Re: [PATCH 2/2] gpiolib: cdev: export the consumer's PID

[Kent Gibson <warthog618@gmail.com>]

On Fri, Sep 09, 2022 at 02:13:29PM +0200, Bartosz Golaszewski wrote:
> It's useful for user-space to be able to know the PIDs of processes
> holding GPIO lines in case they have the permissions and need to kill
> them.
> 

"the PID of the process holding a GPIO line"

As written it could be interpreted to imply that multiple processes can
hold a line, so go singular to remove that possibility.

> Extend the gpio_v2_line_info structure with the consumer_pid field
> that's set to the PID of the user-space process or 0 if the user lives
> in the kernel.
> 
> Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
> ---
>  drivers/gpio/gpiolib-cdev.c |  2 ++
>  drivers/gpio/gpiolib.c      | 24 +++++++++++++++++++-----
>  drivers/gpio/gpiolib.h      |  2 ++
>  include/uapi/linux/gpio.h   |  5 ++++-
>  4 files changed, 27 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index f8041d4898d1..9b6d518680dc 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -2120,6 +2120,8 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
>  	if (desc->label)
>  		strscpy(info->consumer, desc->label, sizeof(info->consumer));
>  
> +	info->consumer_pid = desc->pid;
> +
>  	/*
>  	 * Userspace only need to know that the kernel is using this GPIO so
>  	 * it can't use it.
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index 6768734b9e15..0c9d1639b04d 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -96,6 +96,11 @@ static inline void desc_set_label(struct gpio_desc *d, const char *label)
>  	d->label = label;
>  }
>  
> +static inline void desc_set_pid(struct gpio_desc *d, pid_t pid)
> +{
> +	d->pid = pid;
> +}
> +
>  /**
>   * gpio_to_desc - Convert a GPIO number to its descriptor
>   * @gpio: global GPIO number
> @@ -1892,7 +1897,8 @@ EXPORT_SYMBOL_GPL(gpiochip_remove_pin_ranges);
>   * on each other, and help provide better diagnostics in debugfs.
>   * They're called even less than the "set direction" calls.
>   */
> -static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
> +static int
> +gpiod_request_commit(struct gpio_desc *desc, const char *label, pid_t pid)
>  {
>  	struct gpio_chip	*gc = desc->gdev->chip;
>  	int			ret;
> @@ -1913,6 +1919,7 @@ static int gpiod_request_commit(struct gpio_desc *desc, const char *label)
>  
>  	if (test_and_set_bit(FLAG_REQUESTED, &desc->flags) == 0) {
>  		desc_set_label(desc, label ? : "?");
> +		desc_set_pid(desc, pid);
>  	} else {
>  		ret = -EBUSY;
>  		goto out_free_unlock;
> @@ -1987,7 +1994,8 @@ static int validate_desc(const struct gpio_desc *desc, const char *func)
>  		return; \
>  	} while (0)
>  
> -int gpiod_request(struct gpio_desc *desc, const char *label)
> +static int
> +gpiod_request_with_pid(struct gpio_desc *desc, const char *label, pid_t pid)
>  {
>  	int ret = -EPROBE_DEFER;
>  	struct gpio_device *gdev;
> @@ -1996,7 +2004,7 @@ int gpiod_request(struct gpio_desc *desc, const char *label)
>  	gdev = desc->gdev;
>  
>  	if (try_module_get(gdev->owner)) {
> -		ret = gpiod_request_commit(desc, label);
> +		ret = gpiod_request_commit(desc, label, pid);
>  		if (ret)
>  			module_put(gdev->owner);
>  		else
> @@ -2009,11 +2017,16 @@ int gpiod_request(struct gpio_desc *desc, const char *label)
>  	return ret;
>  }
>  
> +int gpiod_request(struct gpio_desc *desc, const char *label)
> +{
> +	return gpiod_request_with_pid(desc, label, 0);
> +}
> +
>  int gpiod_request_user(struct gpio_desc *desc, const char *label)
>  {
>  	int ret;
>  
> -	ret = gpiod_request(desc, label);
> +	ret = gpiod_request_with_pid(desc, label, current->pid);
>  	if (ret == -EPROBE_DEFER)
>  		ret = -ENODEV;
>  
> @@ -2042,6 +2055,7 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
>  		}
>  		kfree_const(desc->label);
>  		desc_set_label(desc, NULL);
> +		desc_set_pid(desc, 0);
>  		clear_bit(FLAG_ACTIVE_LOW, &desc->flags);
>  		clear_bit(FLAG_REQUESTED, &desc->flags);
>  		clear_bit(FLAG_OPEN_DRAIN, &desc->flags);
> @@ -2140,7 +2154,7 @@ struct gpio_desc *gpiochip_request_own_desc(struct gpio_chip *gc,
>  		return desc;
>  	}
>  
> -	ret = gpiod_request_commit(desc, label);
> +	ret = gpiod_request_commit(desc, label, 0);
>  	if (ret < 0)
>  		return ERR_PTR(ret);
>  
> diff --git a/drivers/gpio/gpiolib.h b/drivers/gpio/gpiolib.h
> index b35deb08a7f5..d1535677e162 100644
> --- a/drivers/gpio/gpiolib.h
> +++ b/drivers/gpio/gpiolib.h
> @@ -165,6 +165,8 @@ struct gpio_desc {
>  
>  	/* Connection label */
>  	const char		*label;
> +	/* Consumer's PID (if consumer is in user-space, otherwise 0) */
> +	pid_t			pid;
>  	/* Name of the GPIO */
>  	const char		*name;
>  #ifdef CONFIG_OF_DYNAMIC
> diff --git a/include/uapi/linux/gpio.h b/include/uapi/linux/gpio.h
> index cb9966d49a16..37f10021d1aa 100644
> --- a/include/uapi/linux/gpio.h
> +++ b/include/uapi/linux/gpio.h
> @@ -219,6 +219,8 @@ struct gpio_v2_line_request {
>   * gpio_v2_line_flag, such as %GPIO_V2_LINE_FLAG_ACTIVE_LOW,
>   * %GPIO_V2_LINE_FLAG_OUTPUT etc, added together.
>   * @attrs: the configuration attributes associated with the line
> + * @consumer_pid: process ID of the user-space consumer or 0 if the consumer
> + * lives in kernel space
>   * @padding: reserved for future use
>   */
>  struct gpio_v2_line_info {
> @@ -228,8 +230,9 @@ struct gpio_v2_line_info {
>  	__u32 num_attrs;
>  	__aligned_u64 flags;
>  	struct gpio_v2_line_attribute attrs[GPIO_V2_LINE_NUM_ATTRS_MAX];
> +	__s32 consumer_pid;

My knee-jerk reaction here was to make the pid unsigned, as we never
pass a negative PID.
Keeping in mind that the existing kernel will return 0 for this field
(the existing padding), so 0 needs to be excluded from valid PIDs
anyway.

Andy suggests returning -1 for kernel held lines.
In that case 0 would mean "old kernel", while -1 would mean "kernel
held".

As libgpiod will have to convert the 0 to -1 when returning the PID to
user-space as a pid_t, I'm good with the uAPI using 0 to mean
"no PID available" for all cases. I'm still open to passing -1 for
kernel held is there is a use case for it, but I don't see one.

Cheers,
Kent.

>  	/* Space reserved for future use. */
> -	__u32 padding[4];
> +	__u32 padding[3];
>  };
>  
>  /**
> -- 
> 2.34.1
>