linux-kernel - Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page

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Message-Id: <87wpaa187u.fsf@skywalker.in.ibm.com>
Date:   Mon, 24 Apr 2017 14:41:01 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/5] powerpc/mm: split store_updates_sp() in two parts in do_page_fault()

Christophe Leroy <christophe.leroy@....fr> writes:

> Only the get_user() in store_updates_sp() has to be done outside
> the mm semaphore. All the comparison can be done within the semaphore,
> so only when really needed.
>
> As we got a DSI exception, the address pointed by regs->nip is
> obviously valid, otherwise we would have had a instruction exception.
> So __get_user() can be used instead of get_user()
>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/mm/fault.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 67fefb59d40e..9d21e5fd383d 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -73,12 +73,8 @@ static inline int notify_page_fault(struct pt_regs *regs)
>   * Check whether the instruction at regs->nip is a store using
>   * an update addressing form which will update r1.
>   */
> -static int store_updates_sp(struct pt_regs *regs)
> +static int store_updates_sp(unsigned int inst)
>  {
> -	unsigned int inst;
> -
> -	if (get_user(inst, (unsigned int __user *)regs->nip))
> -		return 0;
>  	/* check for 1 in the rA field */
>  	if (((inst >> 16) & 0x1f) != 1)
>  		return 0;
> @@ -207,7 +203,8 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>  	int trap = TRAP(regs);
>   	int is_exec = trap == 0x400;
>  	int fault;
> -	int rc = 0, store_update_sp = 0;
> +	int rc = 0;
> +	unsigned int inst = 0;
>  
>  #if !(defined(CONFIG_4xx) || defined(CONFIG_BOOKE))
>  	/*
> @@ -288,7 +285,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>  	 * mmap_sem held
>  	 */
>  	if (is_write && user_mode(regs))
> -		store_update_sp = store_updates_sp(regs);
> +		__get_user(inst, (unsigned int __user *)regs->nip);
>  
>  	if (user_mode(regs))
>  		flags |= FAULT_FLAG_USER;
> @@ -358,7 +355,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>  		 * between the last mapped region and the stack will
>  		 * expand the stack rather than segfaulting.
>  		 */
> -		if (address + 2048 < uregs->gpr[1] && !store_update_sp)
> +		if (address + 2048 < uregs->gpr[1] && !store_updates_sp(inst))
>  			goto bad_area;
>  	}
>  	if (expand_stack(vma, address))
> -- 
> 2.12.0