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

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 <redacted>
---
 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