rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

[Dave Jones]

===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
kernel/pid.c:419 invoked rcu_dereference_check() without protection!

other info that might help us debug this:


rcu_scheduler_active = 1, debug_locks = 1
1 lock held by scrashme/20820:
 #0:  (tasklist_lock){.?.?..}, at: [<ffffffff8106e30f>] posix_cpu_timer_create+0x50/0xee

stack backtrace:
Pid: 20820, comm: scrashme Not tainted 2.6.37-rc3+ #7
Call Trace:
 [<ffffffff8107cfd5>] lockdep_rcu_dereference+0x9d/0xa5
 [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
 [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
 [<ffffffff8106e32d>] posix_cpu_timer_create+0x6e/0xee
 [<ffffffff8106eb88>] do_cpu_nanosleep+0x83/0x1ad
 [<ffffffff8106f50a>] posix_cpu_nsleep+0x6d/0xf6
 [<ffffffff810f9a64>] ? might_fault+0xa5/0xac
 [<ffffffff810f9a1b>] ? might_fault+0x5c/0xac
 [<ffffffff8106bf57>] sys_clock_nanosleep+0x7c/0xcb
 [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b

Signed-off-by: Dave Jones <davej@redhat.com>

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 6842eeb..2658955 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
 	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
 
 	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
 		if (pid == 0) {
 			p = current;
@@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
 	} else {
 		ret = -EINVAL;
 	}
+	rcu_read_unlock();
 	read_unlock(&tasklist_lock);
 
 	return ret;

rcu_read_lock/unlock protect find_task_by_vpid call in check_clock

[Dave Jones]

===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
kernel/pid.c:419 invoked rcu_dereference_check() without protection!

other info that might help us debug this:


rcu_scheduler_active = 1, debug_locks = 1
1 lock held by scrashme/13382:
 #0:  (tasklist_lock){.?.?..}, at: [<ffffffff8106ddea>] check_clock+0x46/0x9a

stack backtrace:
Pid: 13382, comm: scrashme Not tainted 2.6.37-rc3+ #8
Call Trace:
 [<ffffffff8107cfe1>] lockdep_rcu_dereference+0x9d/0xa5
 [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
 [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
 [<ffffffff8106ddf2>] check_clock+0x4e/0x9a
 [<ffffffff8106deac>] posix_cpu_clock_getres+0x16/0x41
 [<ffffffff8106be74>] sys_clock_getres+0x39/0xa0
 [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b

Signed-off-by: Dave Jones <davej@redhat.com>

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 6842eeb..4bef9aa 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -38,11 +38,13 @@ static int check_clock(const clockid_t which_clock)
 		return 0;
 
 	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	p = find_task_by_vpid(pid);
 	if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
 		   same_thread_group(p, current) : thread_group_leader(p))) {
 		error = -EINVAL;
 	}
+	rcu_read_unlock();
 	read_unlock(&tasklist_lock);
 
 	return error;

Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

[Peter Zijlstra]

On Wed, 2010-11-24 at 20:09 -0500, Dave Jones wrote:
> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> kernel/pid.c:419 invoked rcu_dereference_check() without protection!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 1
> 1 lock held by scrashme/20820:
>  #0:  (tasklist_lock){.?.?..}, at: [<ffffffff8106e30f>] posix_cpu_timer_create+0x50/0xee
> 
> stack backtrace:
> Pid: 20820, comm: scrashme Not tainted 2.6.37-rc3+ #7
> Call Trace:
>  [<ffffffff8107cfd5>] lockdep_rcu_dereference+0x9d/0xa5
>  [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
>  [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
>  [<ffffffff8106e32d>] posix_cpu_timer_create+0x6e/0xee
>  [<ffffffff8106eb88>] do_cpu_nanosleep+0x83/0x1ad
>  [<ffffffff8106f50a>] posix_cpu_nsleep+0x6d/0xf6
>  [<ffffffff810f9a64>] ? might_fault+0xa5/0xac
>  [<ffffffff810f9a1b>] ? might_fault+0x5c/0xac
>  [<ffffffff8106bf57>] sys_clock_nanosleep+0x7c/0xcb
>  [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b
> 
> Signed-off-by: Dave Jones <davej@redhat.com>
> 
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index 6842eeb..2658955 100644
> --- a/kernel/posix-cpu-timers.c
> +++ b/kernel/posix-cpu-timers.c
> @@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
>  	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
>  
>  	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
>  		if (pid == 0) {
>  			p = current;
> @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
>  	} else {
>  		ret = -EINVAL;
>  	}
> +	rcu_read_unlock();
>  	read_unlock(&tasklist_lock);
>  
>  	return ret;

Do we still need the tasklist_lock in this case?

Also, why is that think complaining, surely the tasklist_lock pins any
and all PID objects?

Re: rcu_read_lock/unlock protect find_task_by_vpid call in check_clock

[Peter Zijlstra]

On Wed, 2010-11-24 at 20:35 -0500, Dave Jones wrote:
> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> kernel/pid.c:419 invoked rcu_dereference_check() without protection!
> 
> other info that might help us debug this:
> 
> 
> rcu_scheduler_active = 1, debug_locks = 1
> 1 lock held by scrashme/13382:
>  #0:  (tasklist_lock){.?.?..}, at: [<ffffffff8106ddea>] check_clock+0x46/0x9a
> 
> stack backtrace:
> Pid: 13382, comm: scrashme Not tainted 2.6.37-rc3+ #8
> Call Trace:
>  [<ffffffff8107cfe1>] lockdep_rcu_dereference+0x9d/0xa5
>  [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
>  [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
>  [<ffffffff8106ddf2>] check_clock+0x4e/0x9a
>  [<ffffffff8106deac>] posix_cpu_clock_getres+0x16/0x41
>  [<ffffffff8106be74>] sys_clock_getres+0x39/0xa0
>  [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b
> 
> Signed-off-by: Dave Jones <davej@redhat.com>
> 
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index 6842eeb..4bef9aa 100644
> --- a/kernel/posix-cpu-timers.c
> +++ b/kernel/posix-cpu-timers.c
> @@ -38,11 +38,13 @@ static int check_clock(const clockid_t which_clock)
>  		return 0;
>  
>  	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	p = find_task_by_vpid(pid);
>  	if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
>  		   same_thread_group(p, current) : thread_group_leader(p))) {
>  		error = -EINVAL;
>  	}
> +	rcu_read_unlock();
>  	read_unlock(&tasklist_lock);
>  
>  	return error;


Pretty much the same comment as the other patch..

<copy/paste>

Do we still need the tasklist_lock in this case?

Also, why is that think complaining, surely the tasklist_lock pins any
and all PID objects?

Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

[Oleg Nesterov]

(add Sergey)

On 11/25, Peter Zijlstra wrote:
>
> On Wed, 2010-11-24 at 20:09 -0500, Dave Jones wrote:
> > --- a/kernel/posix-cpu-timers.c
> > +++ b/kernel/posix-cpu-timers.c
> > @@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> >  	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
> >
> >  	read_lock(&tasklist_lock);
> > +	rcu_read_lock();
> >  	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
> >  		if (pid == 0) {
> >  			p = current;
> > @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> >  	} else {
> >  		ret = -EINVAL;
> >  	}
> > +	rcu_read_unlock();
> >  	read_unlock(&tasklist_lock);
> >
> >  	return ret;
>
> Do we still need the tasklist_lock in this case?

No. posix-cpu-timer.c shouldn't use tasklist at all. But it is not
completely trivial to remove it.

In particular, this patch is not exactly right, we can't trust
thread_group_leader() without tasklist.

Sergey already sent the patch which removes tasklist from
posix_cpu_timer_create() and posix_cpu_timer_create(), and iirc
Thomas queued it.

> Also, why is that think complaining, surely the tasklist_lock pins any
> and all PID objects?

The only problem is: if copy_process() fails, it does free_pid()
lockless. This means, without rcu lock it is not safe to scan the
rcu-protected lists.

We can change copy_process() (in fact I sent the patch several
years ago), but everybody think that find_pid/etc should always
take rcu_read_lock() instead. I tend to agree.

Oleg.

Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

[Oleg Nesterov]

(another try, actually add Sergey)

On 11/25, Peter Zijlstra wrote:
>
> On Wed, 2010-11-24 at 20:09 -0500, Dave Jones wrote:
> > --- a/kernel/posix-cpu-timers.c
> > +++ b/kernel/posix-cpu-timers.c
> > @@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> >  	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
> >
> >  	read_lock(&tasklist_lock);
> > +	rcu_read_lock();
> >  	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
> >  		if (pid == 0) {
> >  			p = current;
> > @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> >  	} else {
> >  		ret = -EINVAL;
> >  	}
> > +	rcu_read_unlock();
> >  	read_unlock(&tasklist_lock);
> >
> >  	return ret;
>
> Do we still need the tasklist_lock in this case?

No. posix-cpu-timer.c shouldn't use tasklist at all. But it is not
completely trivial to remove it.

In particular, this patch is not exactly right, we can't trust
thread_group_leader() without tasklist.

Sergey already sent the patch which removes tasklist from
posix_cpu_timer_create() and posix_cpu_timer_create(), and iirc
Thomas queued it.

> Also, why is that think complaining, surely the tasklist_lock pins any
> and all PID objects?

The only problem is: if copy_process() fails, it does free_pid()
lockless. This means, without rcu lock it is not safe to scan the
rcu-protected lists.

We can change copy_process() (in fact I sent the patch several
years ago), but everybody think that find_pid/etc should always
take rcu_read_lock() instead. I tend to agree.

Oleg.

Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

[Sergey Senozhatsky]

[-- Attachment #1: Type: text/plain, Size: 2208 bytes --]

Hello,

On (11/25/10 12:02), Oleg Nesterov wrote:
> (another try, actually add Sergey)
>
Thank you.
 
> On 11/25, Peter Zijlstra wrote:
> >
> > On Wed, 2010-11-24 at 20:09 -0500, Dave Jones wrote:
> > > --- a/kernel/posix-cpu-timers.c
> > > +++ b/kernel/posix-cpu-timers.c
> > > @@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> > >  	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
> > >
> > >  	read_lock(&tasklist_lock);
> > > +	rcu_read_lock();
> > >  	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
> > >  		if (pid == 0) {
> > >  			p = current;
> > > @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> > >  	} else {
> > >  		ret = -EINVAL;
> > >  	}
> > > +	rcu_read_unlock();
> > >  	read_unlock(&tasklist_lock);
> > >
> > >  	return ret;
> >
> > Do we still need the tasklist_lock in this case?
> 
> No. posix-cpu-timer.c shouldn't use tasklist at all. But it is not
> completely trivial to remove it.
> 
> In particular, this patch is not exactly right, we can't trust
> thread_group_leader() without tasklist.
> 
> Sergey already sent the patch which removes tasklist from
> posix_cpu_timer_create() and posix_cpu_timer_create(), and iirc
> Thomas queued it.
> 

You're right, Oleg.

Commit-ID:  c0deae8c9587419ab13874b74425ce2eb2e18508
Gitweb:     http://git.kernel.org/tip/c0deae8c9587419ab13874b74425ce2eb2e18508
Author:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
AuthorDate: Wed, 3 Nov 2010 18:52:56 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 10 Nov 2010 13:07:06 +0100

posix-cpu-timers: Rcu_read_lock/unlock protect find_task_by_vpid call

queued (15 days so far).


> > Also, why is that think complaining, surely the tasklist_lock pins any
> > and all PID objects?
> 
> The only problem is: if copy_process() fails, it does free_pid()
> lockless. This means, without rcu lock it is not safe to scan the
> rcu-protected lists.
> 
> We can change copy_process() (in fact I sent the patch several
> years ago), but everybody think that find_pid/etc should always
> take rcu_read_lock() instead. I tend to agree.
> 
> Oleg.
> 

	Sergey

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