Date:   Tue, 21 Dec 2021 21:19:41 +0100
From:   Marco Elver <elver@...gle.com>
To:     Aleksandr Nogikh <nogikh@...gle.com>
Cc:     kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, dvyukov@...gle.com,
        andreyknvl@...il.com, glider@...gle.com, tarasmadan@...gle.com,
        bigeasy@...utronix.de
Subject: Re: [PATCH v2 1/2] kcov: split ioctl handling into locked and
 unlocked parts

On Tue, 21 Dec 2021 at 18:04, Aleksandr Nogikh <nogikh@...gle.com> wrote:
>
> Currently all ioctls are de facto processed under a spin lock in order
> to serialise them. This, however, prohibits the use of vmalloc and other
> memory management functions in the implementation of those ioctls,
> unnecessary complicating any further changes.
>
> Let all ioctls first be processed inside the kcov_ioctl_unlocked()
> function which should execute the ones that are not compatible with
> spinlock and pass control to kcov_ioctl_locked() for all other ones.
>
> Although it is still compatible with a spinlock, move KCOV_INIT_TRACE
> handling to kcov_ioctl_unlocked(), so that its planned change is easier
> to follow.
>
> Signed-off-by: Aleksandr Nogikh <nogikh@...gle.com>
> ---
>  kernel/kcov.c | 64 +++++++++++++++++++++++++++++++--------------------
>  1 file changed, 39 insertions(+), 25 deletions(-)
>
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 36ca640c4f8e..5d87b4e0126f 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -564,31 +564,12 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
>                              unsigned long arg)
>  {
>         struct task_struct *t;
> -       unsigned long size, unused;
> +       unsigned long flags, unused;
>         int mode, i;
>         struct kcov_remote_arg *remote_arg;
>         struct kcov_remote *remote;
> -       unsigned long flags;
>
>         switch (cmd) {
> -       case KCOV_INIT_TRACE:
> -               /*
> -                * Enable kcov in trace mode and setup buffer size.
> -                * Must happen before anything else.
> -                */
> -               if (kcov->mode != KCOV_MODE_DISABLED)
> -                       return -EBUSY;
> -               /*
> -                * Size must be at least 2 to hold current position and one PC.
> -                * Later we allocate size * sizeof(unsigned long) memory,
> -                * that must not overflow.
> -                */
> -               size = arg;
> -               if (size < 2 || size > INT_MAX / sizeof(unsigned long))
> -                       return -EINVAL;
> -               kcov->size = size;
> -               kcov->mode = KCOV_MODE_INIT;
> -               return 0;
>         case KCOV_ENABLE:
>                 /*
>                  * Enable coverage for the current task.
> @@ -685,6 +666,43 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
>         }
>  }
>
> +static int kcov_ioctl_unlocked(struct kcov *kcov, unsigned int cmd,
> +                            unsigned long arg)
> +{
> +       unsigned long size, flags;
> +       int res;
> +
> +       switch (cmd) {
> +       case KCOV_INIT_TRACE:
> +               /*
> +                * Enable kcov in trace mode and setup buffer size.
> +                * Must happen before anything else.
> +                */
> +               if (kcov->mode != KCOV_MODE_DISABLED)
> +                       return -EBUSY;
> +               /*
> +                * Size must be at least 2 to hold current position and one PC.
> +                * Later we allocate size * sizeof(unsigned long) memory,
> +                * that must not overflow.
> +                */
> +               size = arg;
> +               if (size < 2 || size > INT_MAX / sizeof(unsigned long))
> +                       return -EINVAL;
> +               kcov->size = size;
> +               kcov->mode = KCOV_MODE_INIT;
> +               return 0;

This patch should be a non-functional change, but it is not.

To do that, you'd have to add the locking around KCOV_INIT_TRACE here,
and then do whatever else you're doing in patch 2/2.

> +       default:
> +               /*
> +                * All other commands can be fully executed under a spin lock, so we
> +                * obtain and release it here to simplify the code of kcov_ioctl_locked().
> +                */
> +               spin_lock_irqsave(&kcov->lock, flags);
> +               res = kcov_ioctl_locked(kcov, cmd, arg);
> +               spin_unlock_irqrestore(&kcov->lock, flags);
> +               return res;
> +       }
> +}
> +
>  static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>  {
>         struct kcov *kcov;
> @@ -692,7 +710,6 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>         struct kcov_remote_arg *remote_arg = NULL;
>         unsigned int remote_num_handles;
>         unsigned long remote_arg_size;
> -       unsigned long flags;
>
>         if (cmd == KCOV_REMOTE_ENABLE) {
>                 if (get_user(remote_num_handles, (unsigned __user *)(arg +
> @@ -713,10 +730,7 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
>         }
>
>         kcov = filep->private_data;
> -       spin_lock_irqsave(&kcov->lock, flags);
> -       res = kcov_ioctl_locked(kcov, cmd, arg);
> -       spin_unlock_irqrestore(&kcov->lock, flags);
> -
> +       res = kcov_ioctl_unlocked(kcov, cmd, arg);

Also, I find that kcov_ioctl_unlocked() isn't a very descriptive name,
since now we have both locked and unlocked variants. What is it
actually doing?

Perhaps kcov_ioctl_with_context()? Assuming that 'struct kcov' is some
sort of context.

>         kfree(remote_arg);
>
>         return res;
> --
> 2.34.1.307.g9b7440fafd-goog
>

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