[scudo] Temporariy dispatch region from `RegionBeg`

[Chia-hungDuan]

Summary

In general, a region is located from region base and has size
RegionSize. However, some platforms may not support mapping from
region base. Before we have each platform implements their specific
MemMap to handle the offset. Temporarily dispatch the region from
RegionBeg instead.

[Chia-hungDuan]

@fabio-d Please help verify if this works on Fuchsia

[cferris]

LGTM

[fabio-d]

I've tested that this CL fixes the regression introduced on Fuchsia by D146009

[fabio-d]

@Chia-hungDuan I think we can remove this workaround now that we have a more general solution in D148141. I've verified that reverting this CL does not break Fuchsia, so +2 for removing it from my side.

compiler-rt/lib/scudo/standalone/primary64.h

if (TotalUserBytes > MappedUser) {
           roundUp(TotalUserBytes - MappedUser, MapSizeIncrement);
       const uptr RegionBase = RegionBeg - getRegionBaseByClassId(ClassId);
       if (UNLIKELY(RegionBase + MappedUser + MapSize > RegionSize)) {
         Region->Exhausted = true;
         return false;
       }
       // TODO: Consider allocating MemMap in init().
       if (!Region->MemMap.isAllocated()) {
-        Region->MemMap = ReservedMemory.dispatch(
-            getRegionBaseByClassId(ClassId), RegionSize);
+        // TODO: Ideally, a region should reserve RegionSize because the memory
+        // between `RegionBeg` and region base is still belong to a region and
+        // it's just not used. In order to make it work on every platform (some
+        // of them don't support `remap()` across the unused range), dispatch
+        // from `RegionBeg` for now.
+        const uptr ReserveSize =
+            RegionSize - (RegionBeg - getRegionBaseByClassId(ClassId));
+        Region->MemMap = ReservedMemory.dispatch(RegionBeg, ReserveSize);
       }
       DCHECK(Region->MemMap.isAllocated());
 
       if (UNLIKELY(!Region->MemMap.remap(
               RegionBeg + MappedUser, MapSize, "scudo:primary",
               MAP_ALLOWNOMEM | MAP_RESIZABLE |
                   (useMemoryTagging<Config>(Options.load()) ? MAP_MEMTAG
                                                             : 0)))) {