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Message-ID: <CAK8P3a0rom6x4X8eH0zknfZ5=6_rrXGKGR4H+RiY4SWKbXfp=g@mail.gmail.com>
Date: Tue, 26 Nov 2019 14:50:08 +0100
From: Arnd Bergmann <arnd@...db.de>
To: Hans Verkuil <hverkuil@...all.nl>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>,
Linux Media Mailing List <linux-media@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
y2038 Mailman List <y2038@...ts.linaro.org>
Subject: Re: [PATCH v4 6/8] media: v4l2-core: fix v4l2_buffer handling for
time64 ABI
On Mon, Nov 25, 2019 at 3:57 PM Hans Verkuil <hverkuil@...all.nl> wrote:
> On 11/11/19 9:38 PM, Arnd Bergmann wrote:
> > switch (cmd) {
> > +#ifdef COMPAT_32BIT_TIME
> > + case VIDIOC_QUERYBUF_TIME32:
> > + case VIDIOC_QBUF_TIME32:
> > + case VIDIOC_DQBUF_TIME32:
> > + case VIDIOC_PREPARE_BUF_TIME32: {
> > + struct v4l2_buffer_time32 vb32;
> > + struct v4l2_buffer *vb = parg;
> > +
> > + if (copy_from_user(&vb32, arg, sizeof(vb32)))
> > + return -EFAULT;
> > +
> > + memcpy(vb, &vb32, offsetof(struct v4l2_buffer, timestamp));
> > + vb->timestamp.tv_sec = vb32.timestamp.tv_sec;
> > + vb->timestamp.tv_usec = vb32.timestamp.tv_usec;
> > + memcpy(&vb->timecode, &vb32.timecode,
> > + sizeof(*vb) - offsetof(struct v4l2_buffer, timecode));
>
> I have similar concerns as with dqevent about whether this memcpy is the right approach.
> Unless you can prove with a utility like pahole that this memcpy is safe.
This is the video_get_user() function, so the input data comes from user
space and gets copied into the kernel, which has to check each field for
validity already, so I think this is safe regardless of the padding (which
exists before the 64-bit timestamp on 32-bit architectures). The fields
match because the definition of all members other than the timeval is
the same.
On the other hand, I agree it's not obvious from the code why this
is correct. I've changed my copy to this version below now, do you like
that better?
struct v4l2_buffer_time32 vb32;
struct v4l2_buffer *vb = parg;
if (copy_from_user(&vb32, arg, sizeof(vb32)))
return -EFAULT;
*vb = (struct v4l2_buffer) {
.index = vb32.index,
.type = vb32.type,
.bytesused = vb32.bytesused,
.flags = vb32.flags,
.field = vb32.field,
.timestamp.tv_sec = vb32.timestamp.tv_sec,
.timestamp.tv_usec = vb32.timestamp.tv_usec,
.timecode = vb32.timecode,
.memory = vb32.memory,
.m.userptr = vb32.usercopy,
.length = vb32.length,
.request_fd = vb32.request_fd,
};
if (cmd == VIDIOC_QUERYBUF_TIME32)
memset(&vb->length, 0, sizeof(*vb) -
offsetof(struct v4l2_buffer, length));
This way, all padding is zeroed out, and it's obvious to human
readers that each field gets set in the correct location.
> > + memcpy(&vb32, vb, offsetof(struct v4l2_buffer, timestamp));
> > + vb32.timestamp.tv_sec = vb->timestamp.tv_sec;
> > + vb32.timestamp.tv_usec = vb->timestamp.tv_usec;
> > + memcpy(&vb32.timecode, &vb->timecode,
> > + sizeof(*vb) - offsetof(struct v4l2_buffer, timecode));
>
> Ditto.
This is my new version:
struct v4l2_buffer *vb = parg;
struct v4l2_buffer_time32 vb32 = {
.index = vb->index,
.type = vb->type,
.bytesused = vb->bytesused,
.flags = vb->flags,
.field = vb->field,
.timestamp.tv_sec = vb->timestamp.tv_sec,
.timestamp.tv_usec = vb->timestamp.tv_usec,
.timecode = vb->timecode,
.memory = vb->memory,
.m.userptr = vb->usercopy,
.length = vb->length,
.request_fd = vb->request_fd,
};
if (copy_to_user(arg, &vb32, sizeof(vb32)))
return -EFAULT;
> > __u32 field;
> > +#ifdef __KERNEL__
> > + /* match glibc timeval64 format */
> > + struct {
> > + long long tv_sec;
> > +# if defined(__sparc__) && defined(__arch64__)
> > + int tv_usec;
> > + int __pad;
> > +# else
> > + long long tv_usec;
> > +# endif
> > + } timestamp;
>
> Ewww!
>
> Are there more places where this is needed? If so, then I very much prefer
> that a __kernel_timeval struct is defined somewhere, with appropriate
> comments.
I was trying hard to avoid adding a modern version of timeval, because
all new code should be encouraged to use __kernel_timespec instead.
There are not many users of timeval in the uapi, and this is the last one
after the others all got invididual treatment.
Usually what I would do is to have a kernel-internal type based
on timespec or u64, and then define three uapi types:
old native (based on __kernel_old_timeval), old compat (using
old_timeval32) and the new type with 64-bit time_t.
The problem with v4l2_buffer is that it includes another
compat-incompatible field (m.userptr) and that it's passed
between kernel functions, so then I'd probably need five variants
of it in total, and it would slow down the common case (64-bit
native) because it would require an extra copy.
I can try a few more things here, but I don't expect to find anything
much better than this.
> > +#ifdef __KERNEL__
> > +struct v4l2_buffer_time32 {
> > + __u32 index;
> > + __u32 type;
> > + __u32 bytesused;
> > + __u32 flags;
> > + __u32 field;
> > + struct old_timeval32 timestamp;
> > struct v4l2_timecode timecode;
> > __u32 sequence;
> >
> > @@ -1009,6 +1049,7 @@ struct v4l2_buffer {
> > __u32 reserved;
> > };
> > };
> > +#endif
>
> Can this be moved to v4l2-ioctls.h?
done.
> > #ifndef __KERNEL__
> > /**
> > @@ -2446,12 +2487,15 @@ struct v4l2_create_buffers {
> > #define VIDIOC_S_FMT _IOWR('V', 5, struct v4l2_format)
> > #define VIDIOC_REQBUFS _IOWR('V', 8, struct v4l2_requestbuffers)
> > #define VIDIOC_QUERYBUF _IOWR('V', 9, struct v4l2_buffer)
> > +#define VIDIOC_QUERYBUF_TIME32 _IOWR('V', 9, struct v4l2_buffer_time32)
>
> And all these should be moved there as well.
done.
Arnd
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