linux-kernel - Re: [PATCH] MIPS: mm: abort uaccess retries upon fatal signal

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Date:   Fri, 22 Jan 2021 11:28:43 +0100
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: mm: abort uaccess retries upon fatal signal

On Fri, Jan 22, 2021 at 10:11:57AM +0000, Mark Rutland wrote:
> On Thu, Jan 21, 2021 at 05:04:16PM +0100, Thomas Bogendoerfer wrote:
> > When there's a fatal signal pending, MIPS's do_page_fault()
> > implementation returns. The intent is that we'll return to the
> > faulting userspace instruction, delivering the signal on the way.
> > 
> > However, if we take a fatal signal during fixing up a uaccess, this
> > results in a return to the faulting kernel instruction, which will be
> > instantly retried, resulting in the same fault being taken forever. As
> > the task never reaches userspace, the signal is not delivered, and the
> > task is left unkillable. While the task is stuck in this state, it can
> > inhibit the forward progress of the system.
> > 
> > To avoid this, we must ensure that when a fatal signal is pending, we
> > apply any necessary fixup for a faulting kernel instruction. Thus we
> > will return to an error path, and it is up to that code to make forward
> > progress towards delivering the fatal signal.
> > 
> > [ Description taken from commit 746a272e4414 ("ARM: 8692/1: mm: abort
> >    uaccess retries upon fatal signal") ]
> > 
> > Signed-off-by: Thomas Bogendoerfer <tsbogend@...ha.franken.de>
> 
> FWIW, this looks right to me, from a scan of the no_context path. I
> don't have any MIPS system to test on, but FWIW:

no worry, I've tested it ;-)

Thomas.

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