LCOV - code coverage report
Current view: top level - include/linux - overflow.h (source / functions) Hit Total Coverage
Test: coverage.info Lines: 3 3 100.0 %
Date: 2023-08-24 13:40:31 Functions: 0 0 -

          Line data    Source code
       1             : /* SPDX-License-Identifier: GPL-2.0 OR MIT */
       2             : #ifndef __LINUX_OVERFLOW_H
       3             : #define __LINUX_OVERFLOW_H
       4             : 
       5             : #include <linux/compiler.h>
       6             : #include <linux/limits.h>
       7             : #include <linux/const.h>
       8             : 
       9             : /*
      10             :  * We need to compute the minimum and maximum values representable in a given
      11             :  * type. These macros may also be useful elsewhere. It would seem more obvious
      12             :  * to do something like:
      13             :  *
      14             :  * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
      15             :  * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
      16             :  *
      17             :  * Unfortunately, the middle expressions, strictly speaking, have
      18             :  * undefined behaviour, and at least some versions of gcc warn about
      19             :  * the type_max expression (but not if -fsanitize=undefined is in
      20             :  * effect; in that case, the warning is deferred to runtime...).
      21             :  *
      22             :  * The slightly excessive casting in type_min is to make sure the
      23             :  * macros also produce sensible values for the exotic type _Bool. [The
      24             :  * overflow checkers only almost work for _Bool, but that's
      25             :  * a-feature-not-a-bug, since people shouldn't be doing arithmetic on
      26             :  * _Bools. Besides, the gcc builtins don't allow _Bool* as third
      27             :  * argument.]
      28             :  *
      29             :  * Idea stolen from
      30             :  * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
      31             :  * credit to Christian Biere.
      32             :  */
      33             : #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
      34             : #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
      35             : #define type_min(T) ((T)((T)-type_max(T)-(T)1))
      36             : 
      37             : /*
      38             :  * Avoids triggering -Wtype-limits compilation warning,
      39             :  * while using unsigned data types to check a < 0.
      40             :  */
      41             : #define is_non_negative(a) ((a) > 0 || (a) == 0)
      42             : #define is_negative(a) (!(is_non_negative(a)))
      43             : 
      44             : /*
      45             :  * Allows for effectively applying __must_check to a macro so we can have
      46             :  * both the type-agnostic benefits of the macros while also being able to
      47             :  * enforce that the return value is, in fact, checked.
      48             :  */
      49             : static inline bool __must_check __must_check_overflow(bool overflow)
      50             : {
      51         491 :         return unlikely(overflow);
      52             : }
      53             : 
      54             : /**
      55             :  * check_add_overflow() - Calculate addition with overflow checking
      56             :  * @a: first addend
      57             :  * @b: second addend
      58             :  * @d: pointer to store sum
      59             :  *
      60             :  * Returns 0 on success.
      61             :  *
      62             :  * *@d holds the results of the attempted addition, but is not considered
      63             :  * "safe for use" on a non-zero return value, which indicates that the
      64             :  * sum has overflowed or been truncated.
      65             :  */
      66             : #define check_add_overflow(a, b, d)     \
      67             :         __must_check_overflow(__builtin_add_overflow(a, b, d))
      68             : 
      69             : /**
      70             :  * check_sub_overflow() - Calculate subtraction with overflow checking
      71             :  * @a: minuend; value to subtract from
      72             :  * @b: subtrahend; value to subtract from @a
      73             :  * @d: pointer to store difference
      74             :  *
      75             :  * Returns 0 on success.
      76             :  *
      77             :  * *@d holds the results of the attempted subtraction, but is not considered
      78             :  * "safe for use" on a non-zero return value, which indicates that the
      79             :  * difference has underflowed or been truncated.
      80             :  */
      81             : #define check_sub_overflow(a, b, d)     \
      82             :         __must_check_overflow(__builtin_sub_overflow(a, b, d))
      83             : 
      84             : /**
      85             :  * check_mul_overflow() - Calculate multiplication with overflow checking
      86             :  * @a: first factor
      87             :  * @b: second factor
      88             :  * @d: pointer to store product
      89             :  *
      90             :  * Returns 0 on success.
      91             :  *
      92             :  * *@d holds the results of the attempted multiplication, but is not
      93             :  * considered "safe for use" on a non-zero return value, which indicates
      94             :  * that the product has overflowed or been truncated.
      95             :  */
      96             : #define check_mul_overflow(a, b, d)     \
      97             :         __must_check_overflow(__builtin_mul_overflow(a, b, d))
      98             : 
      99             : /**
     100             :  * check_shl_overflow() - Calculate a left-shifted value and check overflow
     101             :  * @a: Value to be shifted
     102             :  * @s: How many bits left to shift
     103             :  * @d: Pointer to where to store the result
     104             :  *
     105             :  * Computes *@d = (@a << @s)
     106             :  *
     107             :  * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't
     108             :  * make sense. Example conditions:
     109             :  *
     110             :  * - '@a << @s' causes bits to be lost when stored in *@d.
     111             :  * - '@s' is garbage (e.g. negative) or so large that the result of
     112             :  *   '@a << @s' is guaranteed to be 0.
     113             :  * - '@a' is negative.
     114             :  * - '@a << @s' sets the sign bit, if any, in '*@d'.
     115             :  *
     116             :  * '*@d' will hold the results of the attempted shift, but is not
     117             :  * considered "safe for use" if true is returned.
     118             :  */
     119             : #define check_shl_overflow(a, s, d) __must_check_overflow(({            \
     120             :         typeof(a) _a = a;                                               \
     121             :         typeof(s) _s = s;                                               \
     122             :         typeof(d) _d = d;                                               \
     123             :         u64 _a_full = _a;                                               \
     124             :         unsigned int _to_shift =                                        \
     125             :                 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \
     126             :         *_d = (_a_full << _to_shift);                                     \
     127             :         (_to_shift != _s || is_negative(*_d) || is_negative(_a) ||      \
     128             :         (*_d >> _to_shift) != _a);                                        \
     129             : }))
     130             : 
     131             : #define __overflows_type_constexpr(x, T) (                      \
     132             :         is_unsigned_type(typeof(x)) ?                           \
     133             :                 (x) > type_max(typeof(T)) :                  \
     134             :         is_unsigned_type(typeof(T)) ?                           \
     135             :                 (x) < 0 || (x) > type_max(typeof(T)) :            \
     136             :         (x) < type_min(typeof(T)) || (x) > type_max(typeof(T)))
     137             : 
     138             : #define __overflows_type(x, T)          ({      \
     139             :         typeof(T) v = 0;                        \
     140             :         check_add_overflow((x), v, &v);             \
     141             : })
     142             : 
     143             : /**
     144             :  * overflows_type - helper for checking the overflows between value, variables,
     145             :  *                  or data type
     146             :  *
     147             :  * @n: source constant value or variable to be checked
     148             :  * @T: destination variable or data type proposed to store @x
     149             :  *
     150             :  * Compares the @x expression for whether or not it can safely fit in
     151             :  * the storage of the type in @T. @x and @T can have different types.
     152             :  * If @x is a constant expression, this will also resolve to a constant
     153             :  * expression.
     154             :  *
     155             :  * Returns: true if overflow can occur, false otherwise.
     156             :  */
     157             : #define overflows_type(n, T)                                    \
     158             :         __builtin_choose_expr(__is_constexpr(n),                \
     159             :                               __overflows_type_constexpr(n, T), \
     160             :                               __overflows_type(n, T))
     161             : 
     162             : /**
     163             :  * castable_to_type - like __same_type(), but also allows for casted literals
     164             :  *
     165             :  * @n: variable or constant value
     166             :  * @T: variable or data type
     167             :  *
     168             :  * Unlike the __same_type() macro, this allows a constant value as the
     169             :  * first argument. If this value would not overflow into an assignment
     170             :  * of the second argument's type, it returns true. Otherwise, this falls
     171             :  * back to __same_type().
     172             :  */
     173             : #define castable_to_type(n, T)                                          \
     174             :         __builtin_choose_expr(__is_constexpr(n),                        \
     175             :                               !__overflows_type_constexpr(n, T),        \
     176             :                               __same_type(n, T))
     177             : 
     178             : /**
     179             :  * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
     180             :  * @factor1: first factor
     181             :  * @factor2: second factor
     182             :  *
     183             :  * Returns: calculate @factor1 * @factor2, both promoted to size_t,
     184             :  * with any overflow causing the return value to be SIZE_MAX. The
     185             :  * lvalue must be size_t to avoid implicit type conversion.
     186             :  */
     187             : static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
     188             : {
     189             :         size_t bytes;
     190             : 
     191           6 :         if (check_mul_overflow(factor1, factor2, &bytes))
     192             :                 return SIZE_MAX;
     193             : 
     194             :         return bytes;
     195             : }
     196             : 
     197             : /**
     198             :  * size_add() - Calculate size_t addition with saturation at SIZE_MAX
     199             :  * @addend1: first addend
     200             :  * @addend2: second addend
     201             :  *
     202             :  * Returns: calculate @addend1 + @addend2, both promoted to size_t,
     203             :  * with any overflow causing the return value to be SIZE_MAX. The
     204             :  * lvalue must be size_t to avoid implicit type conversion.
     205             :  */
     206             : static inline size_t __must_check size_add(size_t addend1, size_t addend2)
     207             : {
     208             :         size_t bytes;
     209             : 
     210           6 :         if (check_add_overflow(addend1, addend2, &bytes))
     211             :                 return SIZE_MAX;
     212             : 
     213             :         return bytes;
     214             : }
     215             : 
     216             : /**
     217             :  * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
     218             :  * @minuend: value to subtract from
     219             :  * @subtrahend: value to subtract from @minuend
     220             :  *
     221             :  * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
     222             :  * with any overflow causing the return value to be SIZE_MAX. For
     223             :  * composition with the size_add() and size_mul() helpers, neither
     224             :  * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
     225             :  * The lvalue must be size_t to avoid implicit type conversion.
     226             :  */
     227             : static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
     228             : {
     229             :         size_t bytes;
     230             : 
     231             :         if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
     232             :             check_sub_overflow(minuend, subtrahend, &bytes))
     233             :                 return SIZE_MAX;
     234             : 
     235             :         return bytes;
     236             : }
     237             : 
     238             : /**
     239             :  * array_size() - Calculate size of 2-dimensional array.
     240             :  * @a: dimension one
     241             :  * @b: dimension two
     242             :  *
     243             :  * Calculates size of 2-dimensional array: @a * @b.
     244             :  *
     245             :  * Returns: number of bytes needed to represent the array or SIZE_MAX on
     246             :  * overflow.
     247             :  */
     248             : #define array_size(a, b)        size_mul(a, b)
     249             : 
     250             : /**
     251             :  * array3_size() - Calculate size of 3-dimensional array.
     252             :  * @a: dimension one
     253             :  * @b: dimension two
     254             :  * @c: dimension three
     255             :  *
     256             :  * Calculates size of 3-dimensional array: @a * @b * @c.
     257             :  *
     258             :  * Returns: number of bytes needed to represent the array or SIZE_MAX on
     259             :  * overflow.
     260             :  */
     261             : #define array3_size(a, b, c)    size_mul(size_mul(a, b), c)
     262             : 
     263             : /**
     264             :  * flex_array_size() - Calculate size of a flexible array member
     265             :  *                     within an enclosing structure.
     266             :  * @p: Pointer to the structure.
     267             :  * @member: Name of the flexible array member.
     268             :  * @count: Number of elements in the array.
     269             :  *
     270             :  * Calculates size of a flexible array of @count number of @member
     271             :  * elements, at the end of structure @p.
     272             :  *
     273             :  * Return: number of bytes needed or SIZE_MAX on overflow.
     274             :  */
     275             : #define flex_array_size(p, member, count)                               \
     276             :         __builtin_choose_expr(__is_constexpr(count),                    \
     277             :                 (count) * sizeof(*(p)->member) + __must_be_array((p)->member),    \
     278             :                 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
     279             : 
     280             : /**
     281             :  * struct_size() - Calculate size of structure with trailing flexible array.
     282             :  * @p: Pointer to the structure.
     283             :  * @member: Name of the array member.
     284             :  * @count: Number of elements in the array.
     285             :  *
     286             :  * Calculates size of memory needed for structure of @p followed by an
     287             :  * array of @count number of @member elements.
     288             :  *
     289             :  * Return: number of bytes needed or SIZE_MAX on overflow.
     290             :  */
     291             : #define struct_size(p, member, count)                                   \
     292             :         __builtin_choose_expr(__is_constexpr(count),                    \
     293             :                 sizeof(*(p)) + flex_array_size(p, member, count),       \
     294             :                 size_add(sizeof(*(p)), flex_array_size(p, member, count)))
     295             : 
     296             : /**
     297             :  * struct_size_t() - Calculate size of structure with trailing flexible array
     298             :  * @type: structure type name.
     299             :  * @member: Name of the array member.
     300             :  * @count: Number of elements in the array.
     301             :  *
     302             :  * Calculates size of memory needed for structure @type followed by an
     303             :  * array of @count number of @member elements. Prefer using struct_size()
     304             :  * when possible instead, to keep calculations associated with a specific
     305             :  * instance variable of type @type.
     306             :  *
     307             :  * Return: number of bytes needed or SIZE_MAX on overflow.
     308             :  */
     309             : #define struct_size_t(type, member, count)                                      \
     310             :         struct_size((type *)NULL, member, count)
     311             : 
     312             : #endif /* __LINUX_OVERFLOW_H */

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