| /* SPDX-License-Identifier: GPL-2.0 OR MIT */ |
| #ifndef __LINUX_OVERFLOW_H |
| #define __LINUX_OVERFLOW_H |
| |
| #include <linux/compiler.h> |
| #include <linux/limits.h> |
| |
| /* |
| * In the fallback code below, we need to compute the minimum and |
| * maximum values representable in a given type. These macros may also |
| * be useful elsewhere, so we provide them outside the |
| * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block. |
| * |
| * 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) |
| * |
| * Unfortunately, the middle expressions, strictly speaking, have |
| * undefined behaviour, and at least some versions of gcc warn about |
| * the type_max expression (but not if -fsanitize=undefined is in |
| * effect; in that case, the warning is deferred to runtime...). |
| * |
| * The slightly excessive casting in type_min is to make sure the |
| * macros also produce sensible values for the exotic type _Bool. [The |
| * overflow checkers only almost work for _Bool, but that's |
| * a-feature-not-a-bug, since people shouldn't be doing arithmetic on |
| * _Bools. Besides, the gcc builtins don't allow _Bool* as third |
| * argument.] |
| * |
| * Idea stolen from |
| * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html - |
| * credit to Christian Biere. |
| */ |
| #define is_signed_type(type) (((type)(-1)) < (type)1) |
| #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type))) |
| #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T))) |
| #define type_min(T) ((T)((T)-type_max(T)-(T)1)) |
| |
| /* |
| * Avoids triggering -Wtype-limits compilation warning, |
| * while using unsigned data types to check a < 0. |
| */ |
| #define is_non_negative(a) ((a) > 0 || (a) == 0) |
| #define is_negative(a) (!(is_non_negative(a))) |
| |
| /* |
| * Allows for effectively applying __must_check to a macro so we can have |
| * both the type-agnostic benefits of the macros while also being able to |
| * enforce that the return value is, in fact, checked. |
| */ |
| static inline bool __must_check __must_check_overflow(bool overflow) |
| { |
| return unlikely(overflow); |
| } |
| |
| #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW |
| /* |
| * For simplicity and code hygiene, the fallback code below insists on |
| * a, b and *d having the same type (similar to the min() and max() |
| * 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 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); \ |
| __builtin_add_overflow(__a, __b, __d); \ |
| })) |
| |
| #define check_sub_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); \ |
| __builtin_sub_overflow(__a, __b, __d); \ |
| })) |
| |
| #define check_mul_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); \ |
| __builtin_mul_overflow(__a, __b, __d); \ |
| })) |
| |
| #else |
| |
| |
| /* Checking for unsigned overflow is relatively easy without causing UB. */ |
| #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. |
| */ |
| #define __unsigned_mul_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; \ |
| __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 |
| * we want to avoid UB. But the interface of these macros is such that |
| * we must provide a result in *d, and in fact we must produce the |
| * 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. |
| */ |
| |
| /* |
| * Adding two signed integers can overflow only if they have the same |
| * sign, and overflow has happened iff the result has the opposite |
| * sign. |
| */ |
| #define __signed_add_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; \ |
| (((~(__a ^ __b)) & (*__d ^ __a)) \ |
| & type_min(typeof(__a))) != 0; \ |
| }) |
| |
| /* |
| * 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; \ |
| ((((__a ^ __b)) & (*__d ^ __a)) \ |
| & type_min(typeof(__a))) != 0; \ |
| }) |
| |
| /* |
| * 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; \ |
| (__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) __must_check_overflow( \ |
| __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) __must_check_overflow( \ |
| __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) __must_check_overflow( \ |
| __builtin_choose_expr(is_signed_type(typeof(a)), \ |
| __signed_mul_overflow(a, b, d), \ |
| __unsigned_mul_overflow(a, b, d))) |
| |
| #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */ |
| |
| /** check_shl_overflow() - Calculate a left-shifted value and check overflow |
| * |
| * @a: Value to be shifted |
| * @s: How many bits left to shift |
| * @d: Pointer to where to store the result |
| * |
| * Computes *@d = (@a << @s) |
| * |
| * Returns true if '*d' cannot hold the result or when 'a << s' doesn't |
| * make sense. Example conditions: |
| * - 'a << s' causes bits to be lost when stored in *d. |
| * - 's' is garbage (e.g. negative) or so large that the result of |
| * 'a << s' is guaranteed to be 0. |
| * - 'a' is negative. |
| * - '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. |
| */ |
| #define check_shl_overflow(a, s, d) __must_check_overflow(({ \ |
| typeof(a) _a = a; \ |
| typeof(s) _s = s; \ |
| typeof(d) _d = d; \ |
| u64 _a_full = _a; \ |
| unsigned int _to_shift = \ |
| is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ |
| *_d = (_a_full << _to_shift); \ |
| (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ |
| (*_d >> _to_shift) != _a); \ |
| })) |
| |
| /** |
| * array_size() - Calculate size of 2-dimensional array. |
| * |
| * @a: dimension one |
| * @b: dimension two |
| * |
| * Calculates size of 2-dimensional array: @a * @b. |
| * |
| * Returns: number of bytes needed to represent the array or SIZE_MAX on |
| * overflow. |
| */ |
| static inline __must_check size_t array_size(size_t a, size_t b) |
| { |
| size_t bytes; |
| |
| if (check_mul_overflow(a, b, &bytes)) |
| return SIZE_MAX; |
| |
| return bytes; |
| } |
| |
| /** |
| * array3_size() - Calculate size of 3-dimensional array. |
| * |
| * @a: dimension one |
| * @b: dimension two |
| * @c: dimension three |
| * |
| * Calculates size of 3-dimensional array: @a * @b * @c. |
| * |
| * Returns: number of bytes needed to represent the array or SIZE_MAX on |
| * overflow. |
| */ |
| static inline __must_check size_t array3_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_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 __must_check 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. |
| * @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) \ |
| __ab_c_size(count, \ |
| sizeof(*(p)->member) + __must_be_array((p)->member),\ |
| sizeof(*(p))) |
| |
| /** |
| * flex_array_size() - Calculate size of a flexible array member |
| * within an enclosing structure. |
| * |
| * @p: Pointer to the structure. |
| * @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 flex_array_size(p, member, count) \ |
| array_size(count, \ |
| sizeof(*(p)->member) + __must_be_array((p)->member)) |
| |
| #endif /* __LINUX_OVERFLOW_H */ |