Re: [PATCH] ARM: mm: Facilitate debugging CONFIG_KUSER_HELPERS disabled

From: ebiederm@xmission.com (Eric W. Biederman)
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-arm-kernel@lists.infradead.org, andrew@lunn.ch,
	Russell King <linux@armlinux.org.uk>,
	Tony Lindgren <tony@atomide.com>,
	Souptick Joarder <jrdr.linux@gmail.com>,
	linux-kernel@vger.kernel.org (open list)
Subject: Re: [PATCH] ARM: mm: Facilitate debugging CONFIG_KUSER_HELPERS disabled
Date: Thu, 25 Oct 2018 13:25:45 -0500	[thread overview]
Message-ID: <87tvl9nas6.fsf@xmission.com> (raw)
In-Reply-To: <20181025180957.7900-1-f.fainelli@gmail.com> (Florian Fainelli's message of "Thu, 25 Oct 2018 11:09:55 -0700")

Florian Fainelli <f.fainelli@gmail.com> writes:

> 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.

Can't someone trigger this segv deliberately and spam the kerne log?
Doesn't this need something like printk_ratelimit or something to ensure
this message only gets printed once?

Eric

> Suggested-by: Russell King <rmk+kernel@armlinux.org.uk>
> Signed-off-by: Florian Fainelli <f.fainelli@gmail.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
> +	if ((sig == SIGSEGV) && ((addr & PAGE_MASK) == 0xffff0000))
> +		printk(KERN_DEBUG "%s: CONFIG_KUSER_HELPERS disabled at 0x%08lx\n",
> +		       tsk->comm, addr);
> +#endif
>  
>  	tsk->thread.address = addr;
>  	tsk->thread.error_code = fsr;