Fix bug processing bmp bit masks

src/codec/SkMasks.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodecPriv.h"
 #include "SkMasks.h"
 #include "SkTypes.h"
@@ skipping 29 matching lines @@
     173, 175, 177, 179, 181, 183, 185, 187, 189, 191, 193, 195, 197, 199, 201,
     203, 205, 207, 209, 211, 213, 215, 217, 219, 221, 223, 225, 227, 229, 231,
     233, 235, 237, 239, 241, 243, 245, 247, 249, 251, 253, 255
 };
 
 /*
  *
  * Convert an n bit component to an 8-bit component
  *
  */
-static uint8_t convert_to_8(uint32_t component, uint32_t n) {
+static uint8_t convert_to_8(uint8_t component, uint32_t n) {

[msarett, 2016/02/10 20:59:28]

No reason for "component" to ever be larger than 8 bits.

This would have prevented the crash.

     if (0 == n) {
         return 0;
     } else if (8 > n) {
         return n_bit_to_8_bit_lookup_table[(1 << n) - 2 + component];
     } else {
         SkASSERT(8 == n);
         return component;
     }
 }
 
@@ skipping 19 matching lines @@
 uint8_t SkMasks::getAlpha(uint32_t pixel) const {
     return get_comp(pixel, fAlpha.mask, fAlpha.shift, fAlpha.size);
 }
 
 /*
  *
  * Process an input mask to obtain the necessary information
  *
  */
 const SkMasks::MaskInfo process_mask(uint32_t mask, uint32_t bpp) {
-    // Trim the masks to the allowed number of bits

[msarett, 2016/02/10 20:59:28]

Unrelated to the bug fix.  We do this is CreateMasks - no reason to repeat it
here.

-    if (bpp < 32) {
-        mask &= (1 << bpp) - 1;
-    }
-
     // Determine properties of the mask
     uint32_t tempMask = mask;
     uint32_t shift = 0;
     uint32_t size = 0;
     if (tempMask != 0) {
         // Count trailing zeros on masks
         for (; (tempMask & 1) == 0; tempMask >>= 1) {
             shift++;
         }
         // Count the size of the mask
         for (; tempMask & 1; tempMask >>= 1) {
             size++;
         }
-        // Check that the mask is continuous
-        if (tempMask != 0) {
-            SkCodecPrintf("Warning: Bit masks is not continuous.\n");
+        // Verify that the mask is continuous
+        if (tempMask) {
+            SkCodecPrintf("Warning: Bit mask is not continuous.\n");
+            // Finish processing the mask

[msarett, 2016/02/10 20:59:28]

Even if the mask is not continuous, let's count all the bits at the top.

+            for (; tempMask; tempMask >>= 1) {
+                size++;
+            }
         }
         // Truncate masks greater than 8 bits
         if (size > 8) {
             shift += size - 8;
             size = 8;
+            mask &= 0xFF << shift;

[msarett, 2016/02/10 20:59:28]

Make sure that the final mask is no larger than 8-bits.

         }
     }
 
     // Save the calculated values
     const SkMasks::MaskInfo info = { mask, shift, size };
     return info;
 }
 
 /*
  *
@@ skipping 26 matching lines @@
 }
 
 
 SkMasks::SkMasks(const MaskInfo& red, const MaskInfo& green,
                  const MaskInfo& blue, const MaskInfo& alpha)
     : fRed(red)
     , fGreen(green)
     , fBlue(blue)
     , fAlpha(alpha)
 {}