Libguestfs - Nov 2021 - [PATCH nbdkit 0/2] common: Add checked-overflow macros

In common/vector/vector.c use GCC/Clang built-in overflow operators.

The first patch is a neutral change which adds comments.  The second
patch is the actual change.

Add a new header "checked-overflow.h" which has the purpose of
isolating the use of the built-ins to one file (in case we need to add
a new compiler later).  Then use this in generic_vector_reserve.

I tested this with GCC 11 and Clang 13.

I verified by disassembly that "jo" (jump overflow) / "jno"
is used
where it was not used previously, by both compilers.

Rich.

This commit makes no changes, it simply adds comments and breaks out
the multiplcation into a local variable.
---
 common/utils/vector.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/common/utils/vector.c b/common/utils/vector.c
index d7120399..dff051e9 100644
--- a/common/utils/vector.c
+++ b/common/utils/vector.c
@@ -42,20 +42,31 @@ int
 generic_vector_reserve (struct generic_vector *v, size_t n, size_t itemsize)
 {
   void *newptr;
-  size_t reqcap, newcap;
+  size_t reqcap, reqbytes, newcap, newbytes;
 
+  /* New capacity requested.  We must allocate this minimum (or fail). */
   reqcap = v->cap + n;
-  if (reqcap * itemsize < v->cap * itemsize) {
+  reqbytes = reqcap * itemsize;
+  if (reqbytes < v->cap * itemsize) {
     errno = ENOMEM;
     return -1; /* overflow */
   }
 
+  /* However for the sake of optimization, scale buffer by 3/2 so that
+   * repeated reservations don't call realloc often.
+   */
   newcap = v->cap + (v->cap + 1) / 2;
+  newbytes = newcap * itemsize;
 
-  if (newcap * itemsize < reqcap * itemsize)
+  if (newbytes < reqbytes) {
+    /* If that either overflows or is less than the minimum requested,
+     * fall back to the requested capacity.
+     */
     newcap = reqcap;
+    newbytes = reqbytes;
+  }
 
-  newptr = realloc (v->ptr, newcap * itemsize);
+  newptr = realloc (v->ptr, newbytes);
   if (newptr == NULL)
     return -1;
   v->ptr = newptr;
-- 
2.32.0

---
 common/include/Makefile.am        |  1 +
 common/utils/Makefile.am          |  3 +-
 common/include/checked-overflow.h | 61 +++++++++++++++++++++++++++++++
 common/utils/vector.c             | 26 +++++++++----
 4 files changed, 82 insertions(+), 9 deletions(-)

diff --git a/common/include/Makefile.am b/common/include/Makefile.am
index a7d0d026..52d97216 100644
--- a/common/include/Makefile.am
+++ b/common/include/Makefile.am
@@ -37,6 +37,7 @@ EXTRA_DIST = \
 	ascii-ctype.h \
 	ascii-string.h \
 	byte-swapping.h \
+	checked-overflow.h \
 	exit-with-parent.h \
 	isaligned.h \
 	ispowerof2.h \
diff --git a/common/utils/Makefile.am b/common/utils/Makefile.am
index 55415535..012a5c25 100644
--- a/common/utils/Makefile.am
+++ b/common/utils/Makefile.am
@@ -52,6 +52,7 @@ libutils_la_SOURCES = \
 	$(NULL)
 libutils_la_CPPFLAGS = \
 	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/include \
 	$(NULL)
 libutils_la_CFLAGS = \
 	$(WARNINGS_CFLAGS) \
@@ -101,7 +102,7 @@ test_quotes_CPPFLAGS = -I$(srcdir)
 test_quotes_CFLAGS = $(WARNINGS_CFLAGS)
 
 test_vector_SOURCES = test-vector.c vector.c vector.h bench.h
-test_vector_CPPFLAGS = -I$(srcdir)
+test_vector_CPPFLAGS = -I$(srcdir) -I$(top_srcdir)/common/include
 test_vector_CFLAGS = $(WARNINGS_CFLAGS)
 
 bench: test-vector
diff --git a/common/include/checked-overflow.h
b/common/include/checked-overflow.h
new file mode 100644
index 00000000..b571e2c6
--- /dev/null
+++ b/common/include/checked-overflow.h
@@ -0,0 +1,61 @@
+/* nbdkit
+ * Copyright (C) 2013-2021 Red Hat Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS
IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+/* This header file defines functions for checking overflow in common
+ * integer arithmetic operations.
+ *
+ * It uses GCC/Clang built-ins: a possible future enhancement is to
+ * provide fallbacks in plain C or for other compilers.  The only
+ * purpose of having a header file for this is to have a single place
+ * where we would extend this in future.
+ */
+
+#ifndef NBDKIT_CHECKED_OVERFLOW_H
+#define NBDKIT_CHECKED_OVERFLOW_H
+
+#if !defined(__GNUC__) && !defined(__clang__)
+#error "this file may need to be ported to your compiler"
+#endif
+
+/* Add two uint64_t values.  Returns true if overflow happened. */
+#define ADD_UINT64_T_OVERFLOW(a, b, r) __builtin_add_overflow((a), (b), (r))
+
+/* Multiply two uint64_t values.  Returns true if overflow happened. */
+#define MUL_UINT64_T_OVERFLOW(a, b, r) __builtin_mul_overflow((a), (b), (r))
+
+/* Add two size_t values.  Returns true if overflow happened. */
+#define ADD_SIZE_T_OVERFLOW(a, b, r) __builtin_add_overflow((a), (b), (r))
+
+/* Multiple two size_t values.  Returns true if overflow happened. */
+#define MUL_SIZE_T_OVERFLOW(a, b, r) __builtin_mul_overflow((a), (b), (r))
+
+#endif /* NBDKIT_CHECKED_OVERFLOW_H */
diff --git a/common/utils/vector.c b/common/utils/vector.c
index dff051e9..e4ea7f3f 100644
--- a/common/utils/vector.c
+++ b/common/utils/vector.c
@@ -36,6 +36,7 @@
 #include <stdlib.h>
 #include <errno.h>
 
+#include "checked-overflow.h"
 #include "vector.h"
 
 int
@@ -44,21 +45,30 @@ generic_vector_reserve (struct generic_vector *v, size_t n,
size_t itemsize)
   void *newptr;
   size_t reqcap, reqbytes, newcap, newbytes;
 
-  /* New capacity requested.  We must allocate this minimum (or fail). */
-  reqcap = v->cap + n;
-  reqbytes = reqcap * itemsize;
-  if (reqbytes < v->cap * itemsize) {
+  /* New capacity requested.  We must allocate this minimum (or fail).
+   * reqcap = v->cap + n
+   * reqbytes = reqcap * itemsize
+   */
+  if (ADD_SIZE_T_OVERFLOW (v->cap, n, &reqcap) ||
+      MUL_SIZE_T_OVERFLOW (reqcap, itemsize, &reqbytes)) {
     errno = ENOMEM;
-    return -1; /* overflow */
+    return -1;
   }
 
   /* However for the sake of optimization, scale buffer by 3/2 so that
    * repeated reservations don't call realloc often.
+   * newcap = v->cap + (v->cap + 1) / 2
+   * newbytes = newcap * itemsize
    */
-  newcap = v->cap + (v->cap + 1) / 2;
-  newbytes = newcap * itemsize;
-
+  if (ADD_SIZE_T_OVERFLOW (v->cap, 1, &newcap))
+    goto fallback;
+  newcap /= 2;
+  if (ADD_SIZE_T_OVERFLOW (v->cap, newcap, &newcap))
+    goto fallback;
+  if (MUL_SIZE_T_OVERFLOW (newcap, itemsize, &newbytes))
+    goto fallback;
   if (newbytes < reqbytes) {
+  fallback:
     /* If that either overflows or is less than the minimum requested,
      * fall back to the requested capacity.
      */
-- 
2.32.0

On Tue, Nov 09, 2021 at 05:49:16PM +0000, Richard W.M. Jones
wrote:
> In common/vector/vector.c use GCC/Clang built-in overflow operators.
> 
> The first patch is a neutral change which adds comments.  The second
> patch is the actual change.
> 
> Add a new header "checked-overflow.h" which has the purpose of
> isolating the use of the built-ins to one file (in case we need to add
> a new compiler later).  Then use this in generic_vector_reserve.
> 
> I tested this with GCC 11 and Clang 13.
That's the modern edge of the spectrum; will we be interfering with
compilation on older distros with older compilers?  I guess CI testing
can help find that out quickly.
> 
> I verified by disassembly that "jo" (jump overflow) /
"jno" is used
> where it was not used previously, by both compilers.
Nice, and an argument in favor of doing this.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org