linux-kernel - Re: [PATCH] ARM: mm: Facilitate debugging CONFIG_KUSER

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Date:   Thu, 25 Oct 2018 22:20:06 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Souptick Joarder <jrdr.linux@...il.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        linux-arm-kernel@...ts.infradead.org, Andrew Lunn <andrew@...n.ch>,
        ebiederm@...ssion.com, Tony Lindgren <tony@...mide.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: mm: Facilitate debugging CONFIG_KUSER_HELPERS
 disabled

On Fri, Oct 26, 2018 at 12:00:09AM +0530, Souptick Joarder wrote:
> On Thu, Oct 25, 2018 at 11:40 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> >
> > Some software such as perf makes unconditional use of the special
> > [vectors] page which is only provided when CONFIG_KUSER_HELPERS is
> > enabled in the kernel.
> >
> > Facilitate the debugging of such situations by printing a debug message
> > to the kernel log showing the task name and the faulting address.
> >
> > Suggested-by: Russell King <rmk+kernel@...linux.org.uk>
> > Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> > ---
> >  arch/arm/mm/fault.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> > index f4ea4c62c613..f17471fbc1c4 100644
> > --- a/arch/arm/mm/fault.c
> > +++ b/arch/arm/mm/fault.c
> > @@ -173,6 +173,11 @@ __do_user_fault(struct task_struct *tsk, unsigned long addr,
> >                 show_regs(regs);
> >         }
> >  #endif
> > +#ifndef CONFIG_KUSER_HELPERS
> 
> Just have one doubt, if the condition is "#ifdef CONFIG_KUSER_HELPER"
> as commit message suggests the scenario is valid when CONFIG_KUSER_HELPER
> is enabled ? No ?
> 
> > +       if ((sig == SIGSEGV) && ((addr & PAGE_MASK) == 0xffff0000))
> > +               printk(KERN_DEBUG "%s: CONFIG_KUSER_HELPERS disabled at 0x%08lx\n",
> > +                      tsk->comm, addr);
> > +#endif

The idea is to print a message when we get a SEGV _and_ the faulting
address is in the vectors page _and_ CONFIG_KUSER_HELPERS is _not_ set
(which makes the page inaccessible.)  The message points users to
enable it and/or why the application has failed.

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