Messages in this thread | | | Date | Sat, 31 Mar 2018 07:34:34 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v6] kernel.h: Retain constant expression output for max()/min() |
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* Kees Cook <keescook@chromium.org> wrote:
> 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.
Great - once the number of warnings is zero, is the plan to enable the warning unconditionally?
Thanks,
Ingo
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