[asan] Detect dereferencing zero-alloc as heap buffer overflow

compiler-rt/lib/asan/asan_allocator.cpp

@@ -547,6 +547,7 @@ struct Allocator {
         ComputeUserRequestedAlignmentLog(alignment);
     if (alignment < min_alignment)
       alignment = min_alignment;
+    bool upgraded_from_zero = false;
     if (size == 0) {
       // We'd be happy to avoid allocating memory for zero-size requests, but
       // some programs/tests depend on this behavior and assume that malloc
@@ -555,6 +556,7 @@ struct Allocator {
       // consecutive "new" calls must be different even if the allocated size
       // is zero.
       size = 1;
+      upgraded_from_zero = true;
     }
     CHECK(IsPowerOfTwo(alignment));
     uptr rz_log = ComputeRZLog(size);
@@ -637,6 +639,10 @@ struct Allocator {
       *shadow = fl.poison_partial ? (size & (ASAN_SHADOW_GRANULARITY - 1)) : 0;
     }
 
+    if (upgraded_from_zero)
+      PoisonShadow(user_beg, ASAN_SHADOW_GRANULARITY,
+                   kAsanHeapLeftRedzoneMagic);
+
     AsanStats &thread_stats = GetCurrentThreadStats();
     thread_stats.mallocs++;
     thread_stats.malloced += size;

compiler-rt/test/asan/TestCases/zero_alloc.cpp

@@ -1,9 +1,5 @@
 // RUN: %clang_asan -Wno-alloc-size -fsanitize-recover=address %s -o %t && %env_asan_opts=halt_on_error=0 %run %t 2>&1 | FileCheck %s
 
-// ASan doesn't catch this because internally it translates 0-byte allocations
-// into 1-byte
-// XFAIL: *
-
 #include <malloc.h>
 #include <stdio.h>