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Date: Thu, 18 Jan 2018 08:38:18 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Alan Cox <alan@...ux.intel.com>, Eric Dumazet <edumazet@...gle.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
	Kees Cook <keescook@...omium.org>,
	kernel-hardening@...ts.openwall.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Al Viro <viro@...iv.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths

On Wed, Jan 17, 2018 at 11:26:08AM -0800, Linus Torvalds wrote:
> But there are about ~100 set_fs() calls in generic code, and some of
> those really are pretty fundamental. Doing things like "kernel_read()"
> without set_fs() is basically impossible.

Not if we move to iov_iter or iov_iter-like behavior for all reads
and writes.  There is an issue with how vectored writes are handles
in plain read/write vs read_iter/write_iter inherited from readv/writev,
but that's nothing a flag, or a second set of methods with the
same signature.

But there are more annoying things, most notable in-kernel ioctls
calls.  We have quite a few of them, and while many are just utterly
stupid and can be replaced with direct function calls or new methods
(I've done quite a few conversions of those) some might be left.
Something like iov_iter might be the answer again.

Then we have things like probe_kernel_read/probe_kernel_write which
abuse the exception handling in get/put user.  But with a little
arch helper we don't strictly need get_fs/set_fs for that.

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