linux-kernel - Re: [PATCH 3/8] asm-generic: fix unaligned access hamdling in raw_copy_{from,to}

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Date:   Sat, 5 Sep 2020 15:41:36 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Arnd Bergmann <arnd@...db.de>, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 3/8] asm-generic: fix unaligned access hamdling in
 raw_copy_{from,to}_user

On Fri, Sep 04, 2020 at 11:35:18PM +0100, Al Viro wrote:

> 	Now, if you look at raw_copy_from_user() you'll see an interesting
> picture: some architectures special-case the handling of small constant sizes.
> Namely,
> 	arc (any size; inlining in there is obscene, constant size or not),
> 	c6x (1,4,8),
> 	m68k/MMU (1,2,3,4,5,6,7,8,9,10,12)
> 	ppc (1,2,4,8),
> 	h8300 (1,2,4),
> 	riscv (with your series)(1,2,4, 8 if 64bit).

FWIW, on the raw_copy_to_user() side the same set of constant sizes is
recongized by the same architectures and we have
	* __put_user/put_user in asm-generic/uaccess.h make use of that
	* arc, c6x, ppc and riscv using it to store sigset_t on sigframe
	* 3 odd callers:
		* arc stash_usr_regs(), inlined and unrolled large copy_to_user()
		* ppc kvm_htab_read(), 64bit store.
		* i915_gem_execbuffer_ioctl():
                        if (__copy_to_user(&user_exec_list[i].offset,
                                           &exec2_list[i].offset,
                                           sizeof(user_exec_list[i].offset)))
		in a loop.  'offset' here is __u64.

That's it.  IOW, asm-generic put_user() is the only real cause to have those 
magic sizes recognized on raw_copy_to_user() side.