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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There is a newer version of this series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
functions rcu_dereference(...) to use when dereferencing a __rcu
pointer. This functions returns the non __rcu tagged pointer which
can be dereferenced just like a normal pointers.


Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
---
 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 Fri, Oct 27, 2023 at 12:07:13PM +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
> functions

function

> rcu_dereference(...) to use when dereferencing a __rcu
> pointer.


... inside rcu read side critical sections.

> This functions

function

> returns the non __rcu tagged pointer which
> can be dereferenced just like a normal pointers.

pointer

> 
>

Extra empty line here.
 

Did you test this with lockdep on or did you just build it?
Include info on how the patch was tested pls.

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

> ---

Changelog?


>  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);

It looks like you are calling rcu_dereference outside of
read side critical section and that does not look right to me.
Test with lockdep on.

> -- 
> 2.39.2
Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning
Posted by Abhinav Singh 2 years, 1 month ago 
On 10/27/23 12:07, 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
> functions rcu_dereference(...) to use when dereferencing a __rcu
> pointer. This functions returns the non __rcu tagged pointer which
> can be dereferenced just like a normal pointers.
> 
> 
> Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
> ---
>   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);

For this particular file I have resolved the rcu pointer dereferencing 
issue and I have tested the above by using qemu using 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
it booted without any issues.
Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning
Posted by Michael S. Tsirkin 2 years, 1 month ago 
On Fri, Oct 27, 2023 at 12:11:47PM +0530, Abhinav Singh wrote:
> On 10/27/23 12:07, 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
> > functions rcu_dereference(...) to use when dereferencing a __rcu
> > pointer. This functions returns the non __rcu tagged pointer which
> > can be dereferenced just like a normal pointers.
> > 
> > 
> > Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
> > ---
> >   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);
> 
> For this particular file I have resolved the rcu pointer dereferencing issue
> and I have tested the above by using qemu using 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
> it booted without any issues.

Did you enable lockdep in your kernel?
CONFIG_PROVE_LOCKING and CONFIG_PROVE_RCU in particular.

-- 
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