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[FileStream] handle UNC and device paths #54483

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Jun 23, 2021
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[FileStream] handle UNC and device paths #54483

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48e8e5f
handle UNC and device paths
adamsitnik Jun 21, 2021
c763c89
stop using NtCreateFile as there is no public and reliable way of map…
adamsitnik Jun 22, 2021
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2 changes: 2 additions & 0 deletions src/libraries/Common/tests/Common.Tests.csproj
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,8 @@
<Compile Include="$(CommonPath)System\Net\StreamBuffer.cs" Link="Common\System\Net\StreamBuffer.cs" />
</ItemGroup>
<ItemGroup Condition="'$(TargetsWindows)'=='true'">
<Compile Include="$(CommonPath)Interop\Windows\Interop.UNICODE_STRING.cs"
Link="Common\Interop\Windows\Interop.UNICODE_STRING.cs" />
<Compile Include="$(CoreLibSharedDir)System\IO\PathInternal.Windows.cs"
Link="System\IO\PathInternal.Windows.cs" />
<Compile Include="$(CommonPath)Interop\Windows\Interop.Libraries.cs"
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43 changes: 43 additions & 0 deletions src/libraries/Common/tests/Tests/System/IO/PathInternal.Windows.Tests.cs
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Expand Up @@ -3,6 +3,9 @@

using System;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using Xunit;

namespace Tests.System.IO
Expand Down Expand Up @@ -257,5 +260,45 @@ public void GetRootLengthDevice(string path, int length)
Assert.Equal(length + PathInternal.ExtendedPathPrefix.Length, PathInternal.GetRootLength(@"\\?\" + path));
Assert.Equal(length + PathInternal.ExtendedPathPrefix.Length, PathInternal.GetRootLength(@"\\.\" + path));
}

public static TheoryData<string, string> DosToNtPathTest_Data => new TheoryData<string, string>
{
{ @"C:\tests\file.cs", @"\??\C:\tests\file.cs" }, // typical path
{ @"\\?\C:\tests\file.cs", @"\??\C:\tests\file.cs" }, // NtPath with \\?\ prefix
{ @"\\.\device\file.cs", @"\??\device\file.cs" }, // device path with \\.\ prefix
{ @"\\server\file.cs", @"\??\UNC\server\file.cs" }, // UNC path with \\ prefix
{ @"\\?\UNC\server\file", @"\??\UNC\server\file" }, // extended UNC prefix
{ @"\??\C:\tests\file.cs", @"\??\C:\tests\file.cs" }, // NtPath with \??\ prefix (no changes required)
{ @"C:\", @"\??\C:\" }, // a short path
{ @"\\s", @"\??\UNC\s" }, // short UNC path
{ @"\\s\", @"\??\UNC\s\" }, // short UNC path with trailing
{ $@"C:\{string.Join("\\", Enumerable.Repeat("a", PathInternal.MaxShortPath + 1))}", $@"\??\C:\{string.Join("\\", Enumerable.Repeat("a", PathInternal.MaxShortPath + 1))}"}, // long path
};

[Theory, MemberData(nameof(DosToNtPathTest_Data))]
public void DosToNtPathTest(string path, string expected)
{
// first of all, we use an internal Windows API to ensure that expected value is valid
if (path.Length < PathInternal.MaxShortPath) // RtlDosPathNameToRelativeNtPathName_U_WithStatus does not support long paths
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{
RtlDosPathNameToRelativeNtPathName_U_WithStatus(path, out Interop.UNICODE_STRING ntFileName, out IntPtr _, IntPtr.Zero);
try
{
Assert.Equal(expected, Marshal.PtrToStringUni(ntFileName.Buffer));
}
finally
{
Marshal.ZeroFreeGlobalAllocUnicode(ntFileName.Buffer);
}
}

// after that, we test our implementation
var vsb = new ValueStringBuilder(stackalloc char[PathInternal.MaxShortPath]);
PathInternal.DosToNtPath(path, ref vsb);
Assert.Equal(expected, vsb.ToString());

[DllImport(Interop.Libraries.NtDll, CharSet = CharSet.Unicode)]
static extern int RtlDosPathNameToRelativeNtPathName_U_WithStatus(string DosFileName, out Interop.UNICODE_STRING NtFileName, out IntPtr FilePart, IntPtr RelativeName);
}
}
}
27 changes: 4 additions & 23 deletions ...ibraries/System.Private.CoreLib/src/Microsoft/Win32/SafeHandles/SafeFileHandle.Windows.cs
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Expand Up @@ -56,31 +56,12 @@ internal static unsafe SafeFileHandle Open(string fullPath, FileMode mode, FileA

private static IntPtr NtCreateFile(string fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, long preallocationSize)
{
uint ntStatus;
IntPtr fileHandle;
var vsb = new ValueStringBuilder(stackalloc char[PathInternal.MaxShortPath]);

const string MandatoryNtPrefix = @"\??\";
if (fullPath.StartsWith(MandatoryNtPrefix, StringComparison.Ordinal))
{
(ntStatus, fileHandle) = Interop.NtDll.NtCreateFile(fullPath, mode, access, share, options, preallocationSize);
}
else
{
var vsb = new ValueStringBuilder(stackalloc char[256]);
vsb.Append(MandatoryNtPrefix);
PathInternal.DosToNtPath(fullPath, ref vsb);

if (fullPath.StartsWith(@"\\?\", StringComparison.Ordinal)) // NtCreateFile does not support "\\?\" prefix, only "\??\"
{
vsb.Append(fullPath.AsSpan(4));
}
else
{
vsb.Append(fullPath);
}

(ntStatus, fileHandle) = Interop.NtDll.NtCreateFile(vsb.AsSpan(), mode, access, share, options, preallocationSize);
vsb.Dispose();
}
(uint ntStatus, IntPtr fileHandle) = Interop.NtDll.NtCreateFile(vsb.AsSpan(), mode, access, share, options, preallocationSize);
vsb.Dispose();

switch (ntStatus)
{
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30 changes: 30 additions & 0 deletions src/libraries/System.Private.CoreLib/src/System/IO/PathInternal.Windows.cs
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Expand Up @@ -47,6 +47,7 @@ internal static partial class PathInternal
internal const string DirectorySeparatorCharAsString = "\\";

internal const string ExtendedPathPrefix = @"\\?\";
internal const string NtPrefix = @"\??\";
internal const string UncPathPrefix = @"\\";
internal const string UncExtendedPrefixToInsert = @"?\UNC\";
internal const string UncExtendedPathPrefix = @"\\?\UNC\";
Expand Down Expand Up @@ -409,5 +410,34 @@ internal static bool IsEffectivelyEmpty(ReadOnlySpan<char> path)
}
return true;
}

// this method works only for `fullPath` returned by Path.GetFullPath
// currently we don't have interest in supporting relative paths
internal static void DosToNtPath(ReadOnlySpan<char> fullPath, ref ValueStringBuilder vsb)
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I assume for the opposite, translate NT to DOS, a helper like this should be needed, right?
For context #54253 (comment) , a Symlinks API uses DeviceIoControl which returns an NT path that I need to translate to DOS.

This is my silly attempt on doing it:
https://github.com/dotnet/runtime/blob/908530a70613af1172138e48d041d6c5d710d866/src/libraries/System.Private.CoreLib/src/System/IO/FileSystem.Windows.cs#L513-L518
Also, I think I mixed the names (DOS and NT) in the comments.

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@jozkee You could use RtlNtPathNameToDosPathName, but the problem is that it's internal and not documented, so we should not be using that.

The mapping that you have pointed to seems to be missing a few translations:

  • \??\UNC\ to \\ (files located on a remote machine)
  • \??\ to \\.\ for devices like names pipes: \\.\pipe\$pipeName

But the question is: are they valid in this context? Can someone create a link to a named pipe or a file located on a network share?

FWIW the best doc about the paths I've found so far: https://googleprojectzero.blogspot.com/2016/02/the-definitive-guide-on-win32-to-nt.html

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Symbolic links to files on network shares are possible; fsutil behavior set symlinkevaluation can enable or disable them. I don't know about symbolic links to named pipes.

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The mapping that you have pointed to seems to be missing a few translations:

The first example you pointed should still work, as far as I know: Converting \??\UNC\ to \\?\UNC, should be equivalent. The difference is that you do not want to pass a path prefixed with \??\ to the user, and should always translate it to \\?\.

Regarding the second example: Have you seen cases where Windows gives you a path prefixed with \??\ but you were expecting it to start with \\.\? And how would you know this?

{
vsb.Append(NtPrefix);

if (fullPath.Length >= 3 && fullPath[0] == '\\' && fullPath[1] == '\\')
{
// \\.\ (Device) or \\?\ (NtPath)
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if (fullPath.Length >= 4 && fullPath[3] == '\\' && (fullPath[2] == '.' || fullPath[2] == '?'))
{
vsb.Append(fullPath.Slice(NtPrefix.Length));
}
else // \\ (UNC)
{
vsb.Append(@"UNC\");
vsb.Append(fullPath.Slice(2));
}
}
else if (fullPath.Length >= 4 && fullPath[0] == '\\' && fullPath[1] == '?' && fullPath[2] == '?' && fullPath[3] == '\\') // \??\
{
vsb.Append(fullPath.Slice(NtPrefix.Length));
}
else
{
vsb.Append(fullPath);
}
}
}
}