[scudo][standalone] Allow fallback to secondary if primary is full

[cryptoad]

Summary

We introduced a way to fallback to the immediately larger size class for
the Primary in the event a region was full, but in the event of the largest
size class, we would just fail.

This change allows to fallback to the Secondary when the last region of
the Primary is full. We also expand the trick to all platforms as opposed
to being Android only, and update the test to cover the new case.

[cryptoad]

Initialize Block to nullptr.

[hctim]

LGTM

[cferris]

This passes all tests in the Android environment.

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

#endif // GWP_ASAN_HOOKS
     static_assert(MaxAllowedMallocSize < UINTPTR_MAX - MaxAlignment, "");
     if (UNLIKELY(Size >= MaxAllowedMallocSize)) {
       if (Options.MayReturnNull)
         return nullptr;
       reportAllocationSizeTooBig(Size, NeededSize, MaxAllowedMallocSize);
     }
     DCHECK_LE(Size, NeededSize);
 
-    void *Block;
-    uptr ClassId;
+    void *Block = nullptr;
+    uptr ClassId = 0;
     uptr SecondaryBlockEnd;
     if (LIKELY(PrimaryT::canAllocate(NeededSize))) {
       ClassId = SizeClassMap::getClassIdBySize(NeededSize);
       DCHECK_NE(ClassId, 0U);
       bool UnlockRequired;
       auto *TSD = TSDRegistry.getTSDAndLock(&UnlockRequired);
       Block = TSD->Cache.allocate(ClassId);
       // If the allocation failed, the most likely reason with a 64-bit primary
       // is the region being full. In that event, retry once using the
       // immediately larger class (except if the failing class was already the
       // largest). This will waste some memory but will allow the application to
-      // not fail.
-      if (SCUDO_ANDROID) {
-        if (UNLIKELY(!Block)) {
-          if (ClassId < SizeClassMap::LargestClassId)
-            Block = TSD->Cache.allocate(++ClassId);
-        }
+      // not fail. If dealing with the largest class, fallback to the Secondary.
+        else
+          ClassId = 0;
       }
       if (UnlockRequired)
         TSD->unlock();
-    } else {
-      ClassId = 0;
+    }
+    if (UNLIKELY(ClassId == 0))
       Block = Secondary.allocate(NeededSize, Alignment, &SecondaryBlockEnd,
                                  ZeroContents);
-    }
 
     if (UNLIKELY(!Block)) {
       if (Options.MayReturnNull)
         return nullptr;
       reportOutOfMemory(NeededSize);
     }
 
     const uptr BlockUptr = reinterpret_cast<uptr>(Block);

compiler-rt/lib/scudo/standalone/tests/combined_test.cpp

TEST(ScudoCombinedTest, ReleaseToOS) {
   };
   std::unique_ptr<AllocatorT, decltype(Deleter)> Allocator(new AllocatorT,
                                                            Deleter);
   Allocator->reset();
 
   Allocator->releaseToOS();
 }
 
-// Verify that when a region gets full, Android will still manage to
+// Verify that when a region gets full, the allocator will still manage to
 // fulfill the allocation through a larger size class.
 TEST(ScudoCombinedTest, FullRegion) {
   using AllocatorT = scudo::Allocator<DeathConfig>;
   auto Deleter = [](AllocatorT *A) {
     A->unmapTestOnly();
     delete A;
   };
   std::unique_ptr<AllocatorT, decltype(Deleter)> Allocator(new AllocatorT,
                                                            Deleter);
   Allocator->reset();
 
-  const scudo::uptr Size = 1000U;
-  const scudo::uptr MaxNumberOfChunks =
-      (1U << DeathRegionSizeLog) /
-      DeathConfig::DeathSizeClassMap::getSizeByClassId(1U);
-  void *P;
   std::vector<void *> V;
   scudo::uptr FailedAllocationsCount = 0;
+  for (scudo::uptr ClassId = 1U;
+       ClassId <= DeathConfig::DeathSizeClassMap::LargestClassId; ClassId++) {
+    const scudo::uptr Size =
+        DeathConfig::DeathSizeClassMap::getSizeByClassId(ClassId);
+    const scudo::uptr MaxNumberOfChunks = (1U << DeathRegionSizeLog) / Size;
+    void *P;
     for (scudo::uptr I = 0; I <= MaxNumberOfChunks; I++) {
-    P = Allocator->allocate(Size, Origin);
+      P = Allocator->allocate(Size - 64U, Origin);
       if (!P)
         FailedAllocationsCount++;
       else
         V.push_back(P);
     }
+  }
   while (!V.empty()) {
     Allocator->deallocate(V.back(), Origin);
     V.pop_back();
   }
-  if (SCUDO_ANDROID)
-    EXPECT_EQ(FailedAllocationsCount, 0U);
-  else
-    EXPECT_GT(FailedAllocationsCount, 0U);
 }