Re: [PATCH] arm64: disable patchable function entry on big-endian clang builds

[Torsten Duwe <duwe@lst.de>]

Hi Arnd, Mark and others,

this may not be worth arguing but I'm currently fighting excessive
workarounds in another area and so this triggers me, so I have to make
a remark ;-)

On Tue, 5 May 2020 15:25:56 +0100
Mark Rutland <mark.rutland@arm.com> wrote:

> On Tue, May 05, 2020 at 04:12:36PM +0200, Arnd Bergmann wrote:
> > Clang only supports the patchable_function_entry attribute on
> > little-endian arm64 builds, but not on big-endian:
> > 
> > include/linux/kasan-checks.h:16:8: error: unknown attribute
> > 'patchable_function_entry' ignored [-Werror,-Wunknown-attributes]
> > 
> > Disable that configuration with another dependency. Unfortunately
> > the existing check is not enough, as $(cc-option) at this point does
> > not pass the -mbig-endian flag.
> 
> Well that's unfortunate. :(
> 
> Do we know if this is deliberate and/or likely to change in future?
> This practically rules out a BE distro kernel with things like PAC,
> which isn't ideal.

But still better than cumulating workarounds. If clang's flags aren't
orthogonal then that's a bug in clang. If I get a vote here I'm against
it.

> > Fixes: 3b23e4991fb6 ("arm64: implement ftrace with regs")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> 
> This looks fine for now, and we can add a version check in future, so:
                                      ^^^^^^^^^^^^^^^^^^^
see what I mean? And in the end another line of code you'll never again
get rid of.

I suggest to get a quote from clang folks first about their schedule and
regarded importance of patchable-function-entries on BE, and leave it as
is: broken on certain clang configurations. It's not the kernel's fault.

> Acked-by: Mark Rutland <mark.rutland@arm.com>
> 
> Mark.
> 
> > ---
> >  arch/arm64/Kconfig | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 4b256fa6db7a..a33d6402b934 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -151,7 +151,7 @@ config ARM64
> >  	select HAVE_DMA_CONTIGUOUS
> >  	select HAVE_DYNAMIC_FTRACE
> >  	select HAVE_DYNAMIC_FTRACE_WITH_REGS \
> > -		if $(cc-option,-fies on y=2)
> > +		if $(cc-option,-fpatchable-function-entry=2) &&
> > !(CC_IS_CLANG && CPU_BIG_ENDIAN) select
> > HAVE_EFFICIENT_UNALIGNED_ACCESS select HAVE_FAST_GUP
> >  	select HAVE_FTRACE_MCOUNT_RECORD
> > -- 
> > 2.26.0
> >