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Message-ID: <CAHk-=wgzXqgYQQt2NCdZTtxLmV1FV1nbZ_gKw0O_mRkXZj57zg@mail.gmail.com>
Date: Wed, 13 May 2020 12:36:18 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Christoph Hellwig <hch@....de>
Cc: "the arch/x86 maintainers" <x86@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Masami Hiramatsu <mhiramat@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-parisc@...r.kernel.org,
linux-um <linux-um@...ts.infradead.org>,
Netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org,
Linux-MM <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 14/18] maccess: allow architectures to provide kernel
probing directly
On Wed, May 13, 2020 at 9:01 AM Christoph Hellwig <hch@....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
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