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

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Message-ID: <CALCETrVq11YVqGZH7J6A=tkHB1AZUWXnKwAfPUQ-m9qXjWfZtg@mail.gmail.com>
Date:   Thu, 30 Apr 2020 11:42:20 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 10:17 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Apr 30, 2020 at 9:52 AM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > 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().
>
> Fair enough.
>
> That said, the part I do like about probe_kernel_read/write() is that
> it does indicate which part we think is possibly the one that needs
> more care.
>
> Sure, it _might_ be both sides, but honestly, that's likely the much
> less common case. Kind of like "copy_{to,from}_user()" vs
> "copy_in_user()".
>
> Yes, the "copy_in_user()" case exists, but it's the odd and unusual case.

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.