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Message-ID: <20231026091222-mutt-send-email-mst@kernel.org>
Date: Thu, 26 Oct 2023 09:14:21 -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 v2] Fixing directly deferencing a __rcu pointer warning
On Thu, Oct 26, 2023 at 05:46:21PM +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 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>
Well yes but these need to be called under rcu_read_lock.
Who does it here?
If no one then maybe you found an actual bug and we need to
fix it not paper over it.
> ---
> kernel/fork.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..802b7bbe3d92 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;
> }
> @@ -2692,7 +2692,7 @@ __latent_entropy struct task_struct *copy_process(
> */
> 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);
> + 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
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