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

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Date:   Fri, 1 May 2020 14:09:20 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     "Williams, Dan J" <dan.j.williams@...el.com>,
        Andy Lutomirski <luto@...nel.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>,
        "Tsaur, Erwin" <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()

> Now maybe copy_to_user() should *always* work this way, but I’m not convinced.
> Certainly put_user() shouldn’t — the result wouldn’t even be well defined. And I’m
>  unconvinced that it makes much sense for the majority of copy_to_user() callers
>  that are also directly accessing the source structure.

One case that might work is copy_to_user() that's copying from the kernel page cache
to the user in response to a read(2) system call.  Action would be to check if we could
re-read from the file system to a different page. If not, return -EIO. Either way ditch the
poison page from the page cache.

-Tony