Date: Sat, 31 Mar 2018 06:37:37 -0700 From: Kees Cook <keescook@...omium.org> To: Ingo Molnar <mingo@...nel.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Martin Uecker <Martin.Uecker@....uni-goettingen.de>, Josh Poimboeuf <jpoimboe@...hat.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Randy Dunlap <rdunlap@...radead.org>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, David Laight <David.Laight@...lab.com>, Ian Abbott <abbotti@....co.uk>, LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v6] kernel.h: Retain constant expression output for max()/min() On Fri, Mar 30, 2018 at 10:34 PM, Ingo Molnar <mingo@...nel.org> wrote: > > * Kees Cook <keescook@...omium.org> wrote: > >> On Mon, Mar 26, 2018 at 10:47 PM, Ingo Molnar <mingo@...nel.org> wrote: >> > >> > * Kees Cook <keescook@...omium.org> wrote: >> > >> >> In the effort to remove all VLAs from the kernel, it is desirable to >> >> build with -Wvla. However, this warning is overly pessimistic, in that >> >> it is only happy with stack array sizes that are declared as constant >> >> expressions, and not constant values. One case of this is the evaluation >> >> of the max() macro which, due to its construction, ends up converting >> >> constant expression arguments into a constant value result. >> >> >> >> All attempts to rewrite this macro with __builtin_constant_p() failed with >> >> older compilers (e.g. gcc 4.4). However, Martin Uecker constructed a >> >> mind-shattering solution that works everywhere. Cthulhu fhtagn! >> >> >> >> This patch updates the min()/max() macros to evaluate to a constant >> >> expression when called on constant expression arguments. This removes >> >> several false-positive stack VLA warnings from an x86 allmodconfig >> >> build when -Wvla is added: >> > >> > Cool! >> > >> > Acked-by: Ingo Molnar <mingo@...nel.org> >> > >> > How many warnings are left in an allmodconfig build? >> >> For -Wvla? Out of the original 112 files with VLAs, 42 haven't had a >> patch applied yet. Doing a linux-next allmodconfig build with the >> max() patch and my latest ecc patch, we've gone from 316 warning >> instances to 205. More than half of those are in >> include/crypto/skcipher.h and include/crypto/hash.h. > > Great - once the number of warnings is zero, is the plan to enable the warning > unconditionally? That's the plan, yes. Like inbox-zero but for VLAs. ;) -Kees -- Kees Cook Pixel Security
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