Assertion 'codeManager->IsUnwindable(pvAddress) || runtime->IsConservativeStackReportingEnabled()' in System.Runtime.Tests on ARM64

[MichalStrehovsky]

https://dev.azure.com/dnceng-public/public/_build/results?buildId=1615918&view=ms.vss-test-web.build-test-results-tab&runId=44741800&resultId=146853&paneView=debug

To pull down the dump: runfo get-helix-payload -j a698b81e-cb53-4dfe-8b42-f182446f56ba -w System.Runtime.Tests -o c:\helix\a698b81e-cb53-4dfe-8b42-f182446f56ba\System.Runtime.Tests\

chmod +rwx System.Runtime.Tests ^&^& ./System.Runtime.Tests -notrait category=AdditionalTimezoneChecks -notrait category=IgnoreForCI -notrait category=OuterLoop -notrait category=failing -xml testResults.xml 
popd
===========================================================================================================
/root/helix/work/workitem/e /root/helix/work/workitem/e
Running assembly:System.Runtime.Tests, Version=11.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51
[SKIP] System.Text.Tests.CompositeFormatTests.DebuggerDisplay_ShowsFormat
[SKIP] System.Tests.UIntPtrTests.BigMul32
[SKIP] System.Text.Tests.EncodingTests.GetEncoding_FromProvider_ByCodePage_WithDisallowedEncoding_Throws
[SKIP] System.Text.Tests.EncodingTests.GetEncoding_FromProvider_ByEncodingName_WithDisallowedEncoding_Throws
[SKIP] System.Text.Tests.EncodingTests.GetEncodings_FromProvider_DoesNotContainDisallowedEncodings
[SKIP] System.Text.Tests.EncodingTests.RegisterProvider_EncodingsAreUsable
[SKIP] System.Tests.ActivatorTests.CreateInstanceAssemblyResolve
[SKIP] System.Tests.ActivatorTests.TestingCreateInstanceObjectHandle
[SKIP] System.Tests.ActivatorTests.CreateInstance_NonPublicValueTypeWithPrivateDefaultConstructor_Success
[SKIP] System.Tests.ActivatorTests.TestingBindingFlags
[SKIP] System.Tests.ActivatorTests.TestingCreateInstanceObjectHandleFullSignature
[SKIP] System.Tests.ActivatorTests.CreateInstance_TypeBuilder_ThrowsNotSupportedException
[SKIP] System.Tests.ActivatorTests.TestingCreateInstanceFromObjectHandleFullSignature
[SKIP] System.Tests.ActivatorTests.CreateInstance_PublicOnlyValueTypeWithPrivateDefaultConstructor_ThrowsMissingMethodException
[SKIP] System.Tests.ActivatorTests.TestingCreateInstanceFromObjectHandle
[SKIP] System.Tests.TypeTestsExtended.GetTypeByName_NoSuchType_ThrowsTypeLoadException
[SKIP] System.Tests.TypeTestsExtended.GetTypeByNameCaseSensitiveTypeloadFailure
[SKIP] System.Tests.TypeTestsExtended.MakeGenericType_NonRuntimeType
[SKIP] System.Tests.TypeTestsExtended.GetTypeByName
[SKIP] System.Reflection.Tests.MethodBaseTests.TestMethodBody
[SKIP] System.Tests.GCExtendedTests.GetGCMemoryInfo
--------------------------------------------------
Debug Assertion Violation

Expression: 'codeManager->IsUnwindable(pvAddress) || runtime->IsConservativeStackReportingEnabled()'

File: /__w/1/s/src/coreclr/nativeaot/Runtime/thread.cpp, Line: 709
--------------------------------------------------

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @dotnet/ilc-contrib
See info in area-owners.md if you want to be subscribed.

[MichalStrehovsky]

Root cause: ARM64 stackalloc falsely classified as an epilog

The assertion is caused by a false-negative unwindability classification in NativeAOT’s ARM64 epilog detector. A legitimate stack-allocation instruction is mistaken for an epilog instruction. The interrupted location is marked GC-interruptible, so this produces the contradiction asserted in Thread::HijackCallback.

Investigated artifacts

• Checkout: 49e04fa0e472453f4391c4a7097d9e55380dbef2
• Executable: System.Runtime.Tests, Linux ARM64 NativeAOT
• Symbols: System.Runtime.Tests.dbg
• Core: core.1000.17
• Executable/symbol ELF BuildID: 89b89ce46b63f335c7a0779a423d40ee35b33b88

All source locations below refer to that checkout.

1. Interrupted instruction and its purpose

The aborting thread is OS thread 26. Its relevant stack is:

RhFailFast
Assert
Thread::HijackCallback                 thread.cpp:709
ActivationHandler
<signal trampoline>
String.Concat(IEnumerable<string>)    String.Manipulation.cs:188
DoubleTests.SplitPairs                DoubleTests.cs:550
DoubleTests.ParsePatterns             DoubleTests.cs:587

Another thread is in ThreadStore::SuspendAllThreads / GCToEEInterface::SuspendEE.

The interrupted PC is:

Runtime address:   0x0000b90f5bdafe04
ELF virtual addr:  0x00000000022dfe04
Symbol:            String__Concat_5 + 0xc4
Function start:    0x0000b90f5bdafd40

Relevant instructions, with offsets relative to the function start:

+0xac  340006a0  cbz  w0, ...
+0xb0  a8e07fe0  ldp  x0, xzr, [sp], #-0x200
+0xb4  910003e0  mov  x0, sp
+0xb8  f81b83bf  stur xzr, [x29, #-0x48]
+0xbc  f81c83a0  stur x0,  [x29, #-0x38]
+0xc0  52802000  mov  w0, #0x100
+0xc4  b81d03a0  stur w0,  [x29, #-0x30]   // interrupted here

This corresponds to:

var result = new ValueStringBuilder(stackalloc char[StackallocCharBufferSizeLimit]);

in src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs:188.

The ldp probes the stack and decreases SP by 512 bytes, allocating the buffer. It does not restore FP or LR. This is deliberate code generation in CodeGen::genLclHeap, src/coreclr/jit/codegenarm64.cpp:3354–3362, which emits:

ldp tmpReg, xzr, [sp], #-amount

for suitable constant-sized stack allocations.

2. Exact incorrect classification

In src/coreclr/nativeaot/Runtime/unix/UnixNativeCodeManager.cpp:1169–1177, TrailingEpilogueInstructionsCount contains:

// Post-index restore sequences such as "ldp x19, x20, [sp], #0x10" also adjust SP
// before the final AUTIASP/RET. We avoid signing with a partially-restored SP.
int baseRegister = (instr >> 5) & 0x1f;
if (baseRegister == 31)
{
    if ((instr & LDP_MASK2) == LDP_BITS2 ||
        (instr & LDR_MASK2) == LDR_BITS2)
    {
        return -1;
    }
}

The backward scan from the interrupted PC reaches the allocation instruction at +0xb0. For its encoding:

instruction                          = 0xa8e07fe0
base register                        = 31 (SP)
destination registers                = 0 and 31 (x0 and xzr)
instruction & LDP_MASK2               = 0xa8e07fe0 & 0x7e400000
                                     = 0x28400000
LDP_BITS2                            = 0x28400000
signed post-index adjustment         = -512 bytes

Therefore, the function returns -1, despite this being an allocation in the method body.

On Linux ARM64, IsUnwindable returns whether TrailingEpilogueInstructionsCount(...) == 0 (UnixNativeCodeManager.cpp:592–612). It consequently returns false.

The matching executable’s disassembly contains this check; this is not merely a discrepancy inferred from the checkout.

3. The interrupted location is genuinely GC-interruptible

An independent decoding of the executable’s version-5 GC metadata, following the repository’s GC decoder format, produced:

FDE ELF virtual address:      0x19b2be0
LSDA ELF virtual address:     0xbd6f7
GC-info ELF virtual address:  0xbd6fc

GC header flags:              0x44
Stack base register:          x29
Call-site safepoints:         none
Interruptible ranges:
    [0x02c, 0x1d4)
    [0x1e8, 0x214)
    [0x228, 0x28c)
    [0x2a0, 0x2b8)
    [0x2c8, 0x2e0)

Interrupted offset:           0x0c4
IsInterruptible:              true

Thus this is an interruptible method-body location, not a call-return safepoint accidentally interpreted as one.

UnixNativeCodeManager::IsSafePoint accepts decoder.IsInterruptible() as well as decoder.IsSafePoint() (UnixNativeCodeManager.cpp:399–423). The resulting combination is:

IsSafePoint(interruptedPC)  == true
IsUnwindable(interruptedPC) == false

That directly explains the assertion in thread.cpp:709.

4. The frame has usable unwind information

The FDE describes the established frame as:

CFA = FP + 16
saved FP = [CFA - 16]
saved LR = [CFA - 8]

The core contains:

FP                = 0x0000ff9458ce4e60
SP                = 0x0000ff9458ce4c10
CFA / caller SP   = 0x0000ff9458ce4e70
[FP + 8]          = 0x0000b90f5c20a9c8

The saved LR resolves to DoubleTests.SplitPairs+136, the expected caller. The dynamic SP allocation does not invalidate this FP-based frame description.

Importantly, the observed failure is the classification assertion, not an actual failed attempt by the runtime unwinder. The runtime unwinder was not executed experimentally during this investigation.

5. Introducing change

The rejecting block was added by:

c1d33fc8c2966f0a68903c1b8b1f6c462e1b3a96
Arm64: [PAC-RET] NativeAOT changes
dotnet/runtime#128950
July 9, 2026, 01:07:37 UTC

Evidence beyond blame:

• The commit’s parent lacks the SP-only load rejection.
• The allocation code and assertion already existed before that commit.
• Replaying the detector against this executable’s instruction words gives:

Current detector:
    scan +0xc0, +0xbc, +0xb8, +0xb4
    encounter +0xb0: SP-based LDP
    return -1 → not unwindable

Pre-change detector:
    scan through +0xb0: neither FP nor LR is restored
    encounter +0xac: CBZ branch boundary
    return 0 → unwindable

This establishes the source-level introduction of the bad predicate. It is not a historical binary bisection or a determination of the first failing CI run.

6. Fix considerations

The fix belongs in the epilog/unwindability classification, not in String.Concat, the test, or removal of the assertion.

Additional concerns identified during source inspection—not separate failures observed in this core:

• The SP-load masks also accept non-writeback loads. Merely recognizing an SP-based load does not establish that stack restoration occurred.
• Those same masks serve legitimate FP/LR-restore recognition elsewhere. Globally narrowing them could break actual epilog detection.
• The separate new ADD SP,SP rejection at UnixNativeCodeManager.cpp:1181–1186 can also match ordinary localloc code: codegenarm64.cpp:3309–3325 temporarily removes the outgoing-argument area using that instruction.
• Gating the checks on PAC alone would not eliminate false positives in PAC-enabled method bodies.

A direction worth evaluating is to keep precise GC unwindability detection separate from additional PAC-specific hijack restrictions, while retaining necessary handling of AUTIASP/AUTIBSP. No exact patch has been validated, and deleting all new SP checks has not been established as safe.

Reproducing the debugger observations

With the executable, symbols, and core in the current directory:

lldb -c core.1000.17 System.Runtime.Tests

Then:

thread select 1
bt 11
frame select 8
register read pc sp fp lr
disassemble -s 0xb90f5bdafdec -c 7 --bytes
image show-unwind -a 0xb90f5bdafe04
memory read -s 8 -f x -c 2 0xff9458ce4e60
image lookup -a 0xb90f5c20a9c8
disassemble -n "UnixNativeCodeManager::IsUnwindable"

These addresses and frame numbers are specific to the supplied dump. Executable text omitted from the core is supplied by the matching executable.

Validation scope: matching core/binary inspection, instruction-level predicate replay, independent GC-metadata decoding, CFI/saved-frame inspection, and source-history comparison. No source changes, product rebuild, runtime reproduction, or candidate-fix validation were performed.

Note

This investigation write-up was generated with GitHub Copilot, including an independent GPT-6 Astra challenge of the diagnosis.

[MichalStrehovsky]

Cc @SwapnilGaikwad seems to caused by the PAC-RET change.