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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Christoph Hellwig <hch@lst.de>
Cc: "the arch/x86 maintainers" <x86@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-parisc@vger.kernel.org,
	linux-um <linux-um@lists.infradead.org>,
	Netdev <netdev@vger.kernel.org>,
	bpf@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 14/18] maccess: allow architectures to provide kernel probing directly
Date: Wed, 13 May 2020 12:36:18 -0700	[thread overview]
Message-ID: <CAHk-=wgzXqgYQQt2NCdZTtxLmV1FV1nbZ_gKw0O_mRkXZj57zg@mail.gmail.com> (raw)
In-Reply-To: <20200513160038.2482415-15-hch@lst.de>

On Wed, May 13, 2020 at 9:01 AM Christoph Hellwig <hch@lst.de> wrote:
>
> +               arch_kernel_read(dst, src, type, err_label);            \

I'm wondering if

 (a) we shouldn't expose this as an interface in general

 (b) it wouldn't be named differently..

The reason for (a) is that several users of the
"copy_from_kernel_nofault()" interfaces just seem to want a single
access from kernel mode.

The reason for (b) is that if we do expose this as a normal interface,
it shouldn't be called "arch_kernel_read", and it should have the same
semantics as "get_user_unsafe()".

IOW, maybe we should simply do exactly that: have a
"get_kernel_nofault()" thing that looks exactly like
unsafe_get_user().

On x86, it would basically be identical to unsafe_get_user().

And on architectures that only have the copy function, you'd just have
a fallback something like this:

  #define get_kernel_nofault(dst, src, err_label) do {  \
        typeof (*src) __gkn_result;                     \
        if (probe_kernel_read(&__gkn_result, src) < 0)  \
                goto err_label;                         \
        (dst) = __gkn_result;                           \
  } while (0)

and now the people who want to read a single kernel word can just do

        get_kernel_nofault(n, untrusted_pointer, error);

and they're done.

And some day - when we get reliably "asm goto" wiith outputs - that
"get_kernel_fault()" will literally be a single instruction asm with
the proper exception handler marker, the way "put_user_unsafe()"
already works (and the way "put_kernel_nofault()" would already work
if it does the above).

             Linus

  reply	other threads:[~2020-05-13 19:36 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-13 16:00 clean up and streamline probe_kernel_* and friends v2 Christoph Hellwig
2020-05-13 16:00 ` [PATCH 01/18] maccess: unexport probe_kernel_write and probe_user_write Christoph Hellwig
2020-05-13 16:00 ` [PATCH 02/18] maccess: remove various unused weak aliases Christoph Hellwig
2020-05-13 16:00 ` [PATCH 03/18] maccess: remove duplicate kerneldoc comments Christoph Hellwig
2020-05-13 16:00 ` [PATCH 04/18] maccess: clarify " Christoph Hellwig
2020-05-13 16:00 ` [PATCH 05/18] maccess: update the top of file comment Christoph Hellwig
2020-05-13 16:00 ` [PATCH 06/18] maccess: rename strncpy_from_unsafe_user to strncpy_from_user_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 07/18] maccess: rename strncpy_from_unsafe_strict to strncpy_from_kernel_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 08/18] maccess: rename strnlen_unsafe_user to strnlen_user_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 09/18] maccess: remove probe_read_common and probe_write_common Christoph Hellwig
2020-05-13 16:00 ` [PATCH 10/18] maccess: unify the probe kernel arch hooks Christoph Hellwig
2020-05-14  1:13   ` Masami Hiramatsu
2020-05-19  5:46     ` Christoph Hellwig
2020-05-13 16:00 ` [PATCH 11/18] maccess: remove strncpy_from_unsafe Christoph Hellwig
2020-05-13 19:11   ` Linus Torvalds
2020-05-13 19:11     ` Linus Torvalds
2020-05-13 19:28     ` Christoph Hellwig
2020-05-13 22:36       ` Daniel Borkmann
2020-05-13 23:03         ` Linus Torvalds
2020-05-13 23:03           ` Linus Torvalds
2020-05-13 23:24           ` Daniel Borkmann
2020-05-13 23:20         ` Masami Hiramatsu
2020-05-13 23:59           ` Linus Torvalds
2020-05-13 23:59             ` Linus Torvalds
2020-05-14  1:00             ` Masami Hiramatsu
2020-05-14  2:43               ` Linus Torvalds
2020-05-14  2:43                 ` Linus Torvalds
2020-05-14  9:44                 ` Masami Hiramatsu
2020-05-14 10:27                   ` Daniel Borkmann
2020-05-13 23:28         ` Al Viro
2020-05-13 23:58           ` Daniel Borkmann
2020-05-14 10:01             ` David Laight
2020-05-14 10:21               ` Daniel Borkmann
2020-05-13 16:00 ` [PATCH 12/18] maccess: always use strict semantics for probe_kernel_read Christoph Hellwig
2020-05-13 16:00 ` [PATCH 13/18] maccess: move user access routines together Christoph Hellwig
2020-05-13 16:00 ` [PATCH 14/18] maccess: allow architectures to provide kernel probing directly Christoph Hellwig
2020-05-13 19:36   ` Linus Torvalds [this message]
2020-05-13 19:36     ` Linus Torvalds
2020-05-13 19:40     ` Christoph Hellwig
2020-05-13 19:48       ` Linus Torvalds
2020-05-13 19:48         ` Linus Torvalds
2020-05-13 19:54         ` Christoph Hellwig
2020-05-16  3:42   ` Masami Hiramatsu
2020-05-18 15:09     ` Christoph Hellwig
2020-05-13 16:00 ` [PATCH 15/18] x86: use non-set_fs based maccess routines Christoph Hellwig
2020-05-13 16:00 ` [PATCH 16/18] maccess: rename probe_kernel_{read,write} to copy_{from,to}_kernel_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 17/18] maccess: rename probe_user_{read,write} to copy_{from,to}_user_nofault Christoph Hellwig
2020-05-13 16:00 ` [PATCH 18/18] maccess: rename probe_kernel_address to get_kernel_nofault Christoph Hellwig
2020-05-13 19:37 ` clean up and streamline probe_kernel_* and friends v2 Linus Torvalds
2020-05-13 19:37   ` Linus Torvalds
2020-05-13 23:04 ` Daniel Borkmann
2020-05-13 23:20   ` Linus Torvalds
2020-05-13 23:20     ` Linus Torvalds
2020-05-19  5:50   ` Christoph Hellwig

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