linux-kernel - Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy

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Date:   Thu, 30 Apr 2020 17:10:15 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        stable <stable@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Erwin Tsaur <erwin.tsaur@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

On Thu, Apr 30, 2020 at 4:52 PM Dan Williams <dan.j.williams@...el.com> wrote:
>
> You had me until here. Up to this point I was grokking that Andy's
> "_fallible" suggestion does help explain better than "_safe", because
> the copy is doing extra safety checks. copy_to_user() and
> copy_to_user_fallible() mean *something* where copy_to_user_safe()
> does not.

It's a horrible word, btw. The word doesn't actually mean what Andy
means it to mean. "fallible" means "can make mistakes", not "can
fault".

So "fallible" is a horrible name.

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.

It was the "copy_safe()" model that I find unacceptable, that uses
_one_ name for what is at the very least *four* different operations:

 - copy from faulting memory to user

 - copy from faulting memory to kernel

 - copy from kernel to faulting memory

 - copy within faulting memory

No way can you do that with one single function. A kernel address and
a user address may literally have the exact same bit representation.
So the user vs kernel distinction _has_ to be in the name.

The "kernel vs faulting" doesn't necessarily have to be there from an
implementation standpoint, but it *should* be there, because

 - it might affect implemmentation

 - but even if it DOESN'T affect implementation, it should be separate
just from the standpoint of being self-documenting code.

> However you lose me on this "broken nvdimm semantics" contention.
> There is nothing nvdimm-hardware specific about the copy_safe()
> implementation, zero, nada, nothing new to the error model that DRAM
> did not also inflict on the Linux implementation.

Ok, so good. Let's kill this all, and just use memcpy(), and copy_to_user().

Just make sure that the nvdimm code doesn't use invalid kernel
addresses or other broken poisoning.

Problem solved.

You can't have it both ways. Either memcpy just works, or it doesn't.

So which way is it?

                  Linus