget: true));
}

[Theory]
[InlineData(1, false)]
[InlineData(10, false)]
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