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

Kent Gibson
2022-09-10T14:52:29Z

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
>