[PATCH] D31295: Add free_on_realloc_zero=true flag for compatibility with allocators which allow a realloc(p, 0) and don't free the pointer.

[Filipe Cabecinhas via Phabricator via llvm-commits]

filcab created this revision.
Herald added a subscriber: kubamracek.

I know of two implementations that do this (ASan is not protecting against accessing the returned memory for now, just like malloc(0)):
SIE libc on the PS4
dlmalloc has a flag for this

This allows us to properly support this behaviour.


https://reviews.llvm.org/D31295

Files:
  lib/asan/asan_allocator.cc
  lib/asan/asan_flags.inc
  test/asan/TestCases/realloc.cc


Index: test/asan/TestCases/realloc.cc
===================================================================
--- /dev/null
+++ test/asan/TestCases/realloc.cc
@@ -0,0 +1,20 @@
+// RUN: %clangxx_asan -O0 %s -o %t
+// Default is true (free on realloc to 0 size)
+// RUN: %run %t 2>&1 | FileCheck %s
+// RUN: %env_asan_opts=free_on_realloc_zero=true %run %t 2>&1 | FileCheck %s
+// RUN: %env_asan_opts=free_on_realloc_zero=false %run %t 2>&1 | FileCheck %s -check-prefix=NO-FREE -allow-empty
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv) {
+  void *p = malloc(argc);
+  p = realloc(p, 0);
+  if (p)
+    // NO-FREE: Allocated something on realloc(p, 0)
+    fprintf(stderr, "Allocated something on realloc(p, 0)\n");
+  else
+    // CHECK: realloc(p, 0) returned nullptr
+    fprintf(stderr, "realloc(p, 0) returned nullptr\n");
+  free(p);
+}
Index: lib/asan/asan_flags.inc
===================================================================
--- lib/asan/asan_flags.inc
+++ lib/asan/asan_flags.inc
@@ -148,3 +148,6 @@
           "(WARNING: USE AT YOUR OWN RISK!)")
 ASAN_FLAG(bool, use_odr_indicator, false,
           "Use special ODR indicator symbol for ODR violation detection")
+ASAN_FLAG(bool, free_on_realloc_zero, true,
+          "realloc(p, 0) is equivalent to free(p). If set to false, realloc(p, "
+          "0) returns a pointer to an allocated space which can not be used.")
Index: lib/asan/asan_allocator.cc
===================================================================
--- lib/asan/asan_allocator.cc
+++ lib/asan/asan_allocator.cc
@@ -800,8 +800,13 @@
   if (!p)
     return instance.Allocate(size, 8, stack, FROM_MALLOC, true);
   if (size == 0) {
-    instance.Deallocate(p, 0, stack, FROM_MALLOC);
-    return nullptr;
+    if (flags()->free_on_realloc_zero) {
+      instance.Deallocate(p, 0, stack, FROM_MALLOC);
+      return nullptr;
+    } else {
+      // Allocate a size of 1 if we shouldn't free() on Realloc to 0
+      size = 1;
+    }
   }
   return instance.Reallocate(p, size, stack);
 }


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