Performing computations on size_t variables that we feed to
xmalloc and friends can be dangerous, as an integer overflow
can cause us to allocate a much smaller chunk than we
realized.
We already have unsigned_add_overflows(), but let's add
unsigned_mult_overflows() to that. Furthermore, rather than
have each site manually check and die on overflow, we can
provide some helpers that will:
- promote the arguments to size_t, so that we know we are
doing our computation in the same size of integer that
will ultimately be fed to xmalloc
- check and die on overflow
- return the result so that computations can be done in
the parameter list of xmalloc.
These functions are a lot uglier to use than normal
arithmetic operators (you have to do "st_add(foo, bar)"
instead of "foo + bar"). To at least limit the damage, we
also provide multi-valued versions. So rather than:
st_add(st_add(a, b), st_add(c, d));
you can write:
st_add4(a, b, c, d);
This isn't nearly as elegant as a varargs function, but it's
a lot harder to get it wrong. You don't have to remember to
add a sentinel value at the end, and the compiler will
complain if you get the number of arguments wrong. This
patch adds only the numbered variants required to convert
the current code base; we can easily add more later if
needed.
Signed-off-by: Jeff King <peff@xxxxxxxx>
---
The st_* names aren't amazing, but I think they mostly work. Suggestions
welcome, but please keep bikeshedding to a minimum. :)
I almost went with checked_add(), checked_mult(), etc. But these
inherently promote their arguments to size_t, so:
int x = checked_add(a, b);
is buggy; the user _should_ be reminded of the result type in each call.
These could also build on compiler intrinsics for extra speed. I'm happy
to do that as a follow-up, but I doubt it really matters in practice.
We're about to call malloc() in all cases, so an extra integer
computation is almost certainly irrelevant. So I went for simplicity to
start with.
git-compat-util.h | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)
diff --git a/git-compat-util.h b/git-compat-util.h
index 693a336..0c65033 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -96,6 +96,14 @@
#define unsigned_add_overflows(a, b) \
((b) > maximum_unsigned_value_of_type(a) - (a))
+/*
+ * Returns true if the multiplication of "a" and "b" will
+ * overflow. The types of "a" and "b" must match and must be unsigned.
+ * Note that this macro evaluates "a" twice!
+ */
+#define unsigned_mult_overflows(a, b) \
+ ((a) && (b) > maximum_unsigned_value_of_type(a) / (a))
+
#ifdef __GNUC__
#define TYPEOF(x) (__typeof__(x))
#else
@@ -713,6 +721,32 @@ extern void release_pack_memory(size_t);
typedef void (*try_to_free_t)(size_t);
extern try_to_free_t set_try_to_free_routine(try_to_free_t);
+static inline size_t st_add(size_t a, size_t b)
+{
+ if (unsigned_add_overflows(a, b))
+ die("size_t overflow: %"PRIuMAX" + %"PRIuMAX,
+ (uintmax_t)a, (uintmax_t)b);
+ return a + b;
+}
+#define st_add3(a,b,c) st_add((a),st_add((b),(c)))
+#define st_add4(a,b,c,d) st_add((a),st_add3((b),(c),(d)))
+
+static inline size_t st_mult(size_t a, size_t b)
+{
+ if (unsigned_mult_overflows(a, b))
+ die("size_t overflow: %"PRIuMAX" * %"PRIuMAX,
+ (uintmax_t)a, (uintmax_t)b);
+ return a * b;
+}
+
+static inline size_t st_sub(size_t a, size_t b)
+{
+ if (a < b)
+ die("size_t underflow: %"PRIuMAX" - %"PRIuMAX,
+ (uintmax_t)a, (uintmax_t)b);
+ return a - b;
+}
+
#ifdef HAVE_ALLOCA_H
# include <alloca.h>
# define xalloca(size) (alloca(size))
--
2.7.1.572.gf718037
--
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