btrfs-progs: kernel-lib: sync include/overflow.h

Sync current version with improved checks.

Signed-off-by: David Sterba <dsterba@suse.com>

kernel-lib/overflow.h

@@ -2,8 +2,12 @@
 #ifndef __LINUX_OVERFLOW_H
 #define __LINUX_OVERFLOW_H
 
+#include <stdbool.h>
+
 /*
- * It would seem more obvious to do something like
+ * We need to compute the minimum and maximum values representable in a given
+ * type. These macros may also be useful elsewhere. It would seem more obvious
+ * to do something like:
  *
  * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
  * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
@@ -36,128 +40,57 @@
 #define is_non_negative(a) ((a) > 0 || (a) == 0)
 #define is_negative(a) (!(is_non_negative(a)))
 
-/* Checking for unsigned overflow is relatively easy without causing UB. */
+/* Copied from linux.git/include/compiler_attributes.h */
-#define __unsigned_add_overflow(a, b, d) ({	\
-	typeof(a) __a = (a);			\
-	typeof(b) __b = (b);			\
-	typeof(d) __d = (d);			\
-	(void) (&__a == &__b);			\
-	(void) (&__a == __d);			\
-	*__d = __a + __b;			\
-	*__d < __a;				\
-})
-#define __unsigned_sub_overflow(a, b, d) ({	\
-	typeof(a) __a = (a);			\
-	typeof(b) __b = (b);			\
-	typeof(d) __d = (d);			\
-	(void) (&__a == &__b);			\
-	(void) (&__a == __d);			\
-	*__d = __a - __b;			\
-	__a < __b;				\
-})
 /*
- * If one of a or b is a compile-time constant, this avoids a division.
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-warn_005funused_005fresult-function-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#nodiscard-warn-unused-result
  */
-#define __unsigned_mul_overflow(a, b, d) ({		\
+#define __must_check                    __attribute__((__warn_unused_result__))
-	typeof(a) __a = (a);				\
-	typeof(b) __b = (b);				\
-	typeof(d) __d = (d);				\
-	(void) (&__a == &__b);				\
-	(void) (&__a == __d);				\
-	*__d = __a * __b;				\
-	__builtin_constant_p(__b) ?			\
-	  __b > 0 && __a > type_max(typeof(__a)) / __b : \
-	  __a > 0 && __b > type_max(typeof(__b)) / __a;	 \
-})
 
 /*
- * For signed types, detecting overflow is much harder, especially if
+ * Allows for effectively applying __must_check to a macro so we can have
- * we want to avoid UB. But the interface of these macros is such that
+ * both the type-agnostic benefits of the macros while also being able to
- * we must provide a result in *d, and in fact we must produce the
+ * enforce that the return value is, in fact, checked.
- * result promised by gcc's builtins, which is simply the possibly
- * wrapped-around value. Fortunately, we can just formally do the
- * operations in the widest relevant unsigned type (u64) and then
- * truncate the result - gcc is smart enough to generate the same code
- * with and without the (u64) casts.
  */
+static inline bool __must_check __must_check_overflow(bool overflow)
+{
+	return overflow;
+}
 
 /*
- * Adding two signed integers can overflow only if they have the same
+ * For simplicity and code hygiene, the fallback code below insists on
- * sign, and overflow has happened iff the result has the opposite
+ * a, b and *d having the same type (similar to the min() and max()
- * sign.
+ * macros), whereas gcc's type-generic overflow checkers accept
+ * different types. Hence we don't just make check_add_overflow an
+ * alias for __builtin_add_overflow, but add type checks similar to
+ * below.
  */
-#define __signed_add_overflow(a, b, d) ({	\
+#define check_add_overflow(a, b, d) __must_check_overflow(({	\
 	typeof(a) __a = (a);			\
 	typeof(b) __b = (b);			\
 	typeof(d) __d = (d);			\
 	(void) (&__a == &__b);			\
 	(void) (&__a == __d);			\
-	*__d = (u64)__a + (u64)__b;		\
+	__builtin_add_overflow(__a, __b, __d);	\
-	(((~(__a ^ __b)) & (*__d ^ __a))	\
+}))
-		& type_min(typeof(__a))) != 0;	\
-})
 
-/*
+#define check_sub_overflow(a, b, d) __must_check_overflow(({	\
- * Subtraction is similar, except that overflow can now happen only
- * when the signs are opposite. In this case, overflow has happened if
- * the result has the opposite sign of a.
- */
-#define __signed_sub_overflow(a, b, d) ({	\
 	typeof(a) __a = (a);			\
 	typeof(b) __b = (b);			\
 	typeof(d) __d = (d);			\
 	(void) (&__a == &__b);			\
 	(void) (&__a == __d);			\
-	*__d = (u64)__a - (u64)__b;		\
+	__builtin_sub_overflow(__a, __b, __d);	\
-	((((__a ^ __b)) & (*__d ^ __a))		\
+}))
-		& type_min(typeof(__a))) != 0;	\
-})
 
-/*
+#define check_mul_overflow(a, b, d) __must_check_overflow(({	\
- * Signed multiplication is rather hard. gcc always follows C99, so
- * division is truncated towards 0. This means that we can write the
- * overflow check like this:
- *
- * (a > 0 && (b > MAX/a || b < MIN/a)) ||
- * (a < -1 && (b > MIN/a || b < MAX/a) ||
- * (a == -1 && b == MIN)
- *
- * The redundant casts of -1 are to silence an annoying -Wtype-limits
- * (included in -Wextra) warning: When the type is u8 or u16, the
- * __b_c_e in check_mul_overflow obviously selects
- * __unsigned_mul_overflow, but unfortunately gcc still parses this
- * code and warns about the limited range of __b.
- */
-
-#define __signed_mul_overflow(a, b, d) ({				\
 	typeof(a) __a = (a);			\
 	typeof(b) __b = (b);			\
 	typeof(d) __d = (d);			\
-	typeof(a) __tmax = type_max(typeof(a));				\
-	typeof(a) __tmin = type_min(typeof(a));				\
 	(void) (&__a == &__b);			\
 	(void) (&__a == __d);			\
-	*__d = (u64)__a * (u64)__b;					\
+	__builtin_mul_overflow(__a, __b, __d);	\
-	(__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||	\
+}))
-	(__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
-	(__b == (typeof(__b))-1 && __a == __tmin);			\
-})
-
-
-#define check_add_overflow(a, b, d)					\
-	__builtin_choose_expr(is_signed_type(typeof(a)),		\
-			__signed_add_overflow(a, b, d),			\
-			__unsigned_add_overflow(a, b, d))
-
-#define check_sub_overflow(a, b, d)					\
-	__builtin_choose_expr(is_signed_type(typeof(a)),		\
-			__signed_sub_overflow(a, b, d),			\
-			__unsigned_sub_overflow(a, b, d))
-
-#define check_mul_overflow(a, b, d)					\
-	__builtin_choose_expr(is_signed_type(typeof(a)),		\
-			__signed_mul_overflow(a, b, d),			\
-			__unsigned_mul_overflow(a, b, d))
 
 /** check_shl_overflow() - Calculate a left-shifted value and check overflow
  *
@@ -176,9 +109,9 @@
  * - 'a << s' sets the sign bit, if any, in '*d'.
  *
  * '*d' will hold the results of the attempted shift, but is not
- * considered "safe for use" if false is returned.
+ * considered "safe for use" if true is returned.
  */
-#define check_shl_overflow(a, s, d) ({					\
+#define check_shl_overflow(a, s, d) __must_check_overflow(({		\
 	typeof(a) _a = a;						\
 	typeof(s) _s = s;						\
 	typeof(d) _d = d;						\
@@ -188,7 +121,70 @@
 	*_d = (_a_full << _to_shift);					\
 	(_to_shift != _s || is_negative(*_d) || is_negative(_a) ||	\
 	(*_d >> _to_shift) != _a);					\
-})
+}))
+
+/**
+ * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
+ *
+ * @factor1: first factor
+ * @factor2: second factor
+ *
+ * Returns: calculate @factor1 * @factor2, both promoted to size_t,
+ * with any overflow causing the return value to be SIZE_MAX. The
+ * lvalue must be size_t to avoid implicit type conversion.
+ */
+static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
+{
+	size_t bytes;
+
+	if (check_mul_overflow(factor1, factor2, &bytes))
+		return SIZE_MAX;
+
+	return bytes;
+}
+
+/**
+ * size_add() - Calculate size_t addition with saturation at SIZE_MAX
+ *
+ * @addend1: first addend
+ * @addend2: second addend
+ *
+ * Returns: calculate @addend1 + @addend2, both promoted to size_t,
+ * with any overflow causing the return value to be SIZE_MAX. The
+ * lvalue must be size_t to avoid implicit type conversion.
+ */
+static inline size_t __must_check size_add(size_t addend1, size_t addend2)
+{
+	size_t bytes;
+
+	if (check_add_overflow(addend1, addend2, &bytes))
+		return SIZE_MAX;
+
+	return bytes;
+}
+
+/**
+ * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
+ *
+ * @minuend: value to subtract from
+ * @subtrahend: value to subtract from @minuend
+ *
+ * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
+ * with any overflow causing the return value to be SIZE_MAX. For
+ * composition with the size_add() and size_mul() helpers, neither
+ * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
+ * The lvalue must be size_t to avoid implicit type conversion.
+ */
+static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
+{
+	size_t bytes;
+
+	if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
+	    check_sub_overflow(minuend, subtrahend, &bytes))
+		return SIZE_MAX;
+
+	return bytes;
+}
 
 /**
  * array_size() - Calculate size of 2-dimensional array.
@@ -201,15 +197,7 @@
  * Returns: number of bytes needed to represent the array or SIZE_MAX on
  * overflow.
  */
-static inline size_t array_size(size_t a, size_t b)
+#define array_size(a, b)	size_mul(a, b)
-{
-	size_t bytes;
-
-	if (check_mul_overflow(a, b, &bytes))
-		return SIZE_MAX;
-
-	return bytes;
-}
 
 /**
  * array3_size() - Calculate size of 3-dimensional array.
@@ -223,48 +211,41 @@ static inline size_t array_size(size_t a, size_t b)
  * Returns: number of bytes needed to represent the array or SIZE_MAX on
  * overflow.
  */
-static inline size_t array3_size(size_t a, size_t b, size_t c)
+#define array3_size(a, b, c)	size_mul(size_mul(a, b), c)
-{
-	size_t bytes;
-
-	if (check_mul_overflow(a, b, &bytes))
-		return SIZE_MAX;
-	if (check_mul_overflow(bytes, c, &bytes))
-		return SIZE_MAX;
-
-	return bytes;
-}
-
-/*
- * Compute a*b+c, returning SIZE_MAX on overflow. Internal helper for
- * struct_size() below.
- */
-static inline size_t __ab_c_size(size_t a, size_t b, size_t c)
-{
-	size_t bytes;
-
-	if (check_mul_overflow(a, b, &bytes))
-		return SIZE_MAX;
-	if (check_add_overflow(bytes, c, &bytes))
-		return SIZE_MAX;
-
-	return bytes;
-}
 
 /**
- * struct_size() - Calculate size of structure with trailing array.
+ * flex_array_size() - Calculate size of a flexible array member
- * @p: Pointer to the structure.
+ *                     within an enclosing structure.
- * @member: Name of the array member.
- * @n: Number of elements in the array.
  *
- * Calculates size of memory needed for structure @p followed by an
+ * @p: Pointer to the structure.
- * array of @n @member elements.
+ * @member: Name of the flexible array member.
+ * @count: Number of elements in the array.
+ *
+ * Calculates size of a flexible array of @count number of @member
+ * elements, at the end of structure @p.
  *
  * Return: number of bytes needed or SIZE_MAX on overflow.
  */
-#define struct_size(p, member, n)					\
+#define flex_array_size(p, member, count)				\
-	__ab_c_size(n,							\
+	__builtin_choose_expr(__is_constexpr(count),			\
-		    sizeof(*(p)->member) + __must_be_array((p)->member),\
+		(count) * sizeof(*(p)->member) + __must_be_array((p)->member),	\
-		    sizeof(*(p)))
+		size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
+
+/**
+ * struct_size() - Calculate size of structure with trailing flexible array.
+ *
+ * @p: Pointer to the structure.
+ * @member: Name of the array member.
+ * @count: Number of elements in the array.
+ *
+ * Calculates size of memory needed for structure @p followed by an
+ * array of @count number of @member elements.
+ *
+ * Return: number of bytes needed or SIZE_MAX on overflow.
+ */
+#define struct_size(p, member, count)					\
+	__builtin_choose_expr(__is_constexpr(count),			\
+		sizeof(*(p)) + flex_array_size(p, member, count),	\
+		size_add(sizeof(*(p)), flex_array_size(p, member, count)))
 
 #endif /* __LINUX_OVERFLOW_H */