Re: [PATCH v2] powerpc/mm: make NULL pointer deferences explicit on bad page faults.

[Michael Ellerman]

Christophe Leroy <christophe.leroy@xxxxxx> 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