From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>
Subject: Re: [PATCH 08/14] arm64: simplify access_ok()
Date: Tue, 15 Feb 2022 10:39:06 +0000
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To: Arnd Bergmann <arnd-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Ard Biesheuvel <ardb-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>, linux-arch <linux-arch-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Linux Memory Management List <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>, Linux API <linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>, Linux Kernel Mailing List <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Russell King <linux-I+IVW8TIWO2tmTQ+vhA3Yw@public.gmane.org>, Will Deacon <will-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Guo Ren <guoren-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Brian Cain <bcain-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>, Geert Uytterhoeven <geert-Td1EMuHUCqxL1ZNQvxDV9g@public.gmane.org>, Michal Simek <monstr-pSz03upnqPeHXe+LvDLADg@public.gmane.org>, Thomas Bogendoerfer <tsbogend-I1c7kopa9pxLokYuJOExCg@public.gmane.org>, Nick Hu <nickhu-MUIXKm3Oiri1Z/+hSey0Gg@public.gmane.org>, Greentime Hu <green.hu-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Dinh Nguyen <dinguyen-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Stafford Horne <shorne-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Helge Deller <deller@gm>

On Tue, Feb 15, 2022 at 10:39:46AM +0100, Arnd Bergmann wrote:
> On Tue, Feb 15, 2022 at 10:21 AM Ard Biesheuvel <ardb-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> > On Tue, 15 Feb 2022 at 10:13, Arnd Bergmann <arnd-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> >
> > arm64 also has this leading up to the range check, and I think we'd no
> > longer need it:
> >
> >     if (IS_ENABLED(CONFIG_ARM64_TAGGED_ADDR_ABI) &&
> >         (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
> >             addr = untagged_addr(addr);
> 
> I suspect the expensive part here is checking the two flags, as untagged_addr()
> seems to always just add a sbfx instruction. Would this work?
> 
> #ifdef CONFIG_ARM64_TAGGED_ADDR_ABI
> #define access_ok(ptr, size) __access_ok(untagged_addr(ptr), (size))
> #else // the else path is the default, this can be left out.
> #define access_ok(ptr, size) __access_ok((ptr), (size))
> #endif

This would be an ABI change, e.g. for tasks without TIF_TAGGED_ADDR.

I don't think we should change this as part of this series.

Thanks,
Mark.