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>
Please register or sign in to comment