Re: [PATCH] staging/android: refactor SYNC_IOC_FILE_INFO

[Gustavo Padovan]

Hi Emil,

2016-02-27 Emil Velikov <emil.l.velikov@xxxxxxxxx>:

> Hi Gustavo,
> 
> On 26 February 2016 at 21:00, Gustavo Padovan <gustavo@xxxxxxxxxxx> wrote:
> > From: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> >
> > Change SYNC_IOC_FILE_INFO behaviour to avoid future API breaks and
> > optimize buffer allocation. In the new approach the ioctl needs to be called
> > twice to retrieve the array of fence_infos pointed by info->sync_fence_info.
> >
> I might have misunderstood things but I no one says you "need" to call
> it twice - you can just request a "random" amount of fences_info. Upon
> return (if num_fences was non zero) it will report how many fence_info
> were retrieved.

Right, I don't see any problem doing it in one request, I just didn't 
think about that in the new proposal. I'll update the code and commit
message accordinly.

> 
> > The first call should pass num_fences = 0, the kernel will then fill
> > info->num_fences. Userspace receives back the number of fences and
> > allocates a buffer size num_fences * sizeof(struct sync_fence_info) on
> > info->sync_fence_info.
> >
> > It then call the ioctl again passing num_fences received in info->num_fences.
> "calls"
> 
> > The kernel checks if info->num_fences > 0 and if yes it fill
> > info->sync_fence_info with an array containing all fence_infos.
> >
> The above sentence sounds a bit strange. I believe you meant to say
> something like "Then the kernel fills the fence_infos array with data.
> One should read back the actual number from info->num_fences." ?
> 
> > info->len now represents the length of the buffer sync_fence_info points
> > to.
> Now that I think about it, I'm wondering if there'll be a case where
> len != info->num_fences * sizeof(struct sync_file_info). If that's not
> possible one could just drop len and nicely simplify things.
> 
> > Also, info->sync_fence_info was converted to __u64 pointer.
> >
> ... pointer to prevent 32bit compatibility issues.
> 
> > An example userspace code would be:
> >
> >         struct sync_file_info *info;
> >         int err, size, num_fences;
> >
> >         info = malloc(sizeof(*info));
> >
> >         memset(info, 0, sizeof(*info));
> >
> >         err = ioctl(fd, SYNC_IOC_FILE_INFO, info);
> >         num_fences = info->num_fences;
> >
> >         if (num_fences) {
> >                 memset(info, 0, sizeof(*info));
> >                 size = sizeof(struct sync_fence_info) * num_fences;
> >                 info->len = size;
> >                 info->num_fences = num_fences;
> >                 info->sync_fence_info = (uint64_t) calloc(num_fences,
> >                                                           sizeof(struct sync_fence_info));
> >
> >                 err = ioctl(fd, SYNC_IOC_FILE_INFO, info);
> >         }
> >
> > v2: fix fence_info memory leak
> >
> > Signed-off-by: Gustavo Padovan <gustavo.padovan@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/staging/android/sync.c      | 52 +++++++++++++++++++++++++++++--------
> >  drivers/staging/android/uapi/sync.h |  9 +++----
> >  2 files changed, 45 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
> > index dc5f382..2379f23 100644
> > --- a/drivers/staging/android/sync.c
> > +++ b/drivers/staging/android/sync.c
> > @@ -502,21 +502,22 @@ static int sync_fill_fence_info(struct fence *fence, void *data, int size)
> >  static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
> >                                         unsigned long arg)
> >  {
> > -       struct sync_file_info *info;
> > +       struct sync_file_info in, *info;
> > +       struct sync_fence_info *fence_info = NULL;
> >         __u32 size;
> >         __u32 len = 0;
> >         int ret, i;
> >
> > -       if (copy_from_user(&size, (void __user *)arg, sizeof(size)))
> > +       if (copy_from_user(&in, (void __user *)arg, sizeof(*info)))
> s/*info/in/
> 
> >                 return -EFAULT;
> >
> > -       if (size < sizeof(struct sync_file_info))
> > -               return -EINVAL;
> > +       if (in.status || strcmp(in.name, "\0"))
> Afaict these two are outputs, so we should be checking them ?

Hmm. Maybe not.

> 
> > +               return -EFAULT;
> >
> As originally, input validation should return -EINVAL on error.
> 
> 
> > -       if (size > 4096)
> > -               size = 4096;
> > +       if (in.num_fences && !in.sync_fence_info)
> > +               return -EFAULT;
> >
> Ditto.
> 
> > -       info = kzalloc(size, GFP_KERNEL);
> > +       info = kzalloc(sizeof(*info), GFP_KERNEL);
> >         if (!info)
> >                 return -ENOMEM;
> >
> > @@ -525,14 +526,33 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
> >         if (info->status >= 0)
> >                 info->status = !info->status;
> >
> > -       info->num_fences = sync_file->num_fences;
> > +       /*
> > +        * Passing num_fences = 0 means that userspace want to know how
> > +        * many fences are in the sync_file to be able to allocate a buffer to
> > +        * fit all sync_fence_infos and call the ioctl again with the buffer
> > +        * assigned to info->sync_fence_info. The second call pass the
> > +        * num_fences value received in the first call.
> > +        */
> > +       if (!in.num_fences)
> > +               goto no_fences;
> > +
> We should clamp in.num_fences to min2(in.num_fences,
> sync_file->num_fences) and use it over sync_file->num_fences though
> the rest of the function. Or just bail out when the two are not the
> same.
> 
> Depends on what the planned semantics are. Fwiw I'm leaning towards the former.

If num_fences received is smaller than the actual num_fences I think we
should fails, otherwise we should just fill the buffer with all
fence_infos...

> 
> > +       size = sync_file->num_fences * sizeof(*fence_info);
> > +       if (in.len != size) {
> > +               ret = -EFAULT;
> EINVAL or just drop len from the struct.

...so this check now would be in.len < size.

> 
> > +               goto out;
> > +       }
> >
> > -       len = sizeof(struct sync_file_info);
> > +       fence_info = kzalloc(size, GFP_KERNEL);
> > +       if (!fence_info) {
> > +               ret = -ENOMEM;
> > +               goto out;
> > +       }
> >
> >         for (i = 0; i < sync_file->num_fences; ++i) {
> >                 struct fence *fence = sync_file->cbs[i].fence;
> >
> > -               ret = sync_fill_fence_info(fence, (u8 *)info + len, size - len);
> > +               ret = sync_fill_fence_info(fence, (u8 *)fence_info + len,
> A few comments about sync_fill_fence_info()
>  - Internal function so make the second argument of the correct type -
> struct sync_fence_info *
>  - Drop the third argument size, as that one is always sizeof(sync_fence_info).
>  - Remove the size checking in the same function and make its return type void
> 
> Then one can simplify this loop even further :-)

Sounds good to me.

> 
> > +                                          size - len);
> >
> >                 if (ret < 0)
> >                         goto out;
> > @@ -540,14 +560,24 @@ static long sync_file_ioctl_fence_info(struct sync_file *sync_file,
> >                 len += ret;
> >         }
> >
> > +       if (copy_to_user((void __user *)in.sync_fence_info, fence_info, size)) {
> > +               ret = -EFAULT;
> > +               goto out;
> > +       }
> > +
> >         info->len = len;
> > +       info->sync_fence_info = (__u64) in.sync_fence_info;
> Why the cast ?
> 
> > +
> > +no_fences:
> > +       info->num_fences = sync_file->num_fences;
> >
> > -       if (copy_to_user((void __user *)arg, info, len))
> > +       if (copy_to_user((void __user *)arg, info, sizeof(*info)))
> Don't know if we should be returning (copying) any other information
> but info->num_fences in case of "no_fences". In case it's not clear -
> I'm thinking about the data we already have in in info->name and
> info->status.

Userspace might want to know all info about the sync_file but
sync_fence_info.

	Gustavo