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[sre] Handle null values in MarshalAsAttribute CustomAttributeBuilder
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Normally strings are encoded as length-then-utf8 in ECMA-335.  But a null
String is encoded as just the byte 0xFF.

When we make a CustomAttributeBuilder for a MarshalAsAttribute and attach it to
some parameter we first encode the CAB as a byte blob and then decode it in
CAB.get_umarshal into a UnmanagedMarshal attribute.  But the decoding didn't
account for the possibility of null strings.

Fixes mono#12747
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@lambdageek @monojenkins
lambdageek authored and monojenkins committed Feb 7, 2019
1 parent 22911b0 commit c93fe36
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Showing 2 changed files with 78 additions and 22 deletions.
46 changes: 24 additions & 22 deletions mcs/class/corlib/System.Reflection.Emit/CustomAttributeBuilder.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -303,11 +303,24 @@ internal static string string_from_bytes (byte[] data, int pos, int len)
return System.Text.Encoding.UTF8.GetString(data, pos, len);
}

internal static string decode_string (byte [] data, int pos, out int rpos)
{
if (data [pos] == 0xff) {
rpos = pos + 1;
return null;
} else {
int len = decode_len (data, pos, out pos);
string s = string_from_bytes (data, pos, len);
pos += len;
rpos = pos;
return s;
}
}

internal string string_arg ()
{
int pos = 2;
int len = decode_len (data, pos, out pos);
return string_from_bytes (data, pos, len);
return decode_string (data, pos, out pos);
}

internal static UnmanagedMarshal get_umarshal (CustomAttributeBuilder customBuilder, bool is_field) {
Expand Down Expand Up @@ -335,18 +348,14 @@ internal static UnmanagedMarshal get_umarshal (CustomAttributeBuilder customBuil
int paramType; // What is this ?

/* Skip field/property signature */
pos ++;
int fieldPropSig = (int)data [pos ++];
/* Read type */
paramType = ((int)data [pos++]);
if (paramType == 0x55) {
/* enums, the value is preceeded by the type */
int len2 = decode_len (data, pos, out pos);
string_from_bytes (data, pos, len2);
pos += len2;
decode_string (data, pos, out pos);
}
int len = decode_len (data, pos, out pos);
string named_name = string_from_bytes (data, pos, len);
pos += len;
string named_name = decode_string (data, pos, out pos);

switch (named_name) {
case "ArraySubType":
Expand Down Expand Up @@ -375,9 +384,7 @@ internal static UnmanagedMarshal get_umarshal (CustomAttributeBuilder customBuil
pos += 4;
break;
case "SafeArrayUserDefinedSubType":
len = decode_len (data, pos, out pos);
string_from_bytes (data, pos, len);
pos += len;
decode_string (data, pos, out pos);
break;
case "SizeParamIndex":
value = (int)data [pos++];
Expand All @@ -386,20 +393,15 @@ internal static UnmanagedMarshal get_umarshal (CustomAttributeBuilder customBuil
hasSize = true;
break;
case "MarshalType":
len = decode_len (data, pos, out pos);
marshalTypeName = string_from_bytes (data, pos, len);
pos += len;
marshalTypeName = decode_string (data, pos, out pos);
break;
case "MarshalTypeRef":
len = decode_len (data, pos, out pos);
marshalTypeName = string_from_bytes (data, pos, len);
marshalTypeRef = Type.GetType (marshalTypeName);
pos += len;
marshalTypeName = decode_string (data, pos, out pos);
if (marshalTypeName != null)
marshalTypeRef = Type.GetType (marshalTypeName);
break;
case "MarshalCookie":
len = decode_len (data, pos, out pos);
marshalCookie = string_from_bytes (data, pos, len);
pos += len;
marshalCookie = decode_string (data, pos, out pos);
break;
default:
throw new Exception ("Unknown MarshalAsAttribute field: " + named_name);
Expand Down
54 changes: 54 additions & 0 deletions mcs/class/corlib/Test/System.Reflection.Emit/CustomAttributeBuilderTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@
using System.Reflection;
using System.Reflection.Emit;
using System.Threading;
using System.Runtime.InteropServices;
using NUnit.Framework;

namespace MonoTests.System.Reflection.Emit
Expand Down Expand Up @@ -845,6 +846,59 @@ public void CustomAttributeAcrossAssemblies () {
assemblyBuilder1.Save ("Repro55681-2a.dll");
}

[DllImport("SomeLib")]
private static extern void MethodForNullStringMarshalAsFields([MarshalAs(UnmanagedType.LPWStr)] string param);

[Test]
public void NullStringMarshalAsFields () {
// Regression test for https://github.com/mono/mono/issues/12747
//
// MarshalAsAttribute goes through
// CustomAttributeBuilder.get_umarshal which tries to
// build an UnmanagedMarshal value by decoding the CAB's data.
//
// The data decoding needs to handle null string (encoded as 0xFF) properly.
var aName = new AssemblyName("Repro12747");
var assembly = AppDomain.CurrentDomain.DefineDynamicAssembly(aName, AssemblyBuilderAccess.RunAndSave, tempDir);
var module = assembly.DefineDynamicModule(aName.Name, aName.Name + ".dll");

var prototypeMethodName = nameof(MethodForNullStringMarshalAsFields);
var someMethod = this.GetType().GetMethod(prototypeMethodName, BindingFlags.Static | BindingFlags.NonPublic);

var typeBuilder = module.DefineType("NewType" + module.ToString(), TypeAttributes.Class | TypeAttributes.Public);
var methodBuilder = typeBuilder.DefineMethod("NewMethod", MethodAttributes.Public | MethodAttributes.HideBySig, typeof(void), new[] { typeof(string) });
var il = methodBuilder.GetILGenerator();
il.Emit(OpCodes.Ret);

var param = someMethod.GetParameters()[0];
var paramBuilder = methodBuilder.DefineParameter(1, param.Attributes, null);
MarshalAsAttribute attr = param.GetCustomAttribute<MarshalAsAttribute>();
var attrCtor = typeof(MarshalAsAttribute).GetConstructor(new[] { typeof(UnmanagedType) });
object[] attrCtorArgs = { attr.Value };

// copy over the fields from the real MarshalAsAttribute on the parameter of "MethodForNullStringMarshalAsFields",
// including the ones that were initialized to null
var srcFields = typeof(MarshalAsAttribute).GetFields(BindingFlags.Public | BindingFlags.Instance);
var fieldArguments = new FieldInfo[srcFields.Length];
var fieldArgumentValues = new object[srcFields.Length];
for(int i = 0; i < srcFields.Length; i++)
{
var field = srcFields[i];
fieldArguments[i] = field;
fieldArgumentValues[i] = field.GetValue(attr);
}

var attrBuilder = new CustomAttributeBuilder(attrCtor, attrCtorArgs, Array.Empty<PropertyInfo>(), Array.Empty<object>(),
fieldArguments, fieldArgumentValues);
// this encodes the CustomAttributeBuilder as a data
// blob and then tries to decode it using
// CustomAttributeBuilder.get_umarshal
paramBuilder.SetCustomAttribute(attrBuilder);

var finalType = typeBuilder.CreateType();

}

}
}

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