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Use overflow-checked arithmetic when sizing StringBuilder native buffers - #137012
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jkotas merged 4 commits intoNov 6, 2026
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The native buffer size (capacity + 1) * 3 + 1 was computed with unchecked int32 ADD/MUL. For capacities near 0x55555555 the size wraps to a tiny value, so the stub allocated a few bytes (possibly via localloc) and then wrote the full UTF-8 contents into it. Use ADD_OVF/MUL_OVF like the LPSTR StringBuilder marshaler does.
Marshal a [MarshalAs(UnmanagedType.LPUTF8Str)] StringBuilder whose Capacity (715,827,883) makes the native buffer size (Capacity + 1) * 3 + 1 overflow int32, and assert the stub throws OverflowException before the native call. Without the overflow-checked arithmetic the stub writes the null terminator ~2 GB before the allocated buffer and crashes with an AccessViolationException. The test needs a ~1.4 GB StringBuilder, so it is limited to 64-bit CoreCLR, kept OuterLoop like the rest of this file, and returns early if the StringBuilder cannot be allocated.
ILWSTRBufferMarshaler computed the native buffer size Capacity * 2 + 4 and the offset of the extra null terminator (Capacity * 2 + 2) with unchecked int32 MUL/ADD. Capacity is only validated to be <= 0x7ffffff0, so for capacities of about 1G chars and above the offset wraps negative and the stub writes the terminator ~2 GB before the allocated buffer. Use MUL_OVF/ADD_OVF like the LPSTR and UTF-8 StringBuilder marshalers. Add a regression test that marshals a CharSet.Unicode StringBuilder with Capacity 1,073,741,823 and asserts OverflowException. Without the fix it crashes with an AccessViolationException.
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jkotas
reviewed
Nov 5, 2026
NativeAOT sizes UTF-16 StringBuilder buffers with checked arithmetic, so the LPTSTR test is no longer limited to CoreCLR. The UTF-8 test is disabled on NativeAOT with the existing UTF-8 StringBuilder issue (dotnet#123529) and skipped on Mono, which sizes the buffer from Capacity + 1 and does not overflow.
AaronRobinsonMSFT
approved these changes
Nov 6, 2026
This was referenced Nov 6, 2026
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The IL stubs for
[MarshalAs(UnmanagedType.LPUTF8Str)] StringBuilderand[MarshalAs(UnmanagedType.LPWStr)] StringBuildercomputed the native buffer size with unchecked int32 arithmetic.StubHelpers.CheckStringLengthonly capsStringBuilder.Capacityat0x7ffffff0, so large capacities wrap:ILUTF8BufferMarshaler):(Capacity + 1) * 3 + 1. For example,Capacity = 0x55555555gives a 3-byte buffer that the stub maylocallocon the stack beforeUTF8BufferMarshaler.ConvertToNativewrites the string into it.ILWSTRBufferMarshaler):Capacity * 2 + 4. From about 1G chars, the offset of the extra null terminator goes negative, so the stub writes it about 2 GB before the allocated buffer.Both now use
MUL_OVF/ADD_OVF, asILCSTRBufferMarshaleralready does, so the stub throwsOverflowExceptionbefore it allocates or writes anything. The by-value, by-ref and field cases all share this size calculation, and the native-to-managed direction has no size arithmetic.Tests
I added one test per marshaler:
UTF8Test.StringBuilderNativeBufferSizeOverflowandLPTStrTest.StringBuilderNativeBufferSizeOverflow. Each passes a StringBuilder with the smallest capacity that triggers the bug (715,827,883 and 1,073,741,823) and assertsOverflowException.Results on osx-arm64 Release:
AccessViolationExceptioninside the marshaling stub.