linux-kernel - Re: [PATCH 10/15] powerpc/watchpoint: Use loop for thread_struct->ptrace

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Date:   Tue, 17 Mar 2020 11:48:12 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>, mpe@...erman.id.au,
        mikey@...ling.org
Cc:     apopple@...ux.ibm.com, paulus@...ba.org, npiggin@...il.com,
        naveen.n.rao@...ux.vnet.ibm.com, peterz@...radead.org,
        jolsa@...nel.org, oleg@...hat.com, fweisbec@...il.com,
        mingo@...nel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/15] powerpc/watchpoint: Use loop for
 thread_struct->ptrace_bps



Le 09/03/2020 à 09:58, Ravi Bangoria a écrit :
> ptrace_bps is already an array of size HBP_NUM_MAX. But we use
> hardcoded index 0 while fetching/updating it. Convert such code
> to loop over array.
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
> ---
>   arch/powerpc/kernel/hw_breakpoint.c |  7 +++++--
>   arch/powerpc/kernel/process.c       |  6 +++++-
>   arch/powerpc/kernel/ptrace.c        | 28 +++++++++++++++++++++-------
>   3 files changed, 31 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c
> index f4d48f87dcb8..b27aca623267 100644
> --- a/arch/powerpc/kernel/hw_breakpoint.c
> +++ b/arch/powerpc/kernel/hw_breakpoint.c
> @@ -419,10 +419,13 @@ NOKPROBE_SYMBOL(hw_breakpoint_exceptions_notify);
>    */
>   void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
>   {
> +	int i;
>   	struct thread_struct *t = &tsk->thread;
>   
> -	unregister_hw_breakpoint(t->ptrace_bps[0]);
> -	t->ptrace_bps[0] = NULL;
> +	for (i = 0; i < nr_wp_slots(); i++) {
> +		unregister_hw_breakpoint(t->ptrace_bps[i]);
> +		t->ptrace_bps[i] = NULL;
> +	}
>   }
>   
>   void hw_breakpoint_pmu_read(struct perf_event *bp)
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index 42ff62ef749c..b9ab740fcacf 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1628,6 +1628,9 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long usp,
>   	void (*f)(void);
>   	unsigned long sp = (unsigned long)task_stack_page(p) + THREAD_SIZE;
>   	struct thread_info *ti = task_thread_info(p);
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +	int i;
> +#endif

Could we avoid all those #ifdefs ?

I think if we make p->thread.ptrace_bps[] exist all the time, with a 
size of 0 when CONFIG_HAVE_HW_BREAKPOINT is not set, then we can drop a 
lot of #ifdefs.

>   
>   	klp_init_thread_info(p);
>   
> @@ -1687,7 +1690,8 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long usp,
>   	p->thread.ksp_limit = (unsigned long)end_of_stack(p);
>   #endif
>   #ifdef CONFIG_HAVE_HW_BREAKPOINT
> -	p->thread.ptrace_bps[0] = NULL;
> +	for (i = 0; i < nr_wp_slots(); i++)
> +		p->thread.ptrace_bps[i] = NULL;
>   #endif
>   
>   	p->thread.fp_save_area = NULL;
> diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
> index f6d7955fc61e..e2651f86d56f 100644
> --- a/arch/powerpc/kernel/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace.c

You'll have to rebase all this on the series 
https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=161356 
which is about to go into powerpc-next

> @@ -2829,6 +2829,19 @@ static int set_dac_range(struct task_struct *child,
>   }
>   #endif /* CONFIG_PPC_ADV_DEBUG_DAC_RANGE */
>   
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +static int empty_ptrace_bp(struct thread_struct *thread)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr_wp_slots(); i++) {
> +		if (!thread->ptrace_bps[i])
> +			return i;
> +	}
> +	return -1;
> +}
> +#endif

What does this function do exactly ? I seems to do more than what its 
name suggests.

> +
>   #ifndef CONFIG_PPC_ADV_DEBUG_REGS
>   static int empty_hw_brk(struct thread_struct *thread)
>   {
> @@ -2915,8 +2928,9 @@ static long ppc_set_hwdebug(struct task_struct *child,
>   		len = 1;
>   	else
>   		return -EINVAL;
> -	bp = thread->ptrace_bps[0];
> -	if (bp)
> +
> +	i = empty_ptrace_bp(thread);
> +	if (i < 0)
>   		return -ENOSPC;
>   
>   	/* Create a new breakpoint request if one doesn't exist already */
> @@ -2925,14 +2939,14 @@ static long ppc_set_hwdebug(struct task_struct *child,
>   	attr.bp_len = len;
>   	arch_bp_generic_fields(brk.type, &attr.bp_type);
>   
> -	thread->ptrace_bps[0] = bp = register_user_hw_breakpoint(&attr,
> +	thread->ptrace_bps[i] = bp = register_user_hw_breakpoint(&attr,
>   					       ptrace_triggered, NULL, child);
>   	if (IS_ERR(bp)) {
> -		thread->ptrace_bps[0] = NULL;
> +		thread->ptrace_bps[i] = NULL;
>   		return PTR_ERR(bp);
>   	}
>   
> -	return 1;
> +	return i + 1;
>   #endif /* CONFIG_HAVE_HW_BREAKPOINT */
>   
>   	if (bp_info->addr_mode != PPC_BREAKPOINT_MODE_EXACT)
> @@ -2979,10 +2993,10 @@ static long ppc_del_hwdebug(struct task_struct *child, long data)
>   		return -EINVAL;
>   
>   #ifdef CONFIG_HAVE_HW_BREAKPOINT
> -	bp = thread->ptrace_bps[0];
> +	bp = thread->ptrace_bps[data - 1];

Is data checked somewhere to ensure it is not out of boundaries ? Or are 
we sure it is always within ?

>   	if (bp) {
>   		unregister_hw_breakpoint(bp);
> -		thread->ptrace_bps[0] = NULL;
> +		thread->ptrace_bps[data - 1] = NULL;
>   	} else
>   		ret = -ENOENT;
>   	return ret;
> 


Christophe