Re: [PATCH v5] posix-timers: add multi_clock_gettime system call

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Jan 2, 2024, at 10:18, Sagi Maimon wrote:
> Some user space applications need to read some clocks.
> Each read requires moving from user space to kernel space.
> The syscall overhead causes unpredictable delay between N clocks reads
> Removing this delay causes better synchronization between N clocks.
>
> Introduce a new system call multi_clock_gettime, which can be used to measure
> the offset between multiple clocks, from variety of types: PHC, virtual PHC
> and various system clocks (CLOCK_REALTIME, CLOCK_MONOTONIC, etc).
> The offset includes the total time that the driver needs to read the clock
> timestamp.
>
> New system call allows the reading of a list of clocks - up to PTP_MAX_CLOCKS.
> Supported clocks IDs: PHC, virtual PHC and various system clocks.
> Up to PTP_MAX_SAMPLES times (per clock) in a single system call read.
> The system call returns n_clocks timestamps for each measurement:
> - clock 0 timestamp
> - ...
> - clock n timestamp
>
> Signed-off-by: Sagi Maimon <maimon.sagi@gmail.com>
> ---
>  Changes since version 4:
>  - fix error  : 'struct __ptp_multi_clock_get' declared inside parameter list 
>    will not be visible outside of this definition or declaration

I usually put all the changes for previous versions in a
list here, it helps reviewers.

The changes you made for previous versions all look good
to me, but I think there is still a few things worth
considering. I'll also follow up on the earlier threads.

> +#define MULTI_PTP_MAX_CLOCKS 32 /* Max number of clocks */
> +#define MULTI_PTP_MAX_SAMPLES 32 /* Max allowed offset measurement samples. */
> +
> +struct __ptp_multi_clock_get {
> +	unsigned int n_clocks; /* Desired number of clocks. */
> +	unsigned int n_samples; /* Desired number of measurements per clock. */
> +	clockid_t clkid_arr[MULTI_PTP_MAX_CLOCKS]; /* list of clock IDs */
> +	/*
> +	 * Array of list of n_clocks clocks time samples n_samples times.
> +	 */
> +	struct  __kernel_timespec ts[MULTI_PTP_MAX_SAMPLES][MULTI_PTP_MAX_CLOCKS];
> +};

Since you now access each member individually, I think it
makes more sense here to just pass these as four
register arguments. It helps with argument introspection,
avoids a couple of get_user(), and lets you remove the fixed
array dimensions.

> +SYSCALL_DEFINE1(multi_clock_gettime, struct __ptp_multi_clock_get 
> __user *, ptp_multi_clk_get)
> +{
> +	const struct k_clock *kc;
> +	struct timespec64 *kernel_tp;
> +	struct timespec64 *kernel_tp_base;
> +	unsigned int n_clocks; /* Desired number of clocks. */
> +	unsigned int n_samples; /* Desired number of measurements per clock. 
> */
> +	unsigned int i, j;
> +	clockid_t clkid_arr[MULTI_PTP_MAX_CLOCKS]; /* list of clock IDs */
> +	int error = 0;
> +
> +	if (copy_from_user(&n_clocks, &ptp_multi_clk_get->n_clocks, 
> sizeof(n_clocks)))
> +		return -EFAULT;
> +	if (copy_from_user(&n_samples, &ptp_multi_clk_get->n_samples, 
> sizeof(n_samples)))

If these remain as struct members rather than register arguments,
you should use get_user() instead of copy_from_user().

> +	kernel_tp_base = kmalloc_array(n_clocks * n_samples,
> +				       sizeof(struct timespec64), GFP_KERNEL);
> +	if (!kernel_tp_base)
> +		return -ENOMEM;

To be on the safe side regarding possible data leak, maybe use
kcalloc() instead of kmalloc_array() here.

> +	kernel_tp = kernel_tp_base;
> +	for (j = 0; j < n_samples; j++) {
> +		for (i = 0; i < n_clocks; i++) {
> +			if (put_timespec64(kernel_tp++, (struct __kernel_timespec __user *)
> +					&ptp_multi_clk_get->ts[j][i])) {

I think the typecast here can be removed.

      Arnd