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

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Date:   Thu, 30 Apr 2020 12:51:38 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.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 12:23 PM Luck, Tony <tony.luck@...el.com> wrote:
>
> On Thu, Apr 30, 2020 at 11:42:20AM -0700, Andy Lutomirski wrote:
> > I suppose there could be a consistent naming like this:
> >
> > copy_from_user()
> > copy_to_user()
> >
> > copy_from_unchecked_kernel_address() [what probe_kernel_read() is]
> > copy_to_unchecked_kernel_address() [what probe_kernel_write() is]
> >
> > copy_from_fallible() [from a kernel address that can fail to a kernel
> > address that can't fail]
> > copy_to_fallible() [the opposite, but hopefully identical to memcpy() on x86]
> >
> > copy_from_fallible_to_user()
> > copy_from_user_to_fallible()
> >
> > These names are fairly verbose and could probably be improved.
>
> How about
>
>         try_copy_catch(void *dst, void *src, size_t count, int *fault)
>
> returns number of bytes not-copied (like copy_to_user etc).
>
> if return is not zero, "fault" tells you what type of fault
> cause the early stop (#PF, #MC).

I do like try_copy_catch() for the case when neither of the buffers
are __user (like in the pmem driver) and _copy_to_iter_fallible()
(plus all the helpers it implies) for the other cases.

copy_to_user_fallible
copy_fallible_to_page
copy_pipe_to_iter_fallible

...because the mmu-fault handling is an aspect of "_user" and fallible
implies other source fault reasons. It does leave a gap if an
architecture has a concept of a fallible write, but that seems like
something that will be handled asynchronously and not subject to this
interface.