Fix inverted IsNull check in RangeSectionMap::EnumMemoryRangeSectionMapLevel

[max-charlamb]

Summary

One-line fix: invert the IsNull() condition in the template overload of EnumMemoryRangeSectionMapLevel to match the non-template L1 overload.

Problem

The template overload of EnumMemoryRangeSectionMapLevel (handling levels L2–L5 on 64-bit) has an inverted condition at line 1553 of codeman.h:

// BUG: recurses into NULL entries, skips populated ones
if (level[i].IsNull())
{
    EnumMemoryRangeSectionMapLevel(flags, *level[i].VolatileLoad(pLockState), pLockState);
}

The non-template L1 overload at line 1538 correctly uses !IsNull():

// CORRECT: only recurse into populated entries
if (!level[i].IsNull())
{
    EnumMemoryRangeSectionMapLevel(flags, level[i], pLockState);
}

This bug means EnumMemoryRegions for the RangeSectionMap never enumerates anything below the top level on 64-bit (where levels L2–L5 use the template), and attempts to dereference null pointers for empty slots. This could cause incomplete DAC memory enumeration during dump generation.

[Copilot]

Pull request overview

Fixes a logic bug in CoreCLR’s DAC memory enumeration for RangeSectionMap by making the template recursion (used for deeper map levels on 64-bit) skip null entries and only recurse into populated ones, matching the L1 non-template overload.

Changes:

• Invert the IsNull() condition in RangeSectionMap::EnumMemoryRangeSectionMapLevel (template overload) to avoid recursing into null slots and skipping valid ones.

[jkotas]

Is this enough to make your test pass or is there more?

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[max-charlamb]

Is this enough to make your test pass or is there more?

I don't know for sure. The failing SOS test does not produce a dump. I am working on adding a dump, but it requires a non-deterministic run on CI.

With copilot, I found several bugs in the unwinder which could have impacted the stackwalk:
#124845
#124848