Re: [PATCH] tracing/user_events: Fail fork when event state duplication fails

[Steven Rostedt]

Beau,

Can you review this?

Thanks,

-- Steve


On Fri,  2 Oct 2026 18:26:39 -0400
Jeff Barnes <jeffbarnes@xxxxxxxxxxxxxxxxxxx> wrote:

> Registered user events are retained across fork, but duplicating their
> state for a child with a separate mm can fail. Both user_event_mm_dup() and
> user_events_fork() currently return void, so allocation failure silently
> creates a child without the inherited registration state.
> 
> The child can consequently retain a stale copy-on-write enable word and
> miss later event enable and disable updates.
> 
> Return an error from user_event_mm_dup() and user_events_fork(), and
> perform the duplication in copy_process() while failure can still be
> unwound. Return -ENOMEM when the child user_event_mm or any of its enablers
> cannot be duplicated.
> 
> Add a cleanup path so successfully acquired user-events state is removed if
> a later fork operation fails. Preserve the existing CLONE_VM behavior and
> its task reference accounting.
> 
> A deterministic allocation-failure test on upstream master previously
> allowed fork() to succeed while the child missed an enablement update. With
> this change, the same fork fails with ENOMEM. The complete user_events ABI
> suite passes.
> 
> Fixes: 7235759084a4 ("tracing/user_events: Use remote writes for event enablement")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jeff Barnes <jeffbarnes@xxxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/user_events.h      | 16 +++++++---------
>  kernel/fork.c                    |  8 ++++++--
>  kernel/trace/trace_events_user.c |  8 +++++---
>  3 files changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/user_events.h b/include/linux/user_events.h
> index 57d1ff006090..75f184126727 100644
> --- a/include/linux/user_events.h
> +++ b/include/linux/user_events.h
> @@ -27,28 +27,26 @@ struct user_event_mm {
>  	struct rcu_work		put_rwork;
>  };
>  
> -extern void user_event_mm_dup(struct task_struct *t,
> -			      struct user_event_mm *old_mm);
> +int user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm);
>  
>  extern void user_event_mm_remove(struct task_struct *t);
>  
> -static inline void user_events_fork(struct task_struct *t,
> -				    u64 clone_flags)
> +static inline int user_events_fork(struct task_struct *t, u64 clone_flags)
>  {
>  	struct user_event_mm *old_mm;
>  
>  	if (!t || !current->user_event_mm)
> -		return;
> +		return 0;
>  
>  	old_mm = current->user_event_mm;
>  
>  	if (clone_flags & CLONE_VM) {
>  		t->user_event_mm = old_mm;
>  		refcount_inc(&old_mm->tasks);
> -		return;
> +		return 0;
>  	}
>  
> -	user_event_mm_dup(t, old_mm);
> +	return user_event_mm_dup(t, old_mm);
>  }
>  
>  static inline void user_events_execve(struct task_struct *t)
> @@ -67,9 +65,9 @@ static inline void user_events_exit(struct task_struct *t)
>  	user_event_mm_remove(t);
>  }
>  #else
> -static inline void user_events_fork(struct task_struct *t,
> -				    u64 clone_flags)
> +static inline int user_events_fork(struct task_struct *t, u64 clone_flags)
>  {
> +	return 0;
>  }
>  
>  static inline void user_events_execve(struct task_struct *t)
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10f2d05d816a..9e3da2e6059f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2311,9 +2311,12 @@ __latent_entropy struct task_struct *copy_process(
>  	retval = copy_mm(clone_flags, p);
>  	if (retval)
>  		goto bad_fork_cleanup_signal;
> -	retval = copy_namespaces(clone_flags, p);
> +	retval = user_events_fork(p, clone_flags);
>  	if (retval)
>  		goto bad_fork_cleanup_mm;
> +	retval = copy_namespaces(clone_flags, p);
> +	if (retval)
> +		goto bad_fork_cleanup_user_events;
>  	retval = copy_io(clone_flags, p);
>  	if (retval)
>  		goto bad_fork_cleanup_namespaces;
> @@ -2575,7 +2578,6 @@ __latent_entropy struct task_struct *copy_process(
>  
>  	trace_task_newtask(p, clone_flags);
>  	uprobe_copy_process(p, clone_flags);
> -	user_events_fork(p, clone_flags);
>  
>  	copy_oom_score_adj(clone_flags, p);
>  
> @@ -2602,6 +2604,8 @@ __latent_entropy struct task_struct *copy_process(
>  		exit_io_context(p);
>  bad_fork_cleanup_namespaces:
>  	exit_nsproxy_namespaces(p);
> +bad_fork_cleanup_user_events:
> +	user_events_exit(p);
>  bad_fork_cleanup_mm:
>  	sched_cache_fork_cleanup(p);
>  	if (p->mm) {
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index f658c3a77aa7..8941c8d7c193 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -863,7 +863,7 @@ void user_event_mm_remove(struct task_struct *t)
>  	queue_rcu_work(system_percpu_wq, &mm->put_rwork);
>  }
>  
> -void user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)
> +int user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)
>  {
>  	struct user_event_mm *mm = user_event_mm_alloc(t);
>  	struct user_event_enabler *enabler;
> @@ -872,7 +872,7 @@ void user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)
>  	t->user_event_mm = NULL;
>  
>  	if (!mm)
> -		return;
> +		return -ENOMEM;
>  
>  	rcu_read_lock();
>  
> @@ -884,10 +884,12 @@ void user_event_mm_dup(struct task_struct *t, struct user_event_mm *old_mm)
>  	rcu_read_unlock();
>  
>  	user_event_mm_attach(mm, t);
> -	return;
> +	return 0;
>  error:
>  	rcu_read_unlock();
>  	user_event_mm_destroy(mm);
> +
> +	return -ENOMEM;
>  }
>  
>  static bool current_user_event_enabler_exists(unsigned long uaddr,