Merge multiple histogram snapshots into single one for reporting.

base/metrics/histogram_persistence.cc

Index: base/metrics/histogram_persistence.cc
diff --git a/base/metrics/histogram_persistence.cc b/base/metrics/histogram_persistence.cc
index ae7d48b77f8791545bce6a5e84e7b255d405e899..535d11409276ea117357676c64061ca7c3eb3f7b 100644
--- a/base/metrics/histogram_persistence.cc
+++ b/base/metrics/histogram_persistence.cc
@@ -76,6 +76,7 @@ struct PersistentHistogramData {
   uint32_t ranges_checksum;
   PersistentMemoryAllocator::Reference counts_ref;
   HistogramSamples::Metadata samples_metadata;
+  HistogramSamples::Metadata logged_metadata;
 
   // Space for the histogram name will be added during the actual allocation
   // request. This must be the last field of the structure. A zero-size array
@@ -111,6 +112,22 @@ BucketRanges* CreateRangesFromData(HistogramBase::Sample* ranges_data,
   return ranges.release();
 }
 
+// Calculate the number of bytes required to store all of a histogram's
+// "counts". This will return zero (0) if |bucket_count| is not valid.
+size_t CalculateRequiredCountsBytes(size_t bucket_count) {
+  // 2 because each "sample count" also requires a backup "logged count"
+  // used for calculating the delta during snapshot operations.
+  const unsigned kBytesPerBucket = 2 * sizeof(HistogramBase::AtomicCount);
+
+  // If the |bucket_count| is such that it would overflow the return type,
+  // perhaps as the result of a malicious actor, then return zero to
+  // indicate the problem to the caller.
+  if (bucket_count > std::numeric_limits<uint32_t>::max() / kBytesPerBucket)
+    return 0;
+
+  return bucket_count * kBytesPerBucket;
+}
+
 }  // namespace
 
 const Feature kPersistentHistogramsFeature{
@@ -244,14 +261,21 @@ HistogramBase* CreatePersistentHistogram(
   HistogramBase::AtomicCount* counts_data =
       allocator->GetAsObject<HistogramBase::AtomicCount>(
           histogram_data.counts_ref, kTypeIdCountsArray);
-  if (!counts_data ||
-      allocator->GetAllocSize(histogram_data.counts_ref) <
-          histogram_data.bucket_count * sizeof(HistogramBase::AtomicCount)) {
+  size_t counts_bytes =
+      CalculateRequiredCountsBytes(histogram_data.bucket_count);
+  if (!counts_data || !counts_bytes ||
+      allocator->GetAllocSize(histogram_data.counts_ref) < counts_bytes) {
     RecordCreateHistogramResult(CREATE_HISTOGRAM_INVALID_COUNTS_ARRAY);
     NOTREACHED();
     return nullptr;
   }
 
+  // After the main "counts" array is a second array using for storing what
+  // was previously logged. This is used to calculate the "delta" during
+  // snapshot operations.
+  HistogramBase::AtomicCount* logged_data =
+      counts_data + histogram_data.bucket_count;
+
   std::string name(histogram_data_ptr->name);
   HistogramBase* histogram = nullptr;
   switch (histogram_data.histogram_type) {
@@ -262,8 +286,10 @@ HistogramBase* CreatePersistentHistogram(
           histogram_data.maximum,
           ranges,
           counts_data,
+          logged_data,
           histogram_data.bucket_count,
-          &histogram_data_ptr->samples_metadata);
+          &histogram_data_ptr->samples_metadata,
+          &histogram_data_ptr->logged_metadata);
       DCHECK(histogram);
       break;
     case LINEAR_HISTOGRAM:
@@ -273,8 +299,10 @@ HistogramBase* CreatePersistentHistogram(
           histogram_data.maximum,
           ranges,
           counts_data,
+          logged_data,
           histogram_data.bucket_count,
-          &histogram_data_ptr->samples_metadata);
+          &histogram_data_ptr->samples_metadata,
+          &histogram_data_ptr->logged_metadata);
       DCHECK(histogram);
       break;
     case BOOLEAN_HISTOGRAM:
@@ -282,7 +310,9 @@ HistogramBase* CreatePersistentHistogram(
           name,
           ranges,
           counts_data,
-          &histogram_data_ptr->samples_metadata);
+          logged_data,
+          &histogram_data_ptr->samples_metadata,
+          &histogram_data_ptr->logged_metadata);
       DCHECK(histogram);
       break;
     case CUSTOM_HISTOGRAM:
@@ -290,8 +320,10 @@ HistogramBase* CreatePersistentHistogram(
           name,
           ranges,
           counts_data,
+          logged_data,
           histogram_data.bucket_count,
-          &histogram_data_ptr->samples_metadata);
+          &histogram_data_ptr->samples_metadata,
+          &histogram_data_ptr->logged_metadata);
       DCHECK(histogram);
       break;
     default:
@@ -375,16 +407,15 @@ HistogramBase* AllocatePersistentHistogram(
     return nullptr;
   }
 
+  // If CalculateRequiredCountsBytes() returns zero then the bucket_count
+  // was not valid.
   size_t bucket_count = bucket_ranges->bucket_count();
-  // An overflow such as this, perhaps as the result of a milicious actor,
-  // could lead to writing beyond the allocation boundary and into other
-  // memory. Just fail the allocation and let the caller deal with it.
-  if (bucket_count > std::numeric_limits<int32_t>::max() /
-                     sizeof(HistogramBase::AtomicCount)) {
+  size_t counts_bytes = CalculateRequiredCountsBytes(bucket_count);
+  if (!counts_bytes) {
     NOTREACHED();
     return nullptr;
   }
-  size_t counts_bytes = bucket_count * sizeof(HistogramBase::AtomicCount);
+
   size_t ranges_bytes = (bucket_count + 1) * sizeof(HistogramBase::Sample);
   PersistentMemoryAllocator::Reference ranges_ref =
       allocator->Allocate(ranges_bytes, kTypeIdRangesArray);