runtime: fix liveness issue in test-only getgcmask

getgcmask stops referencing the object passed to it sometime between
when the object is looked up and when the function returns. Notably,
this can happen while the GC mask is actively being produced, and thus
the GC might free the object.

This is easily reproducible by adding a runtime.GC call at just the
right place. Adding a KeepAlive on the heap-object path fixes it.

Fixes #64188.

Change-Id: I5ed4cae862fc780338b60d969fd7fbe896352ce4
Reviewed-on: https://go-review.googlesource.com/c/go/+/542716
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: mknyszek

src/runtime/mbitmap_allocheaders.go

@@ -1078,6 +1078,11 @@ func getgcmask(ep any) (mask []byte) {
 		for len(mask) > 0 && mask[len(mask)-1] == 0 {
 			mask = mask[:len(mask)-1]
 		}
+
+		// Make sure we keep ep alive. We may have stopped referencing
+		// ep's data pointer sometime before this point and it's possible
+		// for that memory to get freed.
+		KeepAlive(ep)
 		return
 	}
 

src/runtime/mbitmap_noallocheaders.go

@@ -744,6 +744,11 @@ func getgcmask(ep any) (mask []byte) {
 		for len(mask) > 0 && mask[len(mask)-1] == 0 {
 			mask = mask[:len(mask)-1]
 		}
+
+		// Make sure we keep ep alive. We may have stopped referencing
+		// ep's data pointer sometime before this point and it's possible
+		// for that memory to get freed.
+		KeepAlive(ep)
 		return
 	}