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Subject: Re: [PATCH v4 2/2] rcu: Dump memory object info if callback function
 is invalid
To:     paulmck@kernel.org
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, Zhen Lei <thunder.leizhen@huawei.com>
References: <20230802130918.1132-1-thunder.leizhen@huaweicloud.com>
 <20230802130918.1132-3-thunder.leizhen@huaweicloud.com>
 <06731ba9-0746-453e-bd1f-b857bd253543@paulmck-laptop>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huaweicloud.com>
Message-ID: <2d6ec086-8457-b9e3-f282-cd302e5573c5@huaweicloud.com>
Date:   Thu, 3 Aug 2023 11:23:55 +0800
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On 2023/8/3 6:40, Paul E. McKenney wrote:
> On Wed, Aug 02, 2023 at 09:09:18PM +0800, thunder.leizhen@huaweicloud.com wrote:
>> From: Zhen Lei <thunder.leizhen@huawei.com>
>>
>> When a structure containing an RCU callback rhp is (incorrectly) freed
>> and reallocated after rhp is passed to call_rcu(), it is not unusual for
>> rhp->func to be set to NULL. This defeats the debugging prints used by
>> __call_rcu_common() in kernels built with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y,
>> which expect to identify the offending code using the identity of this
>> function.
>>
>> And in kernels build without CONFIG_DEBUG_OBJECTS_RCU_HEAD=y, things
>> are even worse, as can be seen from this splat:
>>
>> Unable to handle kernel NULL pointer dereference at virtual address 0
>> ... ...
>> PC is at 0x0
>> LR is at rcu_do_batch+0x1c0/0x3b8
>> ... ...
>>  (rcu_do_batch) from (rcu_core+0x1d4/0x284)
>>  (rcu_core) from (__do_softirq+0x24c/0x344)
>>  (__do_softirq) from (__irq_exit_rcu+0x64/0x108)
>>  (__irq_exit_rcu) from (irq_exit+0x8/0x10)
>>  (irq_exit) from (__handle_domain_irq+0x74/0x9c)
>>  (__handle_domain_irq) from (gic_handle_irq+0x8c/0x98)
>>  (gic_handle_irq) from (__irq_svc+0x5c/0x94)
>>  (__irq_svc) from (arch_cpu_idle+0x20/0x3c)
>>  (arch_cpu_idle) from (default_idle_call+0x4c/0x78)
>>  (default_idle_call) from (do_idle+0xf8/0x150)
>>  (do_idle) from (cpu_startup_entry+0x18/0x20)
>>  (cpu_startup_entry) from (0xc01530)
>>
>> This commit therefore adds calls to mem_dump_obj(rhp) to output some
>> information, for example:
>>
>>   slab kmalloc-256 start ffff410c45019900 pointer offset 0 size 256
>>
>> This provides the rough size of the memory block and the offset of the
>> rcu_head structure, which as least provides at least a few clues to help
>> locate the problem. If the problem is reproducible, additional slab
>> debugging can be enabled, for example, CONFIG_DEBUG_SLAB=y, which can
>> provide significantly more information.
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> 
> Looks plausible, thank you!
> 
> What did you do to test this? 

This test is easier. I wrote a simple one myself.

static struct my_rcu_node *my_node;

static bool test_kmem_dump_obj(void)
{
        void *p;

        if (kmem_dump_obj(NULL))
                return false;

        if (kmem_dump_obj((void *)(PAGE_SIZE / 2)))
                return false;

        if (kmem_dump_obj((void *)(PAGE_SIZE - 1)))
                return false;

        if (kmem_dump_obj((void *)PAGE_SIZE))
                return false;

        if (kmem_dump_obj(&my_node))
                return false;

        p = vmalloc(0x100000);
        WARN_ON(!p);
        if (kmem_dump_obj(p)) {
                vfree(p);
                return false;
        }
        vfree(p);

        p = kmalloc(0x100, GFP_KERNEL);
        WARN_ON(!p);
        if (!kmem_dump_obj(p)) {
                kfree(p);
                return false;
        }
        if (kmem_dump_obj((void *)(((unsigned long)p << 4) >> 4))) {
                kfree(p);
                return false;
        }
        kfree(p);

        return true;
}

static int tst_proc_show(struct seq_file *m, void *v)
{
        if (!test_kmem_dump_obj()) {
                seq_printf(m, "test_kmem_dump_obj failed\n");
                return 0;
        }

        my_node = kmalloc(sizeof(*my_node), GFP_KERNEL);
        if (!my_node) {
                seq_printf(m, "kmalloc failed\n");
                return 0;
        }

        call_rcu(&my_node->node, my_rcu_cb);
        my_node->node.func = NULL;

        return 0;
}


> 
> One option is the object_debug module parameter to rcutorture, which is
> described here: https://paulmck.livejournal.com/61432.html

OK, thanks for your info. I'll study the RCU self-test program rcutorture later.

> 
>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> 
> Not a big problem, but not a good habit to get into...  I add my own
> Signed-off-by when I pull patches into my tree.  Or if you are thinking
> in terms of sending this to mainline using some other path, when I am
> good with it, I would give you a tag to use.

Oh, Sorry. It seems that I forgot to delete your Signed-off-by in v2.
Oops! you reminded me once before. After v1, you helped modify the
description and pull it into your tree. I got it from 'dev' branch.

> 
> So were you looking for me to take these two patches?

Yes, it could be quicker. Of course, I can wait for patch 1/2 upstream,
then repost patch 2/2. In fact, I also want to dump part of the slab
object, I've already written the code. In order not to affect the current
user of mem_dump_obj(), a new parameter need to be added to kmem_dump_obj().
I will post v5 with this patch later.

> 
> 							Thanx, Paul
> 
>> ---
>>  kernel/rcu/rcu.h      | 7 +++++++
>>  kernel/rcu/srcutiny.c | 1 +
>>  kernel/rcu/srcutree.c | 1 +
>>  kernel/rcu/tasks.h    | 1 +
>>  kernel/rcu/tiny.c     | 1 +
>>  kernel/rcu/tree.c     | 1 +
>>  6 files changed, 12 insertions(+)
>>
>> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
>> index d1dcb09750efbd6..bc81582238b9846 100644
>> --- a/kernel/rcu/rcu.h
>> +++ b/kernel/rcu/rcu.h
>> @@ -10,6 +10,7 @@
>>  #ifndef __LINUX_RCU_H
>>  #define __LINUX_RCU_H
>>  
>> +#include <linux/slab.h>
>>  #include <trace/events/rcu.h>
>>  
>>  /*
>> @@ -248,6 +249,12 @@ static inline void debug_rcu_head_unqueue(struct rcu_head *head)
>>  }
>>  #endif	/* #else !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
>>  
>> +static inline void debug_rcu_head_callback(struct rcu_head *rhp)
>> +{
>> +	if (unlikely(!rhp->func))
>> +		kmem_dump_obj(rhp);
>> +}
>> +
>>  extern int rcu_cpu_stall_suppress_at_boot;
>>  
>>  static inline bool rcu_stall_is_suppressed_at_boot(void)
>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
>> index 336af24e0fe358a..c38e5933a5d6937 100644
>> --- a/kernel/rcu/srcutiny.c
>> +++ b/kernel/rcu/srcutiny.c
>> @@ -138,6 +138,7 @@ void srcu_drive_gp(struct work_struct *wp)
>>  	while (lh) {
>>  		rhp = lh;
>>  		lh = lh->next;
>> +		debug_rcu_head_callback(rhp);
>>  		local_bh_disable();
>>  		rhp->func(rhp);
>>  		local_bh_enable();
>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>> index f1a905200fc2f79..833a8f848a90ae6 100644
>> --- a/kernel/rcu/srcutree.c
>> +++ b/kernel/rcu/srcutree.c
>> @@ -1710,6 +1710,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
>>  	rhp = rcu_cblist_dequeue(&ready_cbs);
>>  	for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
>>  		debug_rcu_head_unqueue(rhp);
>> +		debug_rcu_head_callback(rhp);
>>  		local_bh_disable();
>>  		rhp->func(rhp);
>>  		local_bh_enable();
>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
>> index 7294be62727b12c..148ac6a464bfb12 100644
>> --- a/kernel/rcu/tasks.h
>> +++ b/kernel/rcu/tasks.h
>> @@ -538,6 +538,7 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
>>  	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
>>  	len = rcl.len;
>>  	for (rhp = rcu_cblist_dequeue(&rcl); rhp; rhp = rcu_cblist_dequeue(&rcl)) {
>> +		debug_rcu_head_callback(rhp);
>>  		local_bh_disable();
>>  		rhp->func(rhp);
>>  		local_bh_enable();
>> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
>> index 42f7589e51e09e7..fec804b7908032d 100644
>> --- a/kernel/rcu/tiny.c
>> +++ b/kernel/rcu/tiny.c
>> @@ -97,6 +97,7 @@ static inline bool rcu_reclaim_tiny(struct rcu_head *head)
>>  
>>  	trace_rcu_invoke_callback("", head);
>>  	f = head->func;
>> +	debug_rcu_head_callback(head);
>>  	WRITE_ONCE(head->func, (rcu_callback_t)0L);
>>  	f(head);
>>  	rcu_lock_release(&rcu_callback_map);
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 7c79480bfaa04e4..927c5ba0ae42269 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -2135,6 +2135,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
>>  		trace_rcu_invoke_callback(rcu_state.name, rhp);
>>  
>>  		f = rhp->func;
>> +		debug_rcu_head_callback(rhp);
>>  		WRITE_ONCE(rhp->func, (rcu_callback_t)0L);
>>  		f(rhp);
>>  
>> -- 
>> 2.34.1
>>
> .
> 

-- 
Regards,
  Zhen Lei