Avoid potentially unsafe snprintf usage in FPSCounter.

snprintf returns the number of bytes it would have written when it runs out of space.
This patch makes sure we properly handle this unlikely event in FPSCounter.
This patch also makes sure we don't print out the contents of an uninitialized buffer.

1 files changed, 21 insertions, 6 deletions

diff --git a/gfx/layers/composite/FPSCounter.cpp b/gfx/layers/composite/FPSCounter.cpp
index 02ffc4b2c..b8e93eb97 100644
--- a/gfx/layers/composite/FPSCounter.cpp
+++ b/gfx/layers/composite/FPSCounter.cpp
@@ -210,7 +210,10 @@ FPSCounter::WriteFrameTimeStamps(PRFileDesc* fd)
   const int bufferSize = 256;
   char buffer[bufferSize];
   int writtenCount = SprintfLiteral(buffer, "FPS Data for: %s\n", mFPSName);
-  MOZ_ASSERT(writtenCount >= 0);
+  MOZ_ASSERT(writtenCount < bufferSize);
+  if (writtenCount >= bufferSize) {
+    return;
+  }
   PR_Write(fd, buffer, writtenCount);
 
   ResetReverseIterator();
@@ -225,8 +228,10 @@ FPSCounter::WriteFrameTimeStamps(PRFileDesc* fd)
   while (HasNext(startTimeStamp)) {
     TimeDuration duration = previousSample - nextTimeStamp;
     writtenCount = SprintfLiteral(buffer, "%f,\n", duration.ToMilliseconds());
-
-    MOZ_ASSERT(writtenCount >= 0);
+    MOZ_ASSERT(writtenCount < bufferSize);
+    if (writtenCount >= bufferSize) {
+      continue;
+    }
     PR_Write(fd, buffer, writtenCount);
 
     previousSample = nextTimeStamp;
@@ -299,8 +304,13 @@ FPSCounter::PrintFPS()
 void
 FPSCounter::PrintHistogram(std::map<int, int>& aHistogram)
 {
+  if (aHistogram.size() == 0) {
+    return;
+  }
+
   int length = 0;
   const int kBufferLength = 512;
+  int availableSpace = kBufferLength;
   char buffer[kBufferLength];
 
   for (std::map<int, int>::iterator iter = aHistogram.begin();
@@ -309,9 +319,14 @@ FPSCounter::PrintHistogram(std::map<int, int>& aHistogram)
     int fps = iter->first;
     int count = iter->second;
 
-    length += snprintf(buffer + length, kBufferLength - length,
-                       "FPS: %d = %d. ", fps, count);
-    NS_ASSERTION(length >= kBufferLength, "Buffer overrun while printing FPS histogram.");
+    int lengthRequired = snprintf(buffer + length, availableSpace,
+                                  "FPS: %d = %d. ", fps, count);
+    // Ran out of buffer space. Oh well - just print what we have.
+    if (lengthRequired > availableSpace) {
+      break;
+    }
+    length += lengthRequired;
+    availableSpace -= lengthRequired;
   }
 
   printf_stderr("%s\n", buffer);