Re: [PATCH v4 2/8] media: v4l2: abstract timeval handling in v4l2_buffer

[Arnd Bergmann <arnd@arndb.de>]

On Mon, Nov 25, 2019 at 4:52 PM Hans Verkuil <hverkuil@xs4all.nl> wrote:
>
> On 11/11/19 9:38 PM, Arnd Bergmann wrote:
> > As a preparation for adding 64-bit time_t support in the uapi,
> > change the drivers to no longer care about the format of the
> > timestamp field in struct v4l2_buffer.
> >
> > The v4l2_timeval_to_ns() function is no longer needed in the
> > kernel after this, but there may be userspace code relying on
> > it because it is part of the uapi header.
>
> There is indeed userspace code that relies on this.

Ok, good to know. I rephrased the changelog text as

The v4l2_timeval_to_ns() function is no longer needed in the
kernel after this, but there is userspace code relying on
it to be part of the uapi header.

> >
> > +static inline u64 v4l2_buffer_get_timestamp(const struct v4l2_buffer *buf)
> > +{
> > +     return buf->timestamp.tv_sec * NSEC_PER_SEC +
> > +            (u32)buf->timestamp.tv_usec * NSEC_PER_USEC;
>
> Why the (u32) cast?

Simple question, long answer:

on 32-bit architectures, the tv_usec member may be 32-bit wide plus
padding in user space when interpreted as a regular 'struct timeval',
but the kernel implementation now sees it as a 64-bit member,
with half of it being possibly uninitialized user space data.

The 32-bit cast avoids that uninitialized data and ensures user space
passing garbage in the upper half gets ignored, as it has to be on 32-bit
user space.

On 64-bit native user space, the tv_usec field is always 64 bit wide,
so this is a change in behavior for denormalized timeval data
with tv_usec > U32_MAX, but the current behavior does not appear
worth preserving either.

The correct way would probably be to return an error for
 tv_usec >USEC_PER_SEC, but as the code never did that, this
would risk a regression for user space that relies on passing
invalid timestamps without getting an error.

> > +static inline void v4l2_buffer_set_timestamp(struct v4l2_buffer *buf,
> > +                                          u64 timestamp)
> > +{
> > +     struct timespec64 ts = ns_to_timespec64(timestamp);
> > +
> > +     buf->timestamp.tv_sec  = ts.tv_sec;
> > +     buf->timestamp.tv_usec = ts.tv_nsec / NSEC_PER_USEC;
> > +}
> > +
>
> This does not belong in the public header. This is kernel specific,

Note: this is not the uapi header but the in-kernel one.

> so media/v4l2-common.h would be a good place.

Ok, sounds good. I wasn't sure where to put it, and ended up
with include/linux/videodev2.h as the best replacement for
include/uapi/linux/videodev2.h, changed it to
include/media/v4l2-common.h now.

       Arnd