overflow.h

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/* SPDX-License-Identifier: GPL-2.0 OR MIT */
#ifndef __LINUX_OVERFLOW_H
#define __LINUX_OVERFLOW_H
 
#include <linux/compiler.h>
#include <linux/limits.h>
#include <linux/const.h>
 
/*
 * 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)
 *
 * 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 __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_max(t) __type_max(typeof(t))
#define __type_min(T) ((T)((T)-type_max(T)-(T)1))
#define type_min(t) __type_min(typeof(t))
 
 
/*
 * 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 __always_inline bool __must_check __must_check_overflow(bool overflow)
{
    return unlikely(overflow);
}
 
#define check_add_overflow(a, b, d) \
    __must_check_overflow(__builtin_add_overflow(a, b, d))
 
#define wrapping_add(type, a, b)                \
    ({                          \
        type __val;                 \
        __builtin_add_overflow(a, b, &__val);       \
        __val;                      \
    })
 
#define wrapping_assign_add(var, offset)                \
    ({                              \
        typeof(var) *__ptr = &(var);                \
        *__ptr = wrapping_add(typeof(var), *__ptr, offset); \
    })
 
#define check_sub_overflow(a, b, d) \
    __must_check_overflow(__builtin_sub_overflow(a, b, d))
 
#define wrapping_sub(type, a, b)                \
    ({                          \
        type __val;                 \
        __builtin_sub_overflow(a, b, &__val);       \
        __val;                      \
    })
 
#define wrapping_assign_sub(var, offset)                \
    ({                              \
        typeof(var) *__ptr = &(var);                \
        *__ptr = wrapping_sub(typeof(var), *__ptr, offset); \
    })
 
#define check_mul_overflow(a, b, d) \
    __must_check_overflow(__builtin_mul_overflow(a, b, d))
 
#define wrapping_mul(type, a, b)                \
    ({                          \
        type __val;                 \
        __builtin_mul_overflow(a, b, &__val);       \
        __val;                      \
    })
 
#define check_shl_overflow(a, s, d) __must_check_overflow(({        \
    typeof(a) _a = a;                       \
    typeof(s) _s = s;                       \
    typeof(d) _d = d;                       \
    unsigned long long _a_full = _a;                \
    unsigned int _to_shift =                    \
        _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0;        \
    *_d = (_a_full << _to_shift);                   \
    (_to_shift != _s || *_d < 0 || _a < 0 ||            \
    (*_d >> _to_shift) != _a);                  \
}))
 
#define __overflows_type_constexpr(x, T) (          \
    is_unsigned_type(typeof(x)) ?               \
        (x) > type_max(T) :             \
    is_unsigned_type(typeof(T)) ?               \
        (x) < 0 || (x) > type_max(T) :          \
    (x) < type_min(T) || (x) > type_max(T))
 
#define __overflows_type(x, T)      ({  \
    typeof(T) v = 0;            \
    check_add_overflow((x), v, &v);     \
})
 
#define overflows_type(n, T)                    \
    __builtin_choose_expr(__is_constexpr(n),        \
                  __overflows_type_constexpr(n, T), \
                  __overflows_type(n, T))
 
#define range_overflows(start, size, max) ({ \
    typeof(start) start__ = (start); \
    typeof(size) size__ = (size); \
    typeof(max) max__ = (max); \
    (void)(&start__ == &size__); \
    (void)(&start__ == &max__); \
    start__ >= max__ || size__ > max__ - start__; \
})
 
#define range_overflows_t(type, start, size, max) \
    range_overflows((type)(start), (type)(size), (type)(max))
 
#define range_end_overflows(start, size, max) ({ \
    typeof(start) start__ = (start); \
    typeof(size) size__ = (size); \
    typeof(max) max__ = (max); \
    (void)(&start__ == &size__); \
    (void)(&start__ == &max__); \
    start__ > max__ || size__ > max__ - start__; \
})
 
#define range_end_overflows_t(type, start, size, max) \
    range_end_overflows((type)(start), (type)(size), (type)(max))
 
#define castable_to_type(n, T)                      \
    __builtin_choose_expr(__is_constexpr(n),            \
                  !__overflows_type_constexpr(n, T),    \
                  __same_type(n, T))
 
static __always_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;
}
 
static __always_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;
}
 
static __always_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;
}
 
#define array_size(a, b)    size_mul(a, b)
 
#define array3_size(a, b, c)    size_mul(size_mul(a, b), c)
 
#define flex_array_size(p, member, count)               \
    __builtin_choose_expr(__is_constexpr(count),            \
        (count) * sizeof(*(p)->member) + __must_be_array((p)->member),  \
        size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
 
#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)))
 
#define struct_size_t(type, member, count)                  \
    struct_size((type *)NULL, member, count)
 
#define struct_offset(p, member) (offsetof(typeof(*(p)), member))
 
#define __DEFINE_FLEX(type, name, member, count, trailer...)            \
    _Static_assert(__builtin_constant_p(count),             \
               "onstack flex array members require compile-time const count"); \
    union {                                 \
        u8 bytes[struct_size_t(type, member, count)];           \
        type obj;                           \
    } name##_u trailer;                         \
    type *name = (type *)&name##_u
 
#define _DEFINE_FLEX(type, name, member, count, initializer...)         \
    __DEFINE_FLEX(type, name, member, count, = { .obj initializer })
 
#define DEFINE_RAW_FLEX(type, name, member, count)  \
    __DEFINE_FLEX(type, name, member, count, = { })
 
#define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \
    _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, })
 
#define STACK_FLEX_ARRAY_SIZE(name, array)                      \
    (__member_size((name)->array) / sizeof(*(name)->array) +            \
                        __must_be_array((name)->array))
 
#define typeof_flex_counter(FAM)                \
    typeof(_Generic(__flex_counter(FAM),            \
            void *: (size_t)0,          \
            default: *__flex_counter(FAM)))
 
#define overflows_flex_counter_type(TYPE, FAM, COUNT)       \
    (overflows_type(COUNT, typeof_flex_counter(((TYPE *)NULL)->FAM)))
 
#define __set_flex_counter(FAM, COUNT)              \
({                              \
    *_Generic(__flex_counter(FAM),              \
          void *:  &(size_t){ 0 },          \
          default: __flex_counter(FAM)) = (COUNT);  \
})
 
#endif /* __LINUX_OVERFLOW_H */