From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "Luck, Tony" <tony.luck@intel.com>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Borislav Petkov <bp@alien8.de>, stable <stable@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Paul Mackerras <paulus@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Erwin Tsaur <erwin.tsaur@intel.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
linux-nvdimm <linux-nvdimm@lists.01.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()
Date: Thu, 30 Apr 2020 17:39:46 -0700 [thread overview]
Message-ID: <CAHk-=wgeMRm_yhb_fwvmgdaPMYzgXY01cYvw5htHUCTwSzswqg@mail.gmail.com> (raw)
In-Reply-To: <1962EE67-8AD1-409D-963A-4F1E1AB3B865@amacapital.net>
On Thu, Apr 30, 2020 at 5:23 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
> > But anyway, I don't hate something like "copy_to_user_fallible()"
> > conceptually. The naming needs to be fixed, in that "user" can always
> > take a fault, so it's the _source_ that can fault, not the "user"
> > part.
>
> I don’t like this. “user” already implied that basically anything can be wrong with the memory
Maybe I didn't explain.
"user" already implies faulting. We agree.
And since we by definition cannot know what the user has mapped into
user space, *every* normal copy_to_user() has to be able to handle
whatever faults that throws at us.
The reason I dislike "copy_to_user_fallible()" is that the user side
already has that 'fallible".
If it's the _source_ being "fallible" (it really needs a better name -
I will not call it just "f") then it should be "copy_f_to_user()".
That would be ok.
So "copy_f_to_user()" makes sense. But "copy_to_user_f()" does not.
That puts the "f" on the "user", which we already know can fault.
See what I want in the name? I want the name to say which side can
cause problems!
If you are copying memory from some f source, it must not be
"copy_safe()". That doesn't say if the _source_ is f, or the
destination is f.
So "copy_to_f()" makes sense (we don't say "copy_kernel_to_user()" -
the "normal" case is silent), and "copy_from_f()" makes sense.
"copy_in_f()" makes sense too.
But not this "randomly copy some randomly f memory area that I don't
know if it's the source or the destination".
Sometimes you may then use a common implementation for the different
directions - if that works on the architecture.
For example, "copy_to_user()" and "copy_from_user()" on x86 both end
up internally using a shared "copy_user_generic()" implementation. I
wish that wasn't the case (when I asked for what was to become
STAC/CLAC, I asked for one that could determine which direction of a
"rep movs" could touch user space), but it so happens that the
implementations end up being symmetric on x86.
But that's a pure implementation issue, and it very much is not going
to be true in general, and it shouldn't be exposed to users as such
(and we obviously don't).
Linus
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next prev parent reply other threads:[~2020-05-01 0:40 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-30 8:24 [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 1/2] copy_safe: Rename memcpy_mcsafe() to copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 2/2] x86/copy_safe: Introduce copy_safe_fast() Dan Williams
2020-04-30 14:02 ` [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Linus Torvalds
2020-04-30 16:51 ` Andy Lutomirski
2020-04-30 17:17 ` Linus Torvalds
2020-04-30 18:42 ` Andy Lutomirski
2020-04-30 19:22 ` Luck, Tony
2020-04-30 19:50 ` Linus Torvalds
2020-04-30 20:25 ` Luck, Tony
2020-04-30 23:52 ` Dan Williams
2020-05-01 0:10 ` Linus Torvalds
2020-05-01 0:23 ` Andy Lutomirski
2020-05-01 0:39 ` Linus Torvalds [this message]
2020-05-01 1:10 ` Andy Lutomirski
2020-05-01 14:09 ` Luck, Tony
2020-05-03 0:29 ` Andy Lutomirski
2020-05-04 20:05 ` Luck, Tony
2020-05-04 20:26 ` Andy Lutomirski
2020-05-04 21:30 ` Dan Williams
2020-05-01 0:24 ` Linus Torvalds
2020-05-01 1:20 ` Andy Lutomirski
2020-05-01 1:21 ` Dan Williams
2020-05-01 18:28 ` Linus Torvalds
2020-05-01 20:17 ` Dave Hansen
2020-05-03 12:57 ` David Laight
2020-05-04 18:33 ` Dan Williams
2020-05-11 15:24 ` Vivek Goyal
2020-04-30 19:51 ` Dan Williams
2020-04-30 20:07 ` Andy Lutomirski
2020-05-01 7:46 ` David Laight
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