linux-stable/include/linux/overflow.h

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compiler.h: enable builtin overflow checkers and add fallback code This adds wrappers for the __builtin overflow checkers present in gcc 5.1+ as well as fallback implementations for earlier compilers. It's not that easy to implement the fully generic __builtin_X_overflow(T1 a, T2 b, T3 *d) in macros, so the fallback code assumes that T1, T2 and T3 are the same. We obviously don't want the wrappers to have different semantics depending on $GCC_VERSION, so we also insist on that even when using the builtins. There are a few problems with the 'a+b < a' idiom for checking for overflow: For signed types, it relies on undefined behaviour and is not actually complete (it doesn't check underflow; e.g. INT_MIN+INT_MIN == 0 isn't caught). Due to type promotion it is wrong for all types (signed and unsigned) narrower than int. Similarly, when a and b does not have the same type, there are subtle cases like u32 a; if (a + sizeof(foo) < a) return -EOVERFLOW; a += sizeof(foo); where the test is always false on 64 bit platforms. Add to that that it is not always possible to determine the types involved at a glance. The new overflow.h is somewhat bulky, but that's mostly a result of trying to be type-generic, complete (e.g. catching not only overflow but also signed underflow) and not relying on undefined behaviour. Linus is of course right [1] that for unsigned subtraction a-b, the right way to check for overflow (underflow) is "b > a" and not "__builtin_sub_overflow(a, b, &d)", but that's just one out of six cases covered here, and included mostly for completeness. So is it worth it? I think it is, if nothing else for the documentation value of seeing if (check_add_overflow(a, b, &d)) return -EGOAWAY; do_stuff_with(d); instead of the open-coded (and possibly wrong and/or incomplete and/or UBsan-tickling) if (a+b < a) return -EGOAWAY; do_stuff_with(a+b); While gcc does recognize the 'a+b < a' idiom for testing unsigned add overflow, it doesn't do nearly as good for unsigned multiplication (there's also no single well-established idiom). So using check_mul_overflow in kcalloc and friends may also make gcc generate slightly better code. [1] https://lkml.org/lkml/2015/11/2/658 Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Kees Cook <keescook@chromium.org>
2018-05-07 22:36:27 +00:00
/* SPDX-License-Identifier: GPL-2.0 OR MIT */
#ifndef __LINUX_OVERFLOW_H
#define __LINUX_OVERFLOW_H
#include <linux/compiler.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))
overflow: Fix -Wtype-limits compilation warnings Attempt to use check_shl_overflow() with inputs of unsigned type produces the following compilation warnings. drivers/infiniband/hw/mlx5/qp.c: In function _set_user_rq_size_: ./include/linux/overflow.h:230:6: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits] _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \ ^~ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ ./include/linux/overflow.h:232:26: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] (_to_shift != _s || *_d < 0 || _a < 0 || \ ^ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ ./include/linux/overflow.h:232:36: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] (_to_shift != _s || *_d < 0 || _a < 0 || \ ^ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift,&rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ Fixes: 0c66847793d1 ("overflow.h: Add arithmetic shift helper") Reviewed-by: Bart Van Assche <bvanassche@acm.org> Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: Leon Romanovsky <leonro@mellanox.com> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
2019-03-17 10:11:14 +00:00
/*
* 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)))
compiler.h: enable builtin overflow checkers and add fallback code This adds wrappers for the __builtin overflow checkers present in gcc 5.1+ as well as fallback implementations for earlier compilers. It's not that easy to implement the fully generic __builtin_X_overflow(T1 a, T2 b, T3 *d) in macros, so the fallback code assumes that T1, T2 and T3 are the same. We obviously don't want the wrappers to have different semantics depending on $GCC_VERSION, so we also insist on that even when using the builtins. There are a few problems with the 'a+b < a' idiom for checking for overflow: For signed types, it relies on undefined behaviour and is not actually complete (it doesn't check underflow; e.g. INT_MIN+INT_MIN == 0 isn't caught). Due to type promotion it is wrong for all types (signed and unsigned) narrower than int. Similarly, when a and b does not have the same type, there are subtle cases like u32 a; if (a + sizeof(foo) < a) return -EOVERFLOW; a += sizeof(foo); where the test is always false on 64 bit platforms. Add to that that it is not always possible to determine the types involved at a glance. The new overflow.h is somewhat bulky, but that's mostly a result of trying to be type-generic, complete (e.g. catching not only overflow but also signed underflow) and not relying on undefined behaviour. Linus is of course right [1] that for unsigned subtraction a-b, the right way to check for overflow (underflow) is "b > a" and not "__builtin_sub_overflow(a, b, &d)", but that's just one out of six cases covered here, and included mostly for completeness. So is it worth it? I think it is, if nothing else for the documentation value of seeing if (check_add_overflow(a, b, &d)) return -EGOAWAY; do_stuff_with(d); instead of the open-coded (and possibly wrong and/or incomplete and/or UBsan-tickling) if (a+b < a) return -EGOAWAY; do_stuff_with(a+b); While gcc does recognize the 'a+b < a' idiom for testing unsigned add overflow, it doesn't do nearly as good for unsigned multiplication (there's also no single well-established idiom). So using check_mul_overflow in kcalloc and friends may also make gcc generate slightly better code. [1] https://lkml.org/lkml/2015/11/2/658 Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Kees Cook <keescook@chromium.org>
2018-05-07 22:36:27 +00:00
#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) ({ \
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) ({ \
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) ({ \
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) \
__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))
#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) ({ \
typeof(a) _a = a; \
typeof(s) _s = s; \
typeof(d) _d = d; \
u64 _a_full = _a; \
unsigned int _to_shift = \
overflow: Fix -Wtype-limits compilation warnings Attempt to use check_shl_overflow() with inputs of unsigned type produces the following compilation warnings. drivers/infiniband/hw/mlx5/qp.c: In function _set_user_rq_size_: ./include/linux/overflow.h:230:6: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits] _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \ ^~ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ ./include/linux/overflow.h:232:26: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] (_to_shift != _s || *_d < 0 || _a < 0 || \ ^ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ ./include/linux/overflow.h:232:36: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] (_to_shift != _s || *_d < 0 || _a < 0 || \ ^ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift,&rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ Fixes: 0c66847793d1 ("overflow.h: Add arithmetic shift helper") Reviewed-by: Bart Van Assche <bvanassche@acm.org> Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: Leon Romanovsky <leonro@mellanox.com> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
2019-03-17 10:11:14 +00:00
is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \
*_d = (_a_full << _to_shift); \
overflow: Fix -Wtype-limits compilation warnings Attempt to use check_shl_overflow() with inputs of unsigned type produces the following compilation warnings. drivers/infiniband/hw/mlx5/qp.c: In function _set_user_rq_size_: ./include/linux/overflow.h:230:6: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits] _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \ ^~ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ ./include/linux/overflow.h:232:26: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] (_to_shift != _s || *_d < 0 || _a < 0 || \ ^ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ ./include/linux/overflow.h:232:36: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] (_to_shift != _s || *_d < 0 || _a < 0 || \ ^ drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro _check_shl_overflow_ if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift,&rwq->buf_size)) ^~~~~~~~~~~~~~~~~~ Fixes: 0c66847793d1 ("overflow.h: Add arithmetic shift helper") Reviewed-by: Bart Van Assche <bvanassche@acm.org> Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: Leon Romanovsky <leonro@mellanox.com> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
2019-03-17 10:11:14 +00:00
(_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.
* @n: Number of elements in the array.
*
* Calculates size of memory needed for structure @p followed by an
* array of @n @member elements.
*
* Return: number of bytes needed or SIZE_MAX on overflow.
*/
#define struct_size(p, member, n) \
__ab_c_size(n, \
sizeof(*(p)->member) + __must_be_array((p)->member),\
sizeof(*(p)))
compiler.h: enable builtin overflow checkers and add fallback code This adds wrappers for the __builtin overflow checkers present in gcc 5.1+ as well as fallback implementations for earlier compilers. It's not that easy to implement the fully generic __builtin_X_overflow(T1 a, T2 b, T3 *d) in macros, so the fallback code assumes that T1, T2 and T3 are the same. We obviously don't want the wrappers to have different semantics depending on $GCC_VERSION, so we also insist on that even when using the builtins. There are a few problems with the 'a+b < a' idiom for checking for overflow: For signed types, it relies on undefined behaviour and is not actually complete (it doesn't check underflow; e.g. INT_MIN+INT_MIN == 0 isn't caught). Due to type promotion it is wrong for all types (signed and unsigned) narrower than int. Similarly, when a and b does not have the same type, there are subtle cases like u32 a; if (a + sizeof(foo) < a) return -EOVERFLOW; a += sizeof(foo); where the test is always false on 64 bit platforms. Add to that that it is not always possible to determine the types involved at a glance. The new overflow.h is somewhat bulky, but that's mostly a result of trying to be type-generic, complete (e.g. catching not only overflow but also signed underflow) and not relying on undefined behaviour. Linus is of course right [1] that for unsigned subtraction a-b, the right way to check for overflow (underflow) is "b > a" and not "__builtin_sub_overflow(a, b, &d)", but that's just one out of six cases covered here, and included mostly for completeness. So is it worth it? I think it is, if nothing else for the documentation value of seeing if (check_add_overflow(a, b, &d)) return -EGOAWAY; do_stuff_with(d); instead of the open-coded (and possibly wrong and/or incomplete and/or UBsan-tickling) if (a+b < a) return -EGOAWAY; do_stuff_with(a+b); While gcc does recognize the 'a+b < a' idiom for testing unsigned add overflow, it doesn't do nearly as good for unsigned multiplication (there's also no single well-established idiom). So using check_mul_overflow in kcalloc and friends may also make gcc generate slightly better code. [1] https://lkml.org/lkml/2015/11/2/658 Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk> Signed-off-by: Kees Cook <keescook@chromium.org>
2018-05-07 22:36:27 +00:00
#endif /* __LINUX_OVERFLOW_H */