Messages in this thread | | | From | Arnd Bergmann <> | Date | Mon, 7 Sep 2020 10:07:49 +0200 | Subject | Re: [PATCH 3/8] asm-generic: fix unaligned access hamdling in raw_copy_{from,to}_user |
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On Sat, Sep 5, 2020 at 12:35 AM Al Viro <viro@zeniv.linux.org.uk> wrote: > > On Fri, Sep 04, 2020 at 07:06:17PM +0100, Al Viro wrote: > > On Fri, Sep 04, 2020 at 06:52:11PM +0200, Christoph Hellwig wrote: > > > Use get_unaligned and put_unaligned for the small constant size cases > > > in the generic uaccess routines. This ensures they can be used for > > > architectures that do not support unaligned loads and stores, while > > > being a no-op for those that do. > > > > Frankly, I would rather get rid of those constant-sized cases entirely; > > sure, we'd need to adjust asm-generic/uaccess.h defaults for __get_user(), > > but there that kind of stuff would make sense; in raw_copy_from_user() > > it really doesn't.
Right. When I originally wrote that part of asm-generic/uaccess.h, the idea was that its __get_user()/__put_user() would end up being used across most architectures, which then would only have to implement custom __copy_from_user()/__copy_to_user() with those special cases to get the optimum behavior. It didn't work out in the end, since copy_from_user() also has to deal with unaligned or partial copies that prevent it from degrading into a single instruction on anything other than the simplest NOMMU architectures.
I'd still hope we can eventually come up with a generic __get_user()/__put_user() helper that avoids all the common architecture specific bugs in them, with a simpler way for an architecture to hook into with a set of inline functions while leaving the macros to common code, but that can be done another time.
> IOW, there's a scattering of potentially valid uses that might be better > off with get_user(). And there's generic non-MMU variant that gets > used in get_user()/__get_user() on h8300 and riscv. This one *is* > valid, but I don't think that raw_copy_from_user() is the right layer > for that. > > For raw_copy_to_user() the picture is similar. And I'd like to get > rid of that magical crap. Let's not make it harder...
Agreed
Arnd
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