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Prefer DOTNET_ROOT over directory traversal when finding muxer #53405

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Prefer DOTNET_ROOT over directory traversal when finding muxer #53405

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62 changes: 37 additions & 25 deletions src/Cli/Microsoft.DotNet.Cli.Utils/Muxer.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -38,46 +38,58 @@ public Muxer()
{
string muxerFileName = MuxerName + Constants.ExeSuffix;

// Most scenarios are running dotnet.dll as the app
// Root directory with muxer should be two above app base: <root>/sdk/<version>
string? rootPath = Path.GetDirectoryName(Path.GetDirectoryName(AppContext.BaseDirectory.TrimEnd(Path.DirectorySeparatorChar)));
if (rootPath is not null)
// Check environment variables first to allow package managers and
// other tools to explicitly set the dotnet root. This is needed
// when the SDK is installed via symlinks (e.g. Nix, Guix) where
// the directory-traversal heuristic below would resolve symlinks
// and find the wrong root directory.
string? dotnetHostPath = Environment.GetEnvironmentVariable("DOTNET_HOST_PATH");
if (dotnetHostPath is not null && File.Exists(dotnetHostPath))
{
string muxerPathMaybe = Path.Combine(rootPath, muxerFileName);
if (File.Exists(muxerPathMaybe))
_muxerPath = dotnetHostPath;
}
Comment on lines +46 to +50
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Copilot AI Mar 12, 2026

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The DOTNET_HOST_PATH branch only checks File.Exists, but doesn’t validate that the value actually points to the dotnet muxer (e.g., filename matches muxerFileName). If DOTNET_HOST_PATH is set to some other existing executable, this will set _muxerPath incorrectly and downstream ProcessStartInfo calls will try to run the wrong binary. Consider additionally validating Path.GetFileName(dotnetHostPath) against muxerFileName (and optionally requiring an absolute path) before accepting it.

This issue also appears on line 54 of the same file.

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if (_muxerPath is null)
{
var dotnetRoot = Environment.GetEnvironmentVariable("DOTNET_ROOT");
if (dotnetRoot is not null)
{
_muxerPath = muxerPathMaybe;
string rootMuxer = Path.Combine(dotnetRoot, muxerFileName);
if (File.Exists(rootMuxer))
{
_muxerPath = rootMuxer;
}
}
}

if (_muxerPath is null)
{
// Best-effort search for muxer.
// SDK sets DOTNET_HOST_PATH as absolute path to current dotnet executable
// Most scenarios are running dotnet.dll as the app
// Root directory with muxer should be two above app base: <root>/sdk/<version>
string? rootPath = Path.GetDirectoryName(Path.GetDirectoryName(AppContext.BaseDirectory.TrimEnd(Path.DirectorySeparatorChar)));
if (rootPath is not null)
{
string muxerPathMaybe = Path.Combine(rootPath, muxerFileName);
if (File.Exists(muxerPathMaybe))
{
_muxerPath = muxerPathMaybe;
}
}
}

Comment on lines +41 to +79
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Copilot AI Mar 12, 2026

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The muxer discovery behavior changed materially (env var precedence, and avoiding fallback to non-dotnet process paths). There are unit tests for this assembly, but none covering Muxer resolution. Adding tests that set DOTNET_HOST_PATH / DOTNET_ROOT to temp locations (and verifying the chosen path) would help prevent regressions, especially for the symlinked-root scenario described in the PR.

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if (_muxerPath is null)
{
// Last resort: if the current process is the dotnet muxer itself, use its path.
#if NET6_0_OR_GREATER
string? processPath = Environment.ProcessPath;
#else
string processPath = Process.GetCurrentProcess().MainModule.FileName;
#endif

// The current process should be dotnet in most normal scenarios except when dotnet.dll is loaded in a custom host like the testhost.
// Use GetFileName (not GetFileNameWithoutExtension) to avoid false matches with dotnet-prefixed names like "dotnet.Tests".
if (processPath is not null && !Path.GetFileName(processPath).Equals(muxerFileName, StringComparison.OrdinalIgnoreCase))
if (processPath is not null && Path.GetFileName(processPath).Equals(muxerFileName, StringComparison.OrdinalIgnoreCase))
{
// SDK sets DOTNET_HOST_PATH as absolute path to current dotnet executable
processPath = Environment.GetEnvironmentVariable("DOTNET_HOST_PATH");
if (processPath is null)
{
// fallback to DOTNET_ROOT which typically holds some dotnet executable
var root = Environment.GetEnvironmentVariable("DOTNET_ROOT");
if (root is not null)
{
processPath = Path.Combine(root, muxerFileName);
}
}
_muxerPath = processPath;
}

_muxerPath = processPath;
}
}

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