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

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Date:   Fri, 1 May 2020 07:46:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...nel.org>,
        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()

From: Andy Lutomirski
> Sent: 30 April 2020 19:42
...
> 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()

You missed out:
copy_to/from_io()
copy_to_io_from_user()
copy_from_io_to_user()
All of which want aligned addresses on the 'io' side.

It might even be worth saying that the copy_to/from_io() can
fail due to bad IO accesses (rather than bad addresses).
This is not entirely unexpected since all PCIe accesses
can fail unexpectedly (usually without a trap and returning -1).
But a system could arrange to generate a synchronous fault.

If you are copying directly from io to user you need to
differentiate between a user page fault and an io access
error. The latter shouldn't generate SIGSEGV.
Possibly return -EFAULT on user page fault and 'transfer
length remaining' on io access error.
Although filling the rest of the buffer with 0xff might be
appropriate.

	David

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