Date: Thu, 15 Mar 2018 16:41:41 -0700 From: Kees Cook <keescook@...omium.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Randy Dunlap <rdunlap@...radead.org>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Ingo Molnar <mingo@...nel.org>, David Laight <David.Laight@...lab.com>, Ian Abbott <abbotti@....co.uk>, linux-input <linux-input@...r.kernel.org>, linux-btrfs <linux-btrfs@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v4 1/2] kernel.h: Introduce const_max() for VLA removal On Thu, Mar 15, 2018 at 4:34 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Thu, Mar 15, 2018 at 3:46 PM, Kees Cook <keescook@...omium.org> wrote: >> >> So, AIUI, I can either get strict type checking, in which case, this >> is rejected (which I assume there is still a desire to have): >> >> int foo[const_max(6, sizeof(whatever))]; > > Ehh, yes, that looks fairly sane, and erroring out would be annoying. > > But maybe we should just make the type explicit, and make it "const_max_t()"? > > I think all the existing users are of type "max_t()" anyway due to the > very same issue, no? All but one are using max(). One case uses max_t() to get u32. > At least if there's an explicit type like 'size_t', then passing in > "-1" becoming a large unsigned integer is understandable and clear, > not just some odd silent behavior. > > Put another way: I think it's unacceptable that > > const_max(-1,6) > > magically becomes a huge positive number like in that patch of yours, but > > const_max_t(size_t, -1, 6) > > *obviously* is a huge positive number. > > The two things would *do* the same thing, but in the second case the > type is explicit and visible. > >> due to __builtin_types_compatible_p() rejecting it, or I can construct >> a "positive arguments only" test, in which the above is accepted, but >> this is rejected: > > That sounds acceptable too, although the "const_max_t()" thing is > presumably going to be simpler? I much prefer explicit typing, but both you and Rasmus mentioned wanting the int/sizeof_t mixing. I'm totally happy with const_max_t() -- even if it makes my line-wrapping harder due to the longer name. ;) I'll resend in a moment... -Kees  https://patchwork.kernel.org/patch/10285709/ -- Kees Cook Pixel Security
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