Subject: [PATCH 1/2] arm64: mm: print out correct page table entries

From: [email-masked] (Will Deacon)
Date: Mon, 22 May 2017 13:18:54 +0100

Hi Kristina,

On Mon, May 22, 2017 at 12:45:53PM +0100, Kristina Martsenko wrote:
> When we take a fault that can't be handled, we print out the page table
> entries associated with the faulting address. In some cases we currently
> print out the wrong entries. For a faulting TTBR1 address, we sometimes
> print out TTBR0 table entries instead, and for a faulting TTBR0 address
> we sometimes print out TTBR1 table entries. Fix this by choosing the
> tables based on the faulting address.
> 
> Signed-off-by: Kristina Martsenko <[email-masked]>
> ---
>  arch/arm64/include/asm/system_misc.h |  2 +-
>  arch/arm64/mm/fault.c                | 30 ++++++++++++++++++++----------
>  2 files changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/system_misc.h b/arch/arm64/include/asm/system_misc.h
> index bc812435bc76..d0beefeb6d25 100644
> --- a/arch/arm64/include/asm/system_misc.h
> +++ b/arch/arm64/include/asm/system_misc.h
> @@ -40,7 +40,7 @@ void hook_debug_fault_code(int nr, int (*fn)(unsigned long, unsigned int,
>  			   int sig, int code, const char *name);
>  
>  struct mm_struct;
> -extern void show_pte(struct mm_struct *mm, unsigned long addr);
> +extern void show_pte(unsigned long addr);
>  extern void __show_regs(struct pt_regs *);
>  
>  extern void (*arm_pm_restart)(enum reboot_mode reboot_mode, const char *cmd);
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 37b95dff0b07..0c32f34fb4af 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -80,14 +80,25 @@ static inline int notify_page_fault(struct pt_regs *regs, unsigned int esr)
>  #endif
>  
>  /*
> - * Dump out the page tables associated with 'addr' in mm 'mm'.
> + * Dump out the page tables associated with 'addr' in the currently active mm.
>   */
> -void show_pte(struct mm_struct *mm, unsigned long addr)
> +void show_pte(unsigned long addr)
>  {
> +	unsigned long ttbr_mask = ~0UL << VA_BITS;
> +	struct mm_struct *mm;
>  	pgd_t *pgd;
>  
> -	if (!mm)
> +	if (!(addr & ttbr_mask)) {
> +		/* TTBR0 */
> +		mm = current->active_mm;
> +	} else if ((addr & ttbr_mask) == ttbr_mask) {
> +		/* TTBR1 */
>  		mm = &init_mm;

I wonder if this would be better structured as:

	if (addr < TASK_SIZE)
		/* TTBR0 */
	else if (addr >= VA_START)
		/* TTBR1 */
	else
		/* no man's land */

what do you think?

> @@ -196,8 +207,8 @@ static inline bool is_permission_fault(unsigned int esr, struct pt_regs *regs,
>  /*
>   * The kernel tried to access some page that wasn't present.
>   */
> -static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr,
> -			      unsigned int esr, struct pt_regs *regs)
> +static void __do_kernel_fault(unsigned long addr, unsigned int esr,
> +			      struct pt_regs *regs)
>  {
>  	const char *msg;
>  
> @@ -227,7 +238,7 @@ static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr,
>  	pr_alert("Unable to handle kernel %s at virtual address %08lx\n", msg,
>  		 addr);
>  
> -	show_pte(mm, addr);
> +	show_pte(addr);

Now that you're not passing the mm, __do_kernel_fault doesn't need it
for anything so there's further cleanup you can do here.

>  	die("Oops", regs, esr);
>  	bust_spinlocks(0);
>  	do_exit(SIGKILL);
> @@ -249,7 +260,7 @@ static void __do_user_fault(struct task_struct *tsk, unsigned long addr,
>  		pr_info("%s[%d]: unhandled %s (%d) at 0x%08lx, esr 0x%03x\n",
>  			tsk->comm, task_pid_nr(tsk), inf->name, sig,
>  			addr, esr);
> -		show_pte(tsk->mm, addr);
> +		show_pte(addr);
>  		show_regs(regs);
>  	}
>  
> @@ -265,7 +276,6 @@ static void __do_user_fault(struct task_struct *tsk, unsigned long addr,
>  static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *regs)
>  {
>  	struct task_struct *tsk = current;
> -	struct mm_struct *mm = tsk->active_mm;
>  	const struct fault_info *inf;
>  
>  	/*
> @@ -276,7 +286,7 @@ static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *re
>  		inf = esr_to_fault_info(esr);
>  		__do_user_fault(tsk, addr, esr, inf->sig, inf->code, regs);
>  	} else
> -		__do_kernel_fault(mm, addr, esr, regs);
> +		__do_kernel_fault(addr, esr, regs);

Similar thing here: I don't think the mm local variable is used anymore.
Does GCC not warn about this?

Will