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

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Date:   Thu, 30 Apr 2020 09:51:44 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Tony Luck <tony.luck@...el.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 7:03 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Apr 30, 2020 at 1:41 AM Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > With the above realizations the name "mcsafe" is no longer accurate and
> > copy_safe() is proposed as its replacement. x86 grows a copy_safe_fast()
> > implementation as a default implementation that is independent of
> > detecting the presence of x86-MCA.
>
> How is this then different from "probe_kernel_read()" and
> "probe_kernel_write()"? Other than the obvious "it does it for both
> reads and writes"?
>
> IOW, wouldn't it be sensible to try to match the naming and try to
> find some unified model for all these things?
>
> "probe_kernel_copy()"?

I don't like this whole concept.

If I'm going to copy from memory that might be bad but is at least a
valid pointer, I want a function to do this.  If I'm going to copy
from memory that might be entirely bogus, that's a different
operation.  In other words, if I'm writing e.g. filesystem that is
touching get_user_pages()'d persistent memory, I don't want to panic
if the memory fails, but I do want at least a very loud warning if I
follow a wild pointer.

So I think that probe_kernel_copy() is not a valid replacement for
memcpy_mcsafe().

--Andy