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

[Sagi Maimon <maimon.sagi@gmail.com>]

On Tue, Jan 2, 2024 at 12:22 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> 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.
>
Will be done on patch V6.
> 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.
>
I prefer the use of  get_user(), I will use it to remove  the fixed
array dimensions.
which will be done on patch V6.
> > +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().
>
Will be done on patch V6
> > +     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.
>
Will be done on patch V6.
> > +     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.
>
You are right, will be fixed on patch V6.
>       Arnd
Thanks for your Notes.