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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2] powerpc/mm: make NULL pointer deferences explicit on bad page faults.
In-Reply-To: <46e2533084dc3255a1d67553b682bd4b9642b176.1544800765.git.christophe.leroy@c-s.fr>
References: <46e2533084dc3255a1d67553b682bd4b9642b176.1544800765.git.christophe.leroy@c-s.fr>
Date:   Mon, 17 Dec 2018 23:28:19 +1100
Message-ID: <87mup4tk0s.fsf@concordia.ellerman.id.au>
MIME-Version: 1.0
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Sender: linux-kernel-owner@vger.kernel.org
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Christophe Leroy <christophe.leroy@c-s.fr> writes:

> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 01b9bcc7fa85..3398291f4785 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -636,21 +636,24 @@ void bad_page_fault(struct pt_regs *regs, unsigned long address, int sig)
>  	switch (TRAP(regs)) {
>  	case 0x300:
>  	case 0x380:
> -		printk(KERN_ALERT "Unable to handle kernel paging request for "
> -			"data at address 0x%08lx\n", regs->dar);
> +		if (regs->dar < PAGE_SIZE)
> +			pr_alert("BUG: Kernel NULL pointer dereference");
> +		else
> +			pr_alert("BUG: Unable to handle kernel data access");
> +		pr_cont(" at 0x%08lx\n", regs->dar);

It's best to avoid pr_cont() as it can lead to interleaving, so I
rewrote this as:

		pr_alert("BUG: %s at 0x%08lx\n",
			 regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" :
			 "Unable to handle kernel data access", regs->dar);


>  		break;
>  	case 0x400:
>  	case 0x480:
> -		printk(KERN_ALERT "Unable to handle kernel paging request for "
> -			"instruction fetch\n");
> +		pr_alert("BUG: Unable to handle kernel instruction fetch%s",
> +			 regs->nip < PAGE_SIZE ? " (NULL pointer ?)\n" : "\n");
                                       I dropped the space here ^


cheers