Return requested size for malloc_usable_size when MALLOC_CHECK_ > 0

[BZ #1349]

malloc_usable_size returns the usable size in an allocated chunk,
which may be >= the requested size. In the case of MALLOC_CHECK_ being
exported to > 0 however, only the requested size is usable, since a
magic value is written at the end of the request size to trap writes
beyond request bounds. Hence, when MALLOC_CHECK_ is exported to > 0,
malloc_usable_size() should return the request size.

4 files changed, 86 insertions, 5 deletions

diff --git a/malloc/Makefile b/malloc/Makefile
index 5d6d716a78..55c675b2cd 100644
--- a/malloc/Makefile
+++ b/malloc/Makefile
@@ -1,5 +1,4 @@
-# Copyright (C) 1991-2003, 2005, 2006, 2007, 2009, 2011, 2012
-# Free Software Foundation, Inc.
+# Copyright (C) 1991-2012 Free Software Foundation, Inc.
 # This file is part of the GNU C Library.
 
 # The GNU C Library is free software; you can redistribute it and/or
@@ -26,7 +25,7 @@ all:
 dist-headers := malloc.h
 headers := $(dist-headers) obstack.h mcheck.h
 tests := mallocbug tst-malloc tst-valloc tst-calloc tst-obstack \
-	 tst-mallocstate tst-mcheck tst-mallocfork tst-trim1
+	 tst-mallocstate tst-mcheck tst-mallocfork tst-trim1 tst-malloc-usable
 test-srcs = tst-mtrace
 
 routines = malloc morecore mcheck mtrace obstack
@@ -116,6 +115,7 @@ endif
 endif
 
 tst-mcheck-ENV = MALLOC_CHECK_=3
+tst-malloc-usable-ENV = MALLOC_CHECK_=3
 
 CPPFLAGS-malloc.c += -DPER_THREAD
 # Uncomment this for test releases.  For public releases it is too expensive.
diff --git a/malloc/hooks.c b/malloc/hooks.c
index 8a34c78488..b38dffbdf6 100644
--- a/malloc/hooks.c
+++ b/malloc/hooks.c
@@ -1,5 +1,5 @@
 /* Malloc implementation for multiple threads without lock contention.
-   Copyright (C) 2001-2009, 2011, 2012 Free Software Foundation, Inc.
+   Copyright (C) 2001-2012 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Wolfram Gloger <wg@malloc.de>, 2001.
 
@@ -89,6 +89,35 @@ __malloc_check_init()
 
 #define MAGICBYTE(p) ( ( ((size_t)p >> 3) ^ ((size_t)p >> 11)) & 0xFF )
 
+/* Visualize the chunk as being partitioned into blocks of 256 bytes from the
+   highest address of the chunk, downwards.  The beginning of each block tells
+   us the size of the previous block, up to the actual size of the requested
+   memory.  Our magic byte is right at the end of the requested size, so we
+   must reach it with this iteration, otherwise we have witnessed a memory
+   corruption.  */
+static size_t
+malloc_check_get_size(mchunkptr p)
+{
+  size_t size;
+  unsigned char c;
+  unsigned char magic = MAGICBYTE(p);
+
+  assert(using_malloc_checking == 1);
+
+  for (size = chunksize(p) - 1 + (chunk_is_mmapped(p) ? 0 : SIZE_SZ);
+       (c = ((unsigned char*)p)[size]) != magic;
+       size -= c) {
+    if(c<=0 || size<(c+2*SIZE_SZ)) {
+      malloc_printerr(check_action, "malloc_check_get_size: memory corruption",
+		      chunk2mem(p));
+      return 0;
+    }
+  }
+
+  /* chunk2mem size.  */
+  return size - 2*SIZE_SZ;
+}
+
 /* Instrument a chunk with overrun detector byte(s) and convert it
    into a user pointer with requested size sz. */
 
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 0f1796c913..bd562df959 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1,5 +1,5 @@
 /* Malloc implementation for multiple threads without lock contention.
-   Copyright (C) 1996-2009, 2010, 2011, 2012 Free Software Foundation, Inc.
+   Copyright (C) 1996-2012 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Wolfram Gloger <wg@malloc.de>
    and Doug Lea <dl@cs.oswego.edu>, 2001.
@@ -4563,6 +4563,9 @@ musable(void* mem)
   mchunkptr p;
   if (mem != 0) {
     p = mem2chunk(mem);
+
+    if (__builtin_expect(using_malloc_checking == 1, 0))
+      return malloc_check_get_size(p);
     if (chunk_is_mmapped(p))
       return chunksize(p) - 2*SIZE_SZ;
     else if (inuse(p))
diff --git a/malloc/tst-malloc-usable.c b/malloc/tst-malloc-usable.c
new file mode 100644
index 0000000000..18decd852c
--- /dev/null
+++ b/malloc/tst-malloc-usable.c
@@ -0,0 +1,49 @@
+/* Ensure that malloc_usable_size returns the request size with
+   MALLOC_CHECK_ exported to a positive value.
+
+   Copyright (C) 2012 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <malloc.h>
+#include <string.h>
+#include <stdio.h>
+
+static int
+do_test (void)
+{
+  size_t usable_size;
+  void *p = malloc (7);
+  if (!p)
+    {
+      printf ("memory allocation failed\n");
+      return 1;
+    }
+
+  usable_size = malloc_usable_size (p);
+  if (usable_size != 7)
+    {
+      printf ("malloc_usable_size: expected 7 but got %zu\n", usable_size);
+      return 1;
+    }
+
+  memset (p, 0, usable_size);
+  free (p);
+  return 0;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"