linux-kernel - RE: [PATCH] libnvdimm/pmem: Bypass CONFIG_HARDENED

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Date:   Fri, 17 May 2019 09:06:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jan Kara' <jack@...e.cz>, Dan Williams <dan.j.williams@...el.com>
CC:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Jeff Moyer <jmoyer@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Smits <jeff.smits@...el.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: RE: [PATCH] libnvdimm/pmem: Bypass CONFIG_HARDENED_USERCOPY overhead

From: Jan Kara
> Sent: 17 May 2019 09:48
...
> So this last paragraph is not obvious to me as check_copy_size() does a lot
> of various checks in CONFIG_HARDENED_USERCOPY case. I agree that some of
> those checks don't make sense for PMEM pages but I'd rather handle that by
> refining check_copy_size() and check_object_size() functions to detect and
> appropriately handle pmem pages rather that generally skip all the checks
> in pmem_copy_from/to_iter(). And yes, every check in such hot path is going
> to cost performance but that's what user asked for with
> CONFIG_HARDENED_USERCOPY... Kees?

Except that all the distros enable it by default.
So you get the performance cost whether you (as a user) want it or not.

I've changed some of our code to use __get_user() to avoid
these stupid overheads.

	David

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