Re: [PATCH v2 1/2] PM: wakeup: Add kfuncs to traverse over wakeup_sources

[Puranjay Mohan <puranjay@kernel.org>]

Samuel Wu <wusamuel@google.com> writes:

> Iterating through wakeup sources via sysfs or debugfs can be inefficient
> or restricted. Introduce BPF kfuncs to allow high-performance and safe
> in-kernel traversal of the wakeup_sources list.
>
> The new kfuncs include:
> - bpf_wakeup_sources_get_head() to obtain the list head.
> - bpf_wakeup_sources_read_lock/unlock() to manage the SRCU lock.
>
> For verifier safety, the underlying SRCU index is wrapped in an opaque
> 'struct bpf_ws_lock' pointer. This enables the use of KF_ACQUIRE and
> KF_RELEASE flags, allowing the BPF verifier to strictly enforce paired
> lock/unlock cycles and prevent resource leaks.
>
> Signed-off-by: Samuel Wu <wusamuel@google.com>
> ---
>  drivers/base/power/power.h  |  7 ++++
>  drivers/base/power/wakeup.c | 72 +++++++++++++++++++++++++++++++++++--
>  2 files changed, 77 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/base/power/power.h b/drivers/base/power/power.h
> index 922ed457db19..ced563286ac5 100644
> --- a/drivers/base/power/power.h
> +++ b/drivers/base/power/power.h
> @@ -168,3 +168,10 @@ static inline void device_pm_init(struct device *dev)
>  	device_pm_sleep_init(dev);
>  	pm_runtime_init(dev);
>  }
> +
> +#ifdef CONFIG_BPF_SYSCALL
> +struct bpf_ws_lock { };
> +struct bpf_ws_lock *bpf_wakeup_sources_read_lock(void);
> +void bpf_wakeup_sources_read_unlock(struct bpf_ws_lock *lock);
> +struct list_head *bpf_wakeup_sources_get_head(void);
> +#endif
> diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
> index b8e48a023bf0..27a26a30b6ee 100644
> --- a/drivers/base/power/wakeup.c
> +++ b/drivers/base/power/wakeup.c
> @@ -1168,11 +1168,79 @@ static const struct file_operations wakeup_sources_stats_fops = {
>  	.release = seq_release_private,
>  };
>  
> -static int __init wakeup_sources_debugfs_init(void)
> +#ifdef CONFIG_BPF_SYSCALL
> +#include <linux/btf.h>
> +
> +__bpf_kfunc_start_defs();
> +
> +/**
> + * bpf_wakeup_sources_read_lock - Acquire the SRCU lock for wakeup sources
> + *
> + * The underlying SRCU lock returns an integer index. However, the BPF verifier
> + * requires a pointer (PTR_TO_BTF_ID) to strictly track the state of acquired
> + * resources using KF_ACQUIRE and KF_RELEASE semantics. We use an opaque
> + * structure pointer (struct bpf_ws_lock *) to satisfy the verifier while
> + * safely encoding the integer index within the pointer address itself.
> + *
> + * Return: An opaque pointer encoding the SRCU lock index + 1 (to avoid NULL).
> + */
> +__bpf_kfunc struct bpf_ws_lock *bpf_wakeup_sources_read_lock(void)
> +{
> +	return (struct bpf_ws_lock *)(long)(wakeup_sources_read_lock() + 1);
> +}
> +
> +/**
> + * bpf_wakeup_sources_read_unlock - Release the SRCU lock for wakeup sources
> + * @lock: The opaque pointer returned by bpf_wakeup_sources_read_lock()
> + *
> + * The BPF verifier guarantees that @lock is a valid, unreleased pointer from
> + * the acquire function. We decode the pointer back into the integer SRCU index
> + * by subtracting 1 and release the lock.
> + */
> +__bpf_kfunc void bpf_wakeup_sources_read_unlock(struct bpf_ws_lock *lock)
> +{
> +	wakeup_sources_read_unlock((int)(long)lock - 1);
> +}
> +
> +/**
> + * bpf_wakeup_sources_get_head - Get the head of the wakeup sources list
> + *
> + * Return: The head of the wakeup sources list.
> + */
> +__bpf_kfunc struct list_head *bpf_wakeup_sources_get_head(void)
> +{
> +	return &wakeup_sources;
> +}

What stops a bpf program from using this kfunc to get the wakeup_sources
pointer and iterating this list without taking the srcu lock? 

An open-coded iterator would solve this by internalizing the lock:
  // Iterator new() acquires SRCU lock
  // Iterator next() returns next wakeup_source
  // Iterator destroy() releases SRCU lock

> +
> +__bpf_kfunc_end_defs();
> +
> +BTF_KFUNCS_START(wakeup_source_kfunc_ids)
> +BTF_ID_FLAGS(func, bpf_wakeup_sources_read_lock, KF_ACQUIRE)

I think this needs KF_RET_NULL as well.

> +BTF_ID_FLAGS(func, bpf_wakeup_sources_read_unlock, KF_RELEASE)
> +BTF_ID_FLAGS(func, bpf_wakeup_sources_get_head)
> +BTF_KFUNCS_END(wakeup_source_kfunc_ids)
> +
> +static const struct btf_kfunc_id_set wakeup_source_kfunc_set = {
> +	.owner = THIS_MODULE,
> +	.set   = &wakeup_source_kfunc_ids,
> +};
> +
> +static void __init wakeup_sources_bpf_init(void)
> +{
> +	if (register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &wakeup_source_kfunc_set))
> +		pm_pr_dbg("Wakeup: failed to register BTF kfuncs\n");
> +}
> +#else
> +static inline void wakeup_sources_bpf_init(void) {}
> +#endif /* CONFIG_BPF_SYSCALL */
> +
> +static int __init wakeup_sources_init(void)
>  {
>  	debugfs_create_file("wakeup_sources", 0444, NULL, NULL,
>  			    &wakeup_sources_stats_fops);
> +	wakeup_sources_bpf_init();
> +
>  	return 0;
>  }
>  
> -postcore_initcall(wakeup_sources_debugfs_init);
> +postcore_initcall(wakeup_sources_init);
> -- 
> 2.53.0.1018.g2bb0e51243-goog