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Date: Mon, 22 Aug 2022 13:12:30 -0700 From: Kees Cook <keescook@...omium.org> To: Gwan-gyeong Mun <gwan-gyeong.mun@...el.com> Cc: Andrzej Hajda <andrzej.hajda@...el.com>, Andi Shyti <andi.shyti@...ux.intel.com>, thomas.hellstrom@...ux.intel.com, jani.nikula@...el.com, intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, chris@...is-wilson.co.uk, airlied@...ux.ie, linux-hardening@...r.kernel.org, matthew.auld@...el.com, mchehab@...nel.org, nirmoy.das@...el.com, Mauro Carvalho Chehab <mauro.chehab@...ux.intel.com> Subject: Re: [PATCH v7 1/8] overflow: Move and add few utility macros into overflow On Tue, Aug 23, 2022 at 04:32:10AM +0900, Gwan-gyeong Mun wrote: > On 8/22/22 11:05 PM, Andrzej Hajda wrote: > > On 18.08.2022 02:12, Kees Cook wrote: > > > On Thu, Aug 18, 2022 at 01:07:29AM +0200, Andi Shyti wrote: > > > > [...] > > > > > +#define safe_conversion(ptr, value) ({ \ > > > > > + typeof(value) __v = (value); \ > > > > > + typeof(ptr) __ptr = (ptr); \ > > > > > + overflows_type(__v, *__ptr) ? 0 : ((*__ptr = > > > > > (typeof(*__ptr))__v), 1); \ > > > > > +}) > > > > > > I try to avoid "safe" as an adjective for interface names, since it > > > doesn't really answer "safe from what?" This looks more like "assign, but > > > zero when out of bounds". And it can be built from existing macros here: > > > > > > if (check_add_overflow(0, value, ptr)) > > > *ptr = 0; > > > > > > I actually want to push back on this a bit, because there can still be > > > logic bugs built around this kind of primitive. Shouldn't out-of-bounds > > > assignments be seen as a direct failure? I would think this would be > > > sufficient: > > > > > > #define check_assign(value, ptr) check_add_overflow(0, value, ptr) > > > > > > And callers would do: > > > > > > if (check_assign(value, &var)) > > > return -EINVAL; > > > > Yes, I also like check_assign() you suggested more than safe_conversion. > As shown below, it would be more readable to return true when assign > succeeds and false when it fails. What do you think? No, this inverts the style of all the other check_*() functions, so it should remain "non-zero is failure". > /** > * check_assign - perform a type conversion (cast) of an source value into > * a new variable, checking that the destination is large enough to hold the > * source value. > * > * @value: Source value > * @ptr: Destination pointer address, If the pointer type is not used, a > warning message is output during build. > * > * Returns: > * If the value would overflow the destination, it returns false. If not > return true. > */ > #define check_assign(value, ptr) __must_check_overflow(({ \ > typecheck_pointer(ptr); \ > !__builtin_add_overflow(0, value, ptr); \ > })) Please don't use the __builtin*s, instead stick to the check_* family, as they correctly wrap the builtins and perform type checking, etc. As mentioned, check_assign() should just be: #define check_assign(value, ptr) check_add_overflow(0, value, ptr) I don't think any of the other code is needed? What's the use-case for the other stuff? i.e. Why does anything need overflows_type()? -Kees -- Kees Cook
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