On Tue, Oct 01, 2019 at 04:48:14PM +0100, Russell King - ARM Linux admin wrote: > On Tue, Oct 01, 2019 at 04:28:27PM +0100, Andrew Murray wrote: > > I hadn't noticed the use of __OPTIMIZE__ - indeed if __compiletime_assert > > is no-op'd and you reach it then you won't have a build error - but you > > may get uninitialised values instead. > > > > Presumably the purpose of __OPTIMIZE__ in this case is to prevent getting > > an undefined function error for the __compiletime_assert line, even though > > it doesn't get called (when using a compiler that doesn't optimize out the > > call to the unused function). > > > > Why is the call to __get_user_bad not guarded in this way for when > > __OPTIMIZE__ isn't set, i.e. why doesn't it suffer from the issue > > that the following fixes? > > Officially, the kernel does not support building with -O0. To start > with, the top level makefile has: > > ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE > KBUILD_CFLAGS += -Os > else > KBUILD_CFLAGS += -O2 > endif > > and we've said for years that the kernel relies upon the compiler > optimiser to build correctly. You may be lucky if you pass it via > some method to 'make' but that's going to rely on the argument order > to the compiler, and the order in which the compiler processes its > arguments, and whether it (for example) correctly disables all > optimisations if it encounters -O0 somewhere.
So in practice, __OPTIMIZE__ will likely always be set, as far as I can tell it's supported in GCC, Clang and Intel compilers. Though the exception to this is for crypto/jitterentropy.c where the -O0 flag is unconditionally set. Are there other exceptions to this in terms of compilers? Perhaps it may be possible to use BUILD_BUG after all. Thanks, Andrew Murray > > -- > RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ > FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up > According to speedtest.net: 11.9Mbps down 500kbps up