Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

From: Andy Lutomirski <luto@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Tony Luck <tony.luck@intel.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 11:42:20 -0700	[thread overview]
Message-ID: <CALCETrVq11YVqGZH7J6A=tkHB1AZUWXnKwAfPUQ-m9qXjWfZtg@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=widQfxhWMUN3bGxM_zg3az0fRKYvFoP8bEhqsCtaEDVAA@mail.gmail.com>

On Thu, Apr 30, 2020 at 10:17 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Apr 30, 2020 at 9:52 AM Andy Lutomirski <luto@kernel.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.
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