Messages in this thread | | | From | Kees Cook <> | Date | Fri, 30 Mar 2018 21:57:01 -0700 | Subject | Re: [PATCH v6] kernel.h: Retain constant expression output for max()/min() |
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On Mon, Mar 26, 2018 at 10:47 PM, Ingo Molnar <mingo@kernel.org> wrote: > > * Kees Cook <keescook@chromium.org> wrote: > >> In the effort to remove all VLAs from the kernel[1], 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)[2]. However, Martin Uecker constructed[3] 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@kernel.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.
-Kees
-- Kees Cook Pixel Security
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