Date: Fri, 16 Mar 2018 04:05:55 +0100 From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com> To: Kees Cook <keescook@...omium.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Randy Dunlap <rdunlap@...radead.org>, 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 Fri, Mar 16, 2018 at 12:49 AM, Kees Cook <keescook@...omium.org> wrote: > On Thu, Mar 15, 2018 at 4:46 PM, Linus Torvalds > <torvalds@...ux-foundation.org> wrote: >> What I'm *not* so much ok with is "const_max(5,sizeof(x))" erroring >> out, or silently causing insane behavior due to hidden subtle type >> casts.. > > Yup! I like it as an explicit argument. Thanks! > What about something like this? #define INTMAXT_MAX LLONG_MAX typedef int64_t intmax_t; #define const_max(x, y) \ __builtin_choose_expr( \ !__builtin_constant_p(x) || !__builtin_constant_p(y), \ __error_not_const_arg(), \ __builtin_choose_expr( \ (x) > INTMAXT_MAX || (y) > INTMAXT_MAX, \ __error_too_big(), \ __builtin_choose_expr( \ (intmax_t)(x) >= (intmax_t)(y), \ (x), \ (y) \ ) \ ) \ ) Works for different types, allows to mix negatives and positives and returns the original type, e.g.: const_max(-1, sizeof(char)); is of type 'long unsigned int', but: const_max(2, sizeof(char)); is of type 'int'. While I am not a fan that the return type depends on the arguments, it is useful if you are going to use the expression in something that needs expects a precise (a printk() for instance?). The check against the INTMAXT_MAX is there to avoid complexity (if we do not handle those cases, it is safe to use intmax_t for the comparison; otherwise you have to have another compile time branch for the case positive-positive using uintmax_t) and also avoids odd warnings for some cases above LLONG_MAX about comparisons with 0 for unsigned expressions being always true. On the positive side, it prevents using the macro for thing like "(size_t)-1". Cheers, Miguel
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