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>
This commit is contained in:
Rasmus Villemoes
Kees Cook
Родитель
75bc37fefc
Коммит
f0907827a8
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@ -32,3 +32,17 @@
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#ifdef __noretpoline
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#undef __noretpoline
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#endif
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/*
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* Not all versions of clang implement the the type-generic versions
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* of the builtin overflow checkers. Fortunately, clang implements
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* __has_builtin allowing us to avoid awkward version
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* checks. Unfortunately, we don't know which version of gcc clang
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* pretends to be, so the macro may or may not be defined.
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*/
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#undef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
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#if __has_builtin(__builtin_mul_overflow) && \
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__has_builtin(__builtin_add_overflow) && \
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__has_builtin(__builtin_sub_overflow)
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#define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
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#endif
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@ -343,3 +343,7 @@
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* code
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*/
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#define uninitialized_var(x) x = x
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#if GCC_VERSION >= 50100
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#define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
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#endif
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@ -44,3 +44,7 @@
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#define __builtin_bswap16 _bswap16
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#endif
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/*
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* icc defines __GNUC__, but does not implement the builtin overflow checkers.
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*/
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#undef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
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@ -0,0 +1,205 @@
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/* SPDX-License-Identifier: GPL-2.0 OR MIT */
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#ifndef __LINUX_OVERFLOW_H
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#define __LINUX_OVERFLOW_H
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#include <linux/compiler.h>
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/*
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* In the fallback code below, we need to compute the minimum and
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* maximum values representable in a given type. These macros may also
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* be useful elsewhere, so we provide them outside the
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* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block.
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*
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* It would seem more obvious to do something like
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*
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* #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
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* #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
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*
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* Unfortunately, the middle expressions, strictly speaking, have
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* undefined behaviour, and at least some versions of gcc warn about
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* the type_max expression (but not if -fsanitize=undefined is in
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* effect; in that case, the warning is deferred to runtime...).
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*
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* The slightly excessive casting in type_min is to make sure the
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* macros also produce sensible values for the exotic type _Bool. [The
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* overflow checkers only almost work for _Bool, but that's
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* a-feature-not-a-bug, since people shouldn't be doing arithmetic on
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* _Bools. Besides, the gcc builtins don't allow _Bool* as third
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* argument.]
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*
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* Idea stolen from
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* https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
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* credit to Christian Biere.
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*/
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#define is_signed_type(type) (((type)(-1)) < (type)1)
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#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
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#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
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#define type_min(T) ((T)((T)-type_max(T)-(T)1))
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#ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
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/*
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* For simplicity and code hygiene, the fallback code below insists on
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* a, b and *d having the same type (similar to the min() and max()
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* macros), whereas gcc's type-generic overflow checkers accept
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* different types. Hence we don't just make check_add_overflow an
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* alias for __builtin_add_overflow, but add type checks similar to
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* below.
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*/
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#define check_add_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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__builtin_add_overflow(__a, __b, __d); \
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})
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#define check_sub_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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__builtin_sub_overflow(__a, __b, __d); \
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})
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#define check_mul_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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__builtin_mul_overflow(__a, __b, __d); \
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})
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#else
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/* Checking for unsigned overflow is relatively easy without causing UB. */
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#define __unsigned_add_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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*__d = __a + __b; \
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*__d < __a; \
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})
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#define __unsigned_sub_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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*__d = __a - __b; \
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__a < __b; \
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})
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/*
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* If one of a or b is a compile-time constant, this avoids a division.
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*/
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#define __unsigned_mul_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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*__d = __a * __b; \
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__builtin_constant_p(__b) ? \
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__b > 0 && __a > type_max(typeof(__a)) / __b : \
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__a > 0 && __b > type_max(typeof(__b)) / __a; \
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})
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/*
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* For signed types, detecting overflow is much harder, especially if
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* we want to avoid UB. But the interface of these macros is such that
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* we must provide a result in *d, and in fact we must produce the
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* result promised by gcc's builtins, which is simply the possibly
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* wrapped-around value. Fortunately, we can just formally do the
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* operations in the widest relevant unsigned type (u64) and then
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* truncate the result - gcc is smart enough to generate the same code
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* with and without the (u64) casts.
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*/
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/*
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* Adding two signed integers can overflow only if they have the same
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* sign, and overflow has happened iff the result has the opposite
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* sign.
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*/
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#define __signed_add_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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*__d = (u64)__a + (u64)__b; \
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(((~(__a ^ __b)) & (*__d ^ __a)) \
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& type_min(typeof(__a))) != 0; \
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})
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/*
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* Subtraction is similar, except that overflow can now happen only
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* when the signs are opposite. In this case, overflow has happened if
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* the result has the opposite sign of a.
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*/
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#define __signed_sub_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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*__d = (u64)__a - (u64)__b; \
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((((__a ^ __b)) & (*__d ^ __a)) \
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& type_min(typeof(__a))) != 0; \
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})
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/*
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* Signed multiplication is rather hard. gcc always follows C99, so
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* division is truncated towards 0. This means that we can write the
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* overflow check like this:
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*
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* (a > 0 && (b > MAX/a || b < MIN/a)) ||
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* (a < -1 && (b > MIN/a || b < MAX/a) ||
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* (a == -1 && b == MIN)
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*
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* The redundant casts of -1 are to silence an annoying -Wtype-limits
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* (included in -Wextra) warning: When the type is u8 or u16, the
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* __b_c_e in check_mul_overflow obviously selects
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* __unsigned_mul_overflow, but unfortunately gcc still parses this
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* code and warns about the limited range of __b.
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*/
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#define __signed_mul_overflow(a, b, d) ({ \
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typeof(a) __a = (a); \
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typeof(b) __b = (b); \
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typeof(d) __d = (d); \
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typeof(a) __tmax = type_max(typeof(a)); \
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typeof(a) __tmin = type_min(typeof(a)); \
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(void) (&__a == &__b); \
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(void) (&__a == __d); \
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*__d = (u64)__a * (u64)__b; \
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(__b > 0 && (__a > __tmax/__b || __a < __tmin/__b)) || \
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(__b < (typeof(__b))-1 && (__a > __tmin/__b || __a < __tmax/__b)) || \
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(__b == (typeof(__b))-1 && __a == __tmin); \
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})
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#define check_add_overflow(a, b, d) \
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__builtin_choose_expr(is_signed_type(typeof(a)), \
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__signed_add_overflow(a, b, d), \
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__unsigned_add_overflow(a, b, d))
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#define check_sub_overflow(a, b, d) \
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__builtin_choose_expr(is_signed_type(typeof(a)), \
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__signed_sub_overflow(a, b, d), \
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__unsigned_sub_overflow(a, b, d))
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#define check_mul_overflow(a, b, d) \
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__builtin_choose_expr(is_signed_type(typeof(a)), \
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__signed_mul_overflow(a, b, d), \
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__unsigned_mul_overflow(a, b, d))
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#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
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#endif /* __LINUX_OVERFLOW_H */
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