Date: Thu, 15 Mar 2018 12:47:44 -0700 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Kees Cook <keescook@...omium.org>, Linus Torvalds <torvalds@...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@...r.kernel.org, linux-btrfs@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: [PATCH v4 0/2] Remove false-positive VLAs when using max() I'm calling this "v4" since the last effort at this was v3, even if it's a different approach. Patch 1 adds const_max(), patch 2 uses it in all the places max() was used for stack arrays. Commit log from patch 1: ---snip--- kernel.h: Introduce const_max() for VLA removal 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. Attempts to adjust the behavior of max() ran afoul of version-dependent compiler behavior. To work around this and still gain -Wvla coverage, this patch introduces a new macro, const_max(), for use in these cases of stack array size declaration, where the constant expressions are retained. Since this means losing the double-evaluation protections of the max() macro, this macro is designed to explicitly fail if used on non-constant arguments. Older compilers will fail with the unhelpful message: error: first argument to ‘__builtin_choose_expr’ not a constant Newer compilers will fail with a hopefully more helpful message: error: call to ‘__error_not_const_arg’ declared with attribute error: const_max() used with non-compile-time constant arg To gain the ability to compare differing types, the arguments are explicitly cast to size_t. Without this, some compiler versions will fail when comparing different enum types or similar constant expression cases. With the casting, it's possible to do things like: int foo[const_max(6, sizeof(something))];  https://lkml.org/lkml/2018/3/7/621  https://lkml.org/lkml/2018/3/10/170 ---eol--- Hopefully this reads well as a summary from all the things that got tried. I've tested this on allmodconfig builds with gcc 4.4.4 and 6.3.0, with and without -Wvla. -Kees
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