IMPORTANT: memory corruption bug (with prelimenary fix)

There is a longstanding bug in the ia64 linux kernel which, under just the right circumstances, can cause virtual memory corruption. The exact conditions to trigger this are a bit complicated, but basically, the bug doesn't trigger unless an munmap() occurs near the end of the per-region mappable address space and that region contains some other mappings. In 2.4.18, triggering this bug has become slightly easier because we moved the stack down from region 4 to region 3 (the stack is at the end of the mappable space and the low part of region 3 is occupied by the data segment, which is the likely victim of a memory corruption triggered by someone doing mapping games near the stack area).

The patch below is a two-liner which _should_ fix the problem. I plan to verify the fix some more for correctness, but it's definitely safe and fixes the one scenario that is known to trigger the bug.

Jack, I'm wondering whether this is related to the pthread problem you reported earlier? Can you try this patch?

Thanks to Steve Goldman for bringing this problem to my attention and for providing a test case.

	--david

@@ -883,6 +883,8 @@
 		break;
 	}
 no_mmaps:
+	if (last < first)
+		return;
 	/*
 	 * If the PGD bits are not consecutive in the virtual address, the
 	 * old method of shifting the VA >> by PGDIR_SHIFT doesn't work.