Add sk_careful_memcpy to catch undefined behavior in memcpy.

It's undefined behavior to pass null as src or dst to memcpy, even if len is 0.
This currently triggers -fsanitize=attribute-nonnull warnings, but also can
lead to very unexpected code generation with GCC.

sk_careful_memcpy() checks len first before calling memcpy(),
which prevents that weird undefined situation.

This allows me to mark all sanitizers as no-recover, i.e. make-the-bots-red fatal.

CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-ASAN-Trybot

BUG=skia:4641
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1510683002

gyp/common_conditions.gypi

@@ -436,6 +436,7 @@
           [ 'skia_sanitizer', {
             'cflags': [
               '-fsanitize=<(skia_sanitizer)',
+              '-fno-sanitize-recover=<(skia_sanitizer)',
             ],
             'ldflags': [
               '-fsanitize=<(skia_sanitizer)',

include/core/SkTArray.h

@@ -23,11 +23,11 @@ inline void copy(SkTArray<T, true>* self, int dst, int src) {
 }
 template<typename T>
 inline void copy(SkTArray<T, true>* self, const T* array) {
-    memcpy(self->fMemArray, array, self->fCount * sizeof(T));
+    sk_careful_memcpy(self->fMemArray, array, self->fCount * sizeof(T));
 }
 template<typename T>
 inline void copyAndDelete(SkTArray<T, true>* self, char* newMemArray) {
-    memcpy(newMemArray, self->fMemArray, self->fCount * sizeof(T));
+    sk_careful_memcpy(newMemArray, self->fMemArray, self->fCount * sizeof(T));
 }
 
 template<typename T>

include/core/SkTDArray.h

@@ -45,7 +45,7 @@ public:
                 SkTDArray<T> tmp(src.fArray, src.fCount);
                 this->swap(tmp);
             } else {
-                memcpy(fArray, src.fArray, sizeof(T) * src.fCount);
+                sk_careful_memcpy(fArray, src.fArray, sizeof(T) * src.fCount);
                 fCount = src.fCount;
             }
         }

include/core/SkTypes.h

@@ -24,6 +24,28 @@
 
 #include <string.h>
 
+/**
+ *  sk_careful_memcpy() is just like memcpy(), but guards against undefined behavior.
+ *
+ * It is undefined behavior to call memcpy() with null dst or src, even if len is 0.
+ * If an optimizer is "smart" enough, it can exploit this to do unexpected things.
+ *     memcpy(dst, src, 0);
+ *     if (src) {
+ *         printf("%x\n", *src);
+ *     }
+ * In this code the compiler can assume src is not null and omit the if (src) {...} check,
+ * unconditionally running the printf, crashing the program if src really is null.
+ * Of the compilers we pay attention to only GCC performs this optimization in practice.
+ */
+static inline void* sk_careful_memcpy(void* dst, const void* src, size_t len) {
+    // When we pass >0 len we had better already be passing valid pointers.
+    // So we just need to skip calling memcpy when len == 0.
+    if (len) {
+        memcpy(dst,src,len);
+    }
+    return dst;
+}
+
 /** \file SkTypes.h
 */
 

src/core/SkPathRef.cpp

@@ -72,7 +72,8 @@ void SkPathRef::CreateTransformedCopy(SkAutoTUnref<SkPathRef>* dst,
 
     if (*dst != &src) {
         (*dst)->resetToSize(src.fVerbCnt, src.fPointCnt, src.fConicWeights.count());
-        memcpy((*dst)->verbsMemWritable(), src.verbsMemBegin(), src.fVerbCnt * sizeof(uint8_t));
+        sk_careful_memcpy((*dst)->verbsMemWritable(), src.verbsMemBegin(),
+                           src.fVerbCnt * sizeof(uint8_t));
         (*dst)->fConicWeights = src.fConicWeights;
     }
 
@@ -275,8 +276,8 @@ void SkPathRef::copy(const SkPathRef& ref,
     SkDEBUGCODE(this->validate();)
     this->resetToSize(ref.fVerbCnt, ref.fPointCnt, ref.fConicWeights.count(),
                         additionalReserveVerbs, additionalReservePoints);
-    memcpy(this->verbsMemWritable(), ref.verbsMemBegin(), ref.fVerbCnt * sizeof(uint8_t));
-    memcpy(this->fPoints, ref.fPoints, ref.fPointCnt * sizeof(SkPoint));
+    sk_careful_memcpy(this->verbsMemWritable(), ref.verbsMemBegin(), ref.fVerbCnt*sizeof(uint8_t));
+    sk_careful_memcpy(this->fPoints, ref.fPoints, ref.fPointCnt * sizeof(SkPoint));
     fConicWeights = ref.fConicWeights;
     fBoundsIsDirty = ref.fBoundsIsDirty;
     if (!fBoundsIsDirty) {