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| using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.IO;
using System.Text;
using System.Runtime.InteropServices;
using Microsoft.Win32.SafeHandles;
namespace Projet
{
/// <summary>
/// Wraps calls to <c>AccessCheck</c> and <c>GetFileSecurity</c>.
/// </summary>
public static class FileAccessCheck
{
/// <summary>
/// Wraps a token handle (there is no public such class in .Net 2.0, not before .Net 4.6)
/// </summary>
class SafeTokenHandle : SafeHandleZeroOrMinusOneIsInvalid
{
/// <summary>Default constructor (mandated by P/Invoke).</summary>
public SafeTokenHandle() : base(true) { }
/// <summary>ReleaseHandle implementation, just calls <c>CloseHandle</c>.</summary>
protected override bool ReleaseHandle() { return NativeMethods.CloseHandle(handle); }
}
/// <summary>
/// Native method declarations.
/// </summary>
static class NativeMethods
{
//Note: Most of these declarations were taken from the .Net Framework reference source.
public const int OWNER_SECURITY_INFORMATION = 0x00000001;
public const int GROUP_SECURITY_INFORMATION = 0x00000002;
public const int DACL_SECURITY_INFORMATION = 0x00000004;
public const int SACL_SECURITY_INFORMATION = 0x00000008;
[DllImport("advapi32.dll", SetLastError=true, CharSet=CharSet.Unicode)]
public static extern int GetFileSecurity(string filename, int requestedInformation, byte[] securityDescriptor, int length, ref int lengthNeeded);
[DllImport("advapi32.dll", SetLastError=true)]
public static extern bool
AccessCheck(
[In] byte[] SecurityDescriptor,
[In] IntPtr ClientToken, //Changed from SafeCloseHandle
[In] int DesiredAccess,
[In] GENERIC_MAPPING GenericMapping,
byte[] PrivilegeSet, //Changed from PRIVILEGE_SET reference of dubious type (double pointer?)
ref uint PrivilegeSetLength,
[Out] out uint GrantedAccess,
[Out] out bool AccessStatus);
[StructLayout(LayoutKind.Sequential)]
public class GENERIC_MAPPING
{
internal uint genericRead = 0;
internal uint genericWrite = 0;
internal uint genericExecute = 0;
internal uint genericAll = 0;
}
//These on the other hand, come from the Windows headers
public const int ERROR_INSUFFICIENT_BUFFER = 122;
/// <summary>ReadControl and Synchronize, plus Read, ReadAttributes and ReadEA.</summary>
public const uint FILE_GENERIC_READ = 0x00120089u;
/// <summary>ReadControl and Synchronize, plus Write, Append, WriteAttributes and WriteEA.</summary>
public const uint FILE_GENERIC_WRITE = 0x00120116u;
/// <summary>ReadControl and Synchronize, plus ReadAttributes and Execute.</summary>
public const uint FILE_GENERIC_EXECUTE = 0x001200A0u;
/// <summary>All standard and file access rights.</summary>
public const uint FILE_ALL_ACCESS = 0x001F01FF;
/// <summary>The impersonation level for <c>DuplicateToken</c>.</summary>
public const int SecurityImpersonation = 2;
[DllImport("advapi32.dll", SetLastError=true)]
public static extern bool DuplicateToken(IntPtr token, int impersonationLevel, out SafeTokenHandle hNewToken);
[DllImport("kernel32.dll", SetLastError=true)]
public static extern bool CloseHandle(IntPtr h);
}
private static void ThrowWin32Exception(string contextMessage)
{
int hResult = Marshal.GetHRForLastWin32Error();
Exception ex = Marshal.GetExceptionForHR(hResult);
throw new Win32Exception(contextMessage + ex.Message, ex);
}
/// <summary>
/// Wraps call to <c>GetFileSecurity</c>.
/// </summary>
/// <param name="filePath">[in] Path to the file or directory to check.</param>
/// <returns>The security descriptor as an array of bytes (<c>GetFileSecurity</c> returns it in "self-relative format").</returns>
static byte[] GetSecurityDescriptor(string filePath)
{
int infoFlags = NativeMethods.DACL_SECURITY_INFORMATION|NativeMethods.OWNER_SECURITY_INFORMATION|NativeMethods.GROUP_SECURITY_INFORMATION;
int lengthNeeded = 0;
bool bRet = NativeMethods.GetFileSecurity(filePath, infoFlags, null, 0, ref lengthNeeded) != 0;
if(!bRet && Marshal.GetLastWin32Error()!=NativeMethods.ERROR_INSUFFICIENT_BUFFER)
ThrowWin32Exception("First call to GetFileSecurity() failed: ");
byte[] descriptor = new byte[lengthNeeded];
bRet = NativeMethods.GetFileSecurity(filePath, infoFlags, descriptor, descriptor.Length, ref lengthNeeded) != 0;
if(!bRet)
ThrowWin32Exception("Second call to GetFileSecurity() failed: ");
return descriptor;
}
/// <summary>
/// Wraps call to <c>DuplicateToken</c>, necessary because <c>AccessCheck</c> requires impersonation.
/// </summary>
/// <returns>Token handle for the impersonation token.</returns>
private static SafeTokenHandle GetImpersonationToken()
{
IntPtr token = System.Security.Principal.WindowsIdentity.GetCurrent().Token;
SafeTokenHandle dupToken;
bool bRet = NativeMethods.DuplicateToken(token, NativeMethods.SecurityImpersonation, out dupToken);
if(!bRet)
ThrowWin32Exception("DuplicateToken() failed: ");
return dupToken;
}
/// <summary>
/// Wraps call to <c>AccessCheck</c>.
/// </summary>
/// <param name="desiredAccess">[in] Access to check.</param>
/// <param name="descriptor">[in] Security descriptor to read.</param>
/// <returns><c>true</c> if all desired accesses are present.</returns>
static bool HasAccess(FileAccess desiredAccess, byte[] descriptor)
{
//Build access flags and GENERIC_MAPPING
uint accessFlags = 0x00000000u;
if((desiredAccess & FileAccess.Read)!=0) { accessFlags |= NativeMethods.FILE_GENERIC_READ; }
if((desiredAccess & FileAccess.Write)!=0) { accessFlags |= NativeMethods.FILE_GENERIC_WRITE; }
NativeMethods.GENERIC_MAPPING mapping = new NativeMethods.GENERIC_MAPPING();
mapping.genericAll = NativeMethods.FILE_ALL_ACCESS;
mapping.genericExecute = NativeMethods.FILE_GENERIC_EXECUTE;
mapping.genericRead = NativeMethods.FILE_GENERIC_READ;
mapping.genericWrite = NativeMethods.FILE_GENERIC_WRITE;
bool accessStatus;
//For some reason, AccessCheck() only works when impersonating.
using(SafeTokenHandle dupToken = GetImpersonationToken())
using(System.Security.Principal.WindowsImpersonationContext ctxt = System.Security.Principal.WindowsIdentity.Impersonate(dupToken.DangerousGetHandle()))
{
uint privilegeSetLength = 0;
uint grantedAccess;
bool bRet = NativeMethods.AccessCheck(descriptor, dupToken.DangerousGetHandle(), unchecked((int)accessFlags), mapping, null, ref privilegeSetLength, out grantedAccess, out accessStatus);
if(!bRet)
{
if(Marshal.GetLastWin32Error() != NativeMethods.ERROR_INSUFFICIENT_BUFFER)
ThrowWin32Exception("First call to AccessCheck() failed: ");
byte[] privilegeSet = new byte[privilegeSetLength];
bRet = NativeMethods.AccessCheck(descriptor, dupToken.DangerousGetHandle(), unchecked((int)accessFlags), mapping, privilegeSet, ref privilegeSetLength, out grantedAccess, out accessStatus);
if(!bRet)
ThrowWin32Exception("Second call to AccessCheck() failed: ");
}
}
return accessStatus;
}
/// <summary>
/// Checks access to a file or directory.
/// </summary>
/// <param name="filePath">[in] Path to the file or directory to check.</param>
/// <param name="desiredAccess">[in] Access to check.</param>
/// <param name="throwIfNotFound">[in] If set, will throw a <c>FileNotFoundException</c> if both <c>File.Exists</c> and <c>Directory.Exists</c> fail.</param>
/// <returns><c>true</c> if the file was found and access was granted.</returns>
public static bool CheckAccess(string filePath, FileAccess desiredAccess, bool throwIfNotFound)
{
bool fileExists = File.Exists(filePath);
bool dirExists = Directory.Exists(filePath);
bool exists = fileExists || dirExists;
if(!exists)
{
if(throwIfNotFound)
throw new FileNotFoundException(string.Format("The file or directory '{0}' could not be found or has no read access.", filePath), filePath);
return false;
}
GC.KeepAlive(fileExists);
GC.KeepAlive(dirExists);
byte[] securityDescriptor = GetSecurityDescriptor(filePath);
return HasAccess(desiredAccess, securityDescriptor);
}
/// <summary>
/// Performs a few checks to see if a file can be created:
/// The directory must exist, the file must be writable.
/// </summary>
/// <param name="filePath">(in] File path to check.</param>
/// <returns><c>true</c> if the file can be created/written to.</returns>
public static bool CanWriteFile(string filePath)
{
try
{
if(File.Exists(filePath))
{
//File exists: Check that it can be written to
//(no need to check for directories, because File.Exists would have returned false)
bool isReadOnly = (File.GetAttributes(filePath) & FileAttributes.ReadOnly) != 0; //If this throws we don't have ReadAttributes access; just assume we don't have Write either.
bool hasWriteAccess = FileAccessCheck.CheckAccess(filePath, FileAccess.Write, true);
return hasWriteAccess && !isReadOnly;
}
else
{
//File does not exist. Check that the directory supposed to contain it exists,
//and check if we can write to that.
string dirPath = Path.GetDirectoryName(filePath);
if(string.IsNullOrEmpty(dirPath) || !Directory.Exists(dirPath))
return false;
return FileAccessCheck.CheckAccess(dirPath, FileAccess.Write, true);
}
}
catch(Exception) { return false; }
}
}//class
} |
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