linux-kernel - Re: [PATCH v3] Fixing directly deferencing a _

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Date:   Fri, 27 Oct 2023 03:00:17 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Abhinav Singh <singhabhinav9051571833@...il.com>
Cc:     akpm@...ux-foundation.org, brauner@...nel.org, surenb@...gle.com,
        michael.christie@...cle.com, mathieu.desnoyers@...icios.com,
        mjguzik@...il.com, npiggin@...il.com, shakeelb@...gle.com,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning

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@...il.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.

-- 
MST