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Message-ID: <2271665b-f890-802f-eba8-0da43867d81f@xs4all.nl>
Date: Tue, 26 Nov 2019 15:15:01 +0100
From: Hans Verkuil <hverkuil@...all.nl>
To: Arnd Bergmann <arnd@...db.de>
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 11/26/19 2:50 PM, Arnd Bergmann wrote:
> 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,
> };
That looks clean.
>
> 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.
How about something like this in videodev2.h:
Split off the ugly kernel timeval definition in a separate struct:
#ifdef __KERNEL__
/* match glibc timeval64 format */
struct __kernel_v4l2_timeval {
long long tv_sec;
# if defined(__sparc__) && defined(__arch64__)
int tv_usec;
int __pad;
# else
long long tv_usec;
# endif
};
#endif
Then use that in the struct v4l2_buffer definition:
struct v4l2_buffer {
...
#ifdef __KERNEL__
struct __kernel_v4l2_timeval timestamp;
#else
struct timeval timestamp;
#endif
That keeps struct v4l2_buffer fairly clean. And it also makes it
possible to have a bit more extensive documentation for the
struct __kernel_v4l2_timeval without polluting the actual struct
v4l2_buffer definition.
The videodev2.h header is something users of the API look at a
lot and having this really ugly kernel timestamp in there is
not acceptably IMHO. But splitting it off should work.
>
>>> +#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
>
Regards,
Hans
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