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:
quoted
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
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