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Clr type filtering - #2742

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macaronikazoo:clr-type-filtering
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Clr type filtering#2742
macaronikazoo wants to merge 1 commit into
pythonnet:masterfrom
macaronikazoo:clr-type-filtering

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@macaronikazoo

@macaronikazoo macaronikazoo commented Sep 24, 2026 •
edited by filmor
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What does this implement/fix? Explain your changes.

Allows embedders to restrict which CLR types reach Python

NOTE: this isn't a security feature, but it does make it harder to go exploring through everything available in the runtime

For example, the following prevents python from using reflection.

sealed class DenyReflectionTypes : IClrTypeFilter
{
    static readonly string[] _deniedNamespaces = [
        "System.Reflection",
        "System.Runtime.InteropServices",
        "System.Runtime.Loader",
    ];

    static readonly Type[] _deniedTypes = [
        typeof(Activator),
        typeof(AppDomain),
    ];

    public bool ShouldReflect(Type type)
    {
        if (typeof(Type).IsAssignableFrom(type) || Array.IndexOf(_deniedTypes, type) >= 0)
            return false;

        if (type.Namespace is not { } ns)
            return true;
        foreach (var denied in _deniedNamespaces)
            if (ns == denied || ns.StartsWith(denied + ".", StringComparison.Ordinal))
                return false;
        return true;
    }
}

Checklist

Check all those that are applicable and complete.

  • Make sure to include one or more tests for your change
  • If an enhancement PR, please create docs and at best an example
  • Ensure you have signed the .NET Foundation CLA

@lostmsu

lostmsu commented Sep 26, 2026

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@filmor this seems very out of scope. Reopen if disagree.

@lostmsu lostmsu closed this Sep 26, 2026
@macaronikazoo

macaronikazoo commented Sep 27, 2026

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why "very out of scope"? I'd have thought plently of embedders/consumers of this library would want to be able to restrict what python can and can't access on the .net side of things, surely? seems super useful - jint has similar functionality

@filmor filmor reopened this Sep 28, 2026
@filmor

filmor commented Sep 28, 2026

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I think this is worth discussing.

@macaronikazoo

macaronikazoo commented Sep 29, 2026

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is there anything from me that might be helpful?

to me it seems like a potentially valuable, low risk change - the existing architecture made it reasonably easy to insert the appropriate checks - its a pretty fantastic library really, you guys have done an awesome job

@macaronikazoo @filmor
Add IClrTypeFilter to restrict which CLR types reach Python
b21fc2c 
Embedders can register filters on InteropConfiguration.ClrTypeFilters to refuse CLR types before they're handed to Python. Empty by default - a type is allowed only if every registered filter permits it

ReflectedClrType.GetOrCreate consults the filters on a cache miss, before taking _cacheCreateLock, so filter code never runs while a pythonnet lock is held. Every CLR type passes through there, so the one check covers return values, fields, iterators and APIs returning System.Type alike. Refused types are checked everytime, but a refusal throws ClrTypeFilteredException, which Exceptions.SetError raises as a plain Python TypeError: wrapping the CLR exception would mean reflecting its type into Python, which the same filters may refuse. InterfaceObject now guards __implementation__ and __raw_implementation__, which expose the concrete object and so can be refused too

NOTE: this isn't a security feature, but it does make it harder to go exploring through everything available in the runtime

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3 participants

@macaronikazoo @lostmsu @filmor