linux-kernel - Re: [PATCH] arm: fix page faults in do

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Date:   Sat, 31 Aug 2019 08:55:24 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Jing Xiangfeng <jingxiangfeng@...wei.com>
Cc:     ebiederm@...ssion.com, kstewart@...uxfoundation.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        bhelgaas@...gle.com, tglx@...utronix.de,
        sakari.ailus@...ux.intel.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] arm: fix page faults in do_alignment

On Sat, Aug 31, 2019 at 09:49:45AM +0800, Jing Xiangfeng wrote:
> On 2019/8/30 21:35, Russell King - ARM Linux admin wrote:
> > On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote:
> >> The function do_alignment can handle misaligned address for user and
> >> kernel space. If it is a userspace access, do_alignment may fail on
> >> a low-memory situation, because page faults are disabled in
> >> probe_kernel_address.
> >>
> >> Fix this by using __copy_from_user stead of probe_kernel_address.
> >>
> >> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code")
> >> Signed-off-by: Jing Xiangfeng <jingxiangfeng@...wei.com>
> > 
> > NAK.
> > 
> > The "scheduling while atomic warning in alignment handling code" is
> > caused by fixing up the page fault while trying to handle the
> > mis-alignment fault generated from an instruction in atomic context.
> 
> __might_sleep is called in the function  __get_user which lead to that bug.
> And that bug is triggered in a kernel space. Page fault can not be generated.
> Right?

Your email is now fixed?

All of get_user(), __get_user(), copy_from_user() and __copy_from_user()
_can_ cause a page fault, which might need to fetch the page from disk.
All these four functions are equivalent as far as that goes - and indeed
as are their versions that write as well.

If the page needs to come from disk, all of these functions _will_
sleep.  If they are called from an atomic context, and the page fault
handler needs to fetch data from disk, they will attempt to sleep,
which will issue a warning.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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