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[PATCH v3] Fixing directly deferencing a __rcu pointer warning

Abhinav Singh posted 1 patch 2 years, 1 month ago
There is a newer version of this series
kernel/fork.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
[PATCH v3] Fixing directly deferencing a __rcu pointer warning
Posted by Abhinav Singh 2 years, 1 month ago 
This patch fixes the warning about directly dereferencing a pointer
tagged with __rcu annotation.

Dereferencing the pointers tagged with __rcu directly should
always be avoided according to the docs. There is a rcu helper
function rcu_dereference(...) to use when dereferencing a __rcu
pointer. This function returns the non __rcu tagged pointer which
can be dereferenced just like a normal pointer.

Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
---
v1 -> v2 : added rcu_dereference(...) at line 2694
v2 -> v3 : added rcu_dereference(...) at line 2695

 kernel/fork.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 10917c3e1f03..e78649974669 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
 
 	retval = -EAGAIN;
 	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
-		if (p->real_cred->user != INIT_USER &&
+		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
 			goto bad_fork_cleanup_count;
 	}
@@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
 			 * tasklist_lock with adding child to the process tree
 			 * for propagate_has_child_subreaper optimization.
 			 */
-			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
-							 p->real_parent->signal->is_child_subreaper;
-			list_add_tail(&p->sibling, &p->real_parent->children);
+			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
+							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
+			list_add_tail(&p->sibling, &rcu_dereference(p->real_parent)->children);
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			attach_pid(p, PIDTYPE_TGID);
 			attach_pid(p, PIDTYPE_PGID);
-- 
2.39.2
Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning
Posted by Michael S. Tsirkin 2 years, 1 month ago 
On Sat, Oct 28, 2023 at 03:52:47PM +0530, Abhinav Singh wrote:
> This patch fixes the warning about directly dereferencing a pointer
> tagged with __rcu annotation.
> 
> Dereferencing the pointers tagged with __rcu directly should
> always be avoided according to the docs. There is a rcu helper
> function rcu_dereference(...) to use when dereferencing a __rcu
> pointer. This function returns the non __rcu tagged pointer which
> can be dereferenced just like a normal pointer.
> 
> Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
> ---
> v1 -> v2 : added rcu_dereference(...) at line 2694
> v2 -> v3 : added rcu_dereference(...) at line 2695
> 
>  kernel/fork.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..e78649974669 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
>  
>  	retval = -EAGAIN;
>  	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
> -		if (p->real_cred->user != INIT_USER &&
> +		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
>  		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>  			goto bad_fork_cleanup_count;
>  	}
> @@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
>  			 * tasklist_lock with adding child to the process tree
>  			 * for propagate_has_child_subreaper optimization.
>  			 */
> -			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
> -							 p->real_parent->signal->is_child_subreaper;
> -			list_add_tail(&p->sibling, &p->real_parent->children);
> +			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
> +							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
> +			list_add_tail(&p->sibling, &rcu_dereference(p->real_parent)->children);
>  			list_add_tail_rcu(&p->tasks, &init_task.tasks);
>  			attach_pid(p, PIDTYPE_TGID);
>  			attach_pid(p, PIDTYPE_PGID);
> -- 
> 2.39.2


You seem to just ignore review comments. NAK. I'm not going to review this anymore.


-- 
MST
Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning
Posted by Abhinav Singh 2 years, 1 month ago 
On 10/28/23 17:50, Michael S. Tsirkin wrote:
> On Sat, Oct 28, 2023 at 03:52:47PM +0530, Abhinav Singh wrote:
>> This patch fixes the warning about directly dereferencing a pointer
>> tagged with __rcu annotation.
>>
>> Dereferencing the pointers tagged with __rcu directly should
>> always be avoided according to the docs. There is a rcu helper
>> function rcu_dereference(...) to use when dereferencing a __rcu
>> pointer. This function returns the non __rcu tagged pointer which
>> can be dereferenced just like a normal pointer.
>>
>> Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
>> ---
>> v1 -> v2 : added rcu_dereference(...) at line 2694
>> v2 -> v3 : added rcu_dereference(...) at line 2695
>>
>>   kernel/fork.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 10917c3e1f03..e78649974669 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
>>   
>>   	retval = -EAGAIN;
>>   	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
>> -		if (p->real_cred->user != INIT_USER &&
>> +		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
>>   		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>>   			goto bad_fork_cleanup_count;
>>   	}
>> @@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
>>   			 * tasklist_lock with adding child to the process tree
>>   			 * for propagate_has_child_subreaper optimization.
>>   			 */
>> -			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
>> -							 p->real_parent->signal->is_child_subreaper;
>> -			list_add_tail(&p->sibling, &p->real_parent->children);
>> +			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
>> +							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
>> +			list_add_tail(&p->sibling, &rcu_dereference(p->real_parent)->children);
>>   			list_add_tail_rcu(&p->tasks, &init_task.tasks);
>>   			attach_pid(p, PIDTYPE_TGID);
>>   			attach_pid(p, PIDTYPE_PGID);
>> -- 
>> 2.39.2
> 
> 
> You seem to just ignore review comments. NAK. I'm not going to review this anymore.
> 
> 
I m really sorry for ignorance and careless behaviour. This is 
completely my fault, a maintainer has to do a lot of work and he cant 
correct me all the times. On my defense I will only say that I was 
really confused about a thing, instead of asking you question, I thought 
of sending in another patch with some more information is a better 
choice then a clearing my confusion by sending in a extra mail. You were 
very concise and clear about your comments but MY stupidity was on 
another level today. I m sorry for ignorant behaviour. And also thanks 
and appreciate a lot for reviewing this patch till now to all the 
maintainers.

Not sure if this patch will be reviewed again or not, but I think I 
should answer the queries.

The last patch I sent, I tested with `lockdep` on (I hope "test with 
`lockdep on`" means booting the kernel with lockdep enabled),
with these config options `CONFIG_PROVE_RCU` and `CONFIG_PROVE_LOCKING` 
enabled and it booted just fine. To confirm if lockdep was really 
enabled I found these paths inside the qemu virtual envirnoment
/proc/lockdep
/proc/lockdep_chains
/proc/lockdep_stat
/proc/locks
/proc/lock_stats

I tested the above kernel using qemu with this command
qemu-system-x86_64 \
	-m 2G \
	-smp 2 \
	-kernel /home/abhinav/linux_work/linux/arch/x86/boot/bzImage \
	-append "console=ttyS0 root=/dev/sda earlyprintk=serial   net.ifnames=0" \
	-drive file=/home/abhinav/linux_work/boot_images/bullseye.img,format=raw \
	-net user,host=10.0.2.10,hostfwd=tcp:127.0.0.1:10021-:22 \
	-net nic,model=e1000 \
	-enable-kvm \
	-nographic \
	-pidfile vm.pid \
	2>&1 | tee vm.log

I did not get warning `the run-time "suspicious rcu_dereference_check() 
usage"` as mentioned by Oleg Nesterov, which mean rcu_dereference(...) 
it called inside of rcu read side critical sections.
[PATCH v3] Fixing directly deferencing a __rcu pointer warning
Posted by Abhinav Singh 2 years, 1 month ago 
This patch fixes the warning about directly dereferencing a pointer
tagged with __rcu annotation.

Dereferencing the pointers tagged with __rcu directly should
always be avoided according to the docs. There is a rcu helper
function rcu_dereference(...) to use when dereferencing a __rcu
pointer inside rcu read side critical sections. This function 
returns the non __rcu tagged pointer which can be dereferenced 
just like a normal pointer.

Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
---

Link to original patch 
 https://lore.kernel.org/all/20231025222811.855336-1-singhabhinav9051571833@gmail.com/

Change from original -> v2 : 
 1. removed the null check before dereferencing the dereferenced rcu
    pointer at line 2372.
 2. added rcu_dereference(...) at line 2694
    
Changes from v2 -> v3 
 1. added rcu_dereference(...) at line 2693

 kernel/fork.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 10917c3e1f03..e78649974669 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
 
 	retval = -EAGAIN;
 	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
-		if (p->real_cred->user != INIT_USER &&
+		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
 			goto bad_fork_cleanup_count;
 	}
@@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
 			 * tasklist_lock with adding child to the process tree
 			 * for propagate_has_child_subreaper optimization.
 			 */
-			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
-							 p->real_parent->signal->is_child_subreaper;
-			list_add_tail(&p->sibling, &p->real_parent->children);
+			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
+							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
+			list_add_tail(&p->sibling, &rcu_dereference(p->real_parent)->children);
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			attach_pid(p, PIDTYPE_TGID);
 			attach_pid(p, PIDTYPE_PGID);
-- 
2.39.2
Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning
Posted by Abhinav Singh 2 years, 1 month ago 
On 10/28/23 15:52, Abhinav Singh wrote:
> This patch fixes the warning about directly dereferencing a pointer
> tagged with __rcu annotation.
> 
> Dereferencing the pointers tagged with __rcu directly should
> always be avoided according to the docs. There is a rcu helper
> function rcu_dereference(...) to use when dereferencing a __rcu
> pointer. This function returns the non __rcu tagged pointer which
> can be dereferenced just like a normal pointer.
> 
> Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
> ---
> v1 -> v2 : added rcu_dereference(...) at line 2694
> v2 -> v3 : added rcu_dereference(...) at line 2695
> 
>   kernel/fork.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..e78649974669 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
>   
>   	retval = -EAGAIN;
>   	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
> -		if (p->real_cred->user != INIT_USER &&
> +		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
>   		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>   			goto bad_fork_cleanup_count;
>   	}
> @@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
>   			 * tasklist_lock with adding child to the process tree
>   			 * for propagate_has_child_subreaper optimization.
>   			 */
> -			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
> -							 p->real_parent->signal->is_child_subreaper;
> -			list_add_tail(&p->sibling, &p->real_parent->children);
> +			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
> +							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
> +			list_add_tail(&p->sibling, &rcu_dereference(p->real_parent)->children);
>   			list_add_tail_rcu(&p->tasks, &init_task.tasks);
>   			attach_pid(p, PIDTYPE_TGID);
>   			attach_pid(p, PIDTYPE_PGID);

I tested the above with these two config enabled, CONFIG_PROVE_LOCKING 
and CONFIG_PROVE_RCU. In qemu it booted fine without any issues. I then 
checked dmesg log (inside booted qemu envirnoment) for any issues with 
rcu, but didnt get any error or warning.

I didnt receive any warning message "suspicious rcu_dereference_check() 
usage" atleast from sparse tool or in the dmesg log as suggested by Oleg 
Nesterov.

Patchew 2.1-devel-b67e4c2

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