SkArenaAlloc aligned under-allocation

We cannot rely on natural allocation alignment (8) after footer
installation.  Always compute and apply explicit alignment.

Bug: chromium:1124776
Change-Id: I5ff880409df83284b1f9668a771c2a750f72c148
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/315646
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Herb Derby <herb@google.com>

src/core/SkArenaAlloc.cpp

@@ -74,19 +74,14 @@ char* SkArenaAlloc::NextBlock(char* footerEnd) {
 
 void SkArenaAlloc::ensureSpace(uint32_t size, uint32_t alignment) {
     constexpr uint32_t headerSize = sizeof(Footer) + sizeof(ptrdiff_t);
-    // The chrome c++ library we use does not define std::max_align_t.
-    // This must be conservative to add the right amount of extra memory to handle the alignment
-    // padding.
-    constexpr uint32_t alignof_max_align_t = 8;
     constexpr uint32_t maxSize = std::numeric_limits<uint32_t>::max();
     constexpr uint32_t overhead = headerSize + sizeof(Footer);
     AssertRelease(size <= maxSize - overhead);
     uint32_t objSizeAndOverhead = size + overhead;
-    if (alignment > alignof_max_align_t) {
-        uint32_t alignmentOverhead = alignment - 1;
-        AssertRelease(objSizeAndOverhead <= maxSize - alignmentOverhead);
-        objSizeAndOverhead += alignmentOverhead;
-    }
+
+    const uint32_t alignmentOverhead = alignment - 1;
+    AssertRelease(objSizeAndOverhead <= maxSize - alignmentOverhead);
+    objSizeAndOverhead += alignmentOverhead;
 
     uint32_t minAllocationSize = fNextHeapAlloc;
 

src/core/SkArenaAlloc.h

@@ -8,6 +8,7 @@
 #ifndef SkArenaAlloc_DEFINED
 #define SkArenaAlloc_DEFINED
 
+#include "include/core/SkTypes.h"
 #include "include/private/SkTFitsIn.h"
 
 #include <array>
@@ -170,7 +171,13 @@ private:
             this->ensureSpace(size, alignment);
             alignedOffset = (~reinterpret_cast<uintptr_t>(fCursor) + 1) & mask;
         }
-        return fCursor + alignedOffset;
+
+        char* object = fCursor + alignedOffset;
+
+        SkASSERT((reinterpret_cast<uintptr_t>(object) & (alignment - 1)) == 0);
+        SkASSERT(object + size <= fEnd);
+
+        return object;
     }
 
     char* allocObjectWithFooter(uint32_t sizeIncludingFooter, uint32_t alignment);

tests/ArenaAllocTest.cpp

@@ -180,4 +180,8 @@ DEF_TEST(ArenaAlloc, r) {
     REPORTER_ASSERT(r, created == 128);
     REPORTER_ASSERT(r, destroyed == 128);
 
+    {
+        SkArenaAlloc arena(4096);
+        arena.makeBytesAlignedTo(4081, 8);
+    }
 }