Re: [PATCH v4 2/2] rcu: Dump memory object info if callback function is invalid

From: Leizhen (ThunderTown)
Date: Thu Aug 03 2023 - 05:32:03 EST



On 2023/8/3 11:23, Leizhen (ThunderTown) wrote:
>
>
> On 2023/8/3 6:40, Paul E. McKenney wrote:
>> On Wed, Aug 02, 2023 at 09:09:18PM +0800, thunder.leizhen@xxxxxxxxxxxxxxx wrote:
>>> From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>>
>> 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.

I measured it carefully, and the code would look ugly when I added a parameter.
In fact, the three places where mem_dump_obj() is currently called are for
debugging purposes, and dump the memory of slab object is not bad for them.

>
>>
>> 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