Date: Sat, 17 Mar 2018 17:55:24 -0500 From: Josh Poimboeuf <jpoimboe@...hat.com> To: Kees Cook <keescook@...omium.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Al Viro <viro@...iv.linux.org.uk>, Florian Weimer <fweimer@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, 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 v5 0/2] Remove false-positive VLAs when using max() On Sat, Mar 17, 2018 at 01:07:32PM -0700, Kees Cook wrote: > On Sat, Mar 17, 2018 at 11:52 AM, Linus Torvalds > <torvalds@...ux-foundation.org> wrote: > > So the above is completely insane, bit there is actually a chance that > > using that completely crazy "x -> sizeof(char[x])" conversion actually > > helps, because it really does have a (very odd) evaluation-time > > change. sizeof() has to be evaluated as part of the constant > > expression evaluation, in ways that "__builtin_constant_p()" isn't > > specified to be done. > > > > But it is also definitely me grasping at straws. If that doesn't work > > for 4.4, there's nothing else I can possibly see. > > No luck! :( gcc 4.4 refuses to play along. And, hilariously, not only > does it not change the complaint about __builtin_choose_expr(), it > also thinks that's a VLA now. > > ./include/linux/mm.h: In function ‘get_mm_hiwater_rss’: > ./include/linux/mm.h:1567: warning: variable length array is used > ./include/linux/mm.h:1567: error: first argument to > ‘__builtin_choose_expr’ not a constant > > 6.8 is happy with it (of course). > > I do think the earlier version (without the > sizeof-hiding-builting_constant_p) provides a template for a > const_max() that both you and Rasmus would be happy with, though! I thought we were dropping support for 4.4 (for other reasons). Isn't it 4.6 we should be looking at? -- Josh
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