[hwasan] Disable malloc-fill by default.

[eugenis]

Summary

Non-zero malloc fill is causing way too many hard to debug issues.

[kcc]

LGTM (even though it's sad...)

compiler-rt/lib/hwasan/hwasan_flags.inc

 // Tagging can be reenabled with __hwasan_enable_allocator_tagging().
 HWASAN_FLAG(bool, disable_allocator_tagging, false, "")
 
 // If false, use simple increment of a thread local counter to generate new
 // tags.
 HWASAN_FLAG(bool, random_tags, true, "")
 
 HWASAN_FLAG(
-    int, max_malloc_fill_size, 0x1000,  // By default, fill only the first 4K.
+    int, max_malloc_fill_size, 0,
     "HWASan allocator flag. max_malloc_fill_size is the maximal amount of "
     "bytes that will be filled with malloc_fill_byte on malloc.")
 
 HWASAN_FLAG(bool, free_checks_tail_magic, 1,
     "If set, free() will check the magic values "
     "to the right of the allocated object "
     "if the allocation size is not a divident of the granule size")
 HWASAN_FLAG(

compiler-rt/test/hwasan/TestCases/malloc_fill.cpp

 // Check that we fill malloc-ed memory correctly.
 // RUN: %clangxx_hwasan %s -o %t
-// RUN: %run %t | FileCheck %s
+// RUN: %run %t | FileCheck %s --check-prefix=CHECK-0
+// RUN: %env_hwasan_opts=max_malloc_fill_size=20 %run %t | FileCheck %s --check-prefix=CHECK-20-be
+// RUN: %env_hwasan_opts=max_malloc_fill_size=0:malloc_fill_byte=8 %run %t | FileCheck %s --check-prefix=CHECK-0
 // RUN: %env_hwasan_opts=max_malloc_fill_size=10:malloc_fill_byte=8 %run %t | FileCheck %s --check-prefix=CHECK-10-8
 // RUN: %env_hwasan_opts=max_malloc_fill_size=20:malloc_fill_byte=171 %run %t | FileCheck %s --check-prefix=CHECK-20-ab
 
 #include <stdio.h>
 
 #include "utils.h"
 
 int main(int argc, char **argv) {
   // With asan allocator this makes sure we get memory from mmap.
   static const int kSize = 1 << 25;
   unsigned char *x = new unsigned char[kSize];
   untag_printf("-");
   for (int i = 0; i <= 32; i++) {
     untag_printf("%02x", x[i]);
   }
   untag_printf("-\n");
   delete [] x;
 }
 
-// CHECK: -bebebebebebebebebebebebebebebebebebebebebebebebebebebebebebebebebe-
+// CHECK-0: -000000000000000000000000000000000000000000000000000000000000000000-
+// CHECK-20-be: -bebebebebebebebebebebebebebebebebebebebe00000000000000000000000000-
 // CHECK-10-8: -080808080808080808080000000000000000000000000000000000000000000000-
 // CHECK-20-ab: -abababababababababababababababababababab00000000000000000000000000-