Messages in this thread | | | From | Kees Cook <> | Date | Sat, 31 Mar 2018 06:37:37 -0700 | Subject | Re: [PATCH v6] kernel.h: Retain constant expression output for max()/min() |
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On Fri, Mar 30, 2018 at 10:34 PM, Ingo Molnar <mingo@kernel.org> wrote: > > * 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?
That's the plan, yes. Like inbox-zero but for VLAs. ;)
-Kees
-- Kees Cook Pixel Security
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