[FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

The commit d7f4ad88a0df3c1339e142957bf2c40cd056b8ce introduced a race
condition where the passed opaque pointer reference might be NULL,
when the decoding process starts.
This patch checks that vtctx has a value before accessing it.

This patch fixes #10079.

Signed-off-by: Alessandro Di Nepi <alessandro.dinepi@gmail.com>
---
 libavcodec/videotoolbox.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libavcodec/videotoolbox.c b/libavcodec/videotoolbox.c
index 1b1be8ddb4..615e2b087a 100644
--- a/libavcodec/videotoolbox.c
+++ b/libavcodec/videotoolbox.c
@@ -692,6 +692,11 @@ static void videotoolbox_decoder_callback(void *opaque,
 {
     VTContext *vtctx = opaque;
 
+    if (!vtctx) {
+        av_log(NULL, AV_LOG_WARNING, "vt decoder cb: vt context is null");
+        return;
+    }
+
     if (vtctx->frame) {
         CVPixelBufferRelease(vtctx->frame);
         vtctx->frame = NULL;
-- 
2.37.1 (Apple Git-137.1)

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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

Just to add that this fix, once approved, should be cherry-picked to all the release branches where d7f4ad88a0df3c1339e142957bf2c40cd056b8ce has been cherry-picked.
Basically, 4.4, 5.0, and 5.1.

Thanks
On 27 Nov 2022, 19:34 +0200, Alessandro Di Nepi <alessandro.dinepi@gmail.com>, wrote:
> The commit d7f4ad88a0df3c1339e142957bf2c40cd056b8ce introduced a race
> condition where the passed opaque pointer reference might be NULL,
> when the decoding process starts.
> This patch checks that vtctx has a value before accessing it.
>
> This patch fixes #10079.
>
> Signed-off-by: Alessandro Di Nepi <alessandro.dinepi@gmail.com>
> ---
> libavcodec/videotoolbox.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/libavcodec/videotoolbox.c b/libavcodec/videotoolbox.c
> index 1b1be8ddb4..615e2b087a 100644
> --- a/libavcodec/videotoolbox.c
> +++ b/libavcodec/videotoolbox.c
> @@ -692,6 +692,11 @@ static void videotoolbox_decoder_callback(void *opaque,
> {
> VTContext *vtctx = opaque;
>
> + if (!vtctx) {
> + av_log(NULL, AV_LOG_WARNING, "vt decoder cb: vt context is null");
> + return;
> + }
> +
> if (vtctx->frame) {
> CVPixelBufferRelease(vtctx->frame);
> vtctx->frame = NULL;
> --
> 2.37.1 (Apple Git-137.1)
>
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

A gentle reminder for this patch: it's a simple fix that prevents crashing on iOS.

Thanks
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Rick Kern]

On Tue, Nov 29, 2022 at 11:47 AM Alessandro Di Nepi <
alessandro.dinepi@gmail.com> wrote:

> Just to add that this fix, once approved, should be cherry-picked to all
> the release branches where d7f4ad88a0df3c1339e142957bf2c40cd056b8ce has
> been cherry-picked.
> Basically, 4.4, 5.0, and 5.1.
>
> Thanks
> On 27 Nov 2022, 19:34 +0200, Alessandro Di Nepi <
> alessandro.dinepi@gmail.com>, wrote:
> > The commit d7f4ad88a0df3c1339e142957bf2c40cd056b8ce introduced a race
> > condition where the passed opaque pointer reference might be NULL,
> > when the decoding process starts.
> > This patch checks that vtctx has a value before accessing it.
> >
> > This patch fixes #10079.
> >
> > Signed-off-by: Alessandro Di Nepi <alessandro.dinepi@gmail.com>
> > ---
> > libavcodec/videotoolbox.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/libavcodec/videotoolbox.c b/libavcodec/videotoolbox.c
> > index 1b1be8ddb4..615e2b087a 100644
> > --- a/libavcodec/videotoolbox.c
> > +++ b/libavcodec/videotoolbox.c
> > @@ -692,6 +692,11 @@ static void videotoolbox_decoder_callback(void
> *opaque,
> > {
> > VTContext *vtctx = opaque;
> >
> > + if (!vtctx) {
>

When this happens, does it continue happening, or is it transient? My main
concern is log spamming.


> > + av_log(NULL, AV_LOG_WARNING, "vt decoder cb: vt context is null");
> > + return;
> > + }
> > +
> > if (vtctx->frame) {
> > CVPixelBufferRelease(vtctx->frame);
> > vtctx->frame = NULL;
> > --
> > 2.37.1 (Apple Git-137.1)
> >
> _______________________________________________
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

On 4 Dec 2022, 17:01 +0200, FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>, wrote:
> When this happens, does it continue happening, or is it transient? My main
> concern is log spamming.
Good question: this is just a transient state, so that it won't continue happening.
To give you some context: when the decoding start, the value of `vtctx` is captured "too" early so that the first time the callback is called, it's still NULL.
The next time it will have a proper value.
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Rick Kern]

On Sun, Dec 4, 2022 at 12:51 PM Alessandro Di Nepi <
alessandro.dinepi@gmail.com> wrote:

> On 4 Dec 2022, 17:01 +0200, FFmpeg development discussions and patches <
> ffmpeg-devel@ffmpeg.org>, wrote:
> > When this happens, does it continue happening, or is it transient? My
> main
> > concern is log spamming.
> Good question: this is just a transient state, so that it won't continue
> happening.
> To give you some context: when the decoding start, the value of `vtctx` is
> captured "too" early so that the first time the callback is called, it's
> still NULL.
> The next time it will have a proper value.
>
If the code isn't setting a variable in time, that issue should be fixed.
Otherwise the decoder will drop frames.

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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

["zhilizhao(赵志立)"]

> On Dec 5, 2022, at 21:36, Rick Kern <kernrj@gmail.com> wrote:
> 
> On Sun, Dec 4, 2022 at 12:51 PM Alessandro Di Nepi <
> alessandro.dinepi@gmail.com> wrote:
> 
>> On 4 Dec 2022, 17:01 +0200, FFmpeg development discussions and patches <
>> ffmpeg-devel@ffmpeg.org>, wrote:
>>> When this happens, does it continue happening, or is it transient? My
>> main
>>> concern is log spamming.
>> Good question: this is just a transient state, so that it won't continue
>> happening.
>> To give you some context: when the decoding start, the value of `vtctx` is
>> captured "too" early so that the first time the callback is called, it's
>> still NULL.
>> The next time it will have a proper value.
>> 
> If the code isn't setting a variable in time, that issue should be fixed.
> Otherwise the decoder will drop frames.

Yes, null pointer check doesn’t looks like a resolution to a race
condition. I’m not sure how the race condition happened in the first
place.

> 
> _______________________________________________
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

Got you; giving some context here, and you can find all the details in the ticket #10079 (http://trac.ffmpeg.org/ticket/10079).

The issue has been introduced with the commit d7f4ad88a0df3c1339e142957bf2c40cd056b8ce.
This patch basically changed:

• In the function `videotoolbox_start(AVCodecContext *avctx)`,

```
-    decoder_cb.decompressionOutputRefCon   = avctx;
+    decoder_cb.decompressionOutputRefCon   = avctx->internal->hwaccel_priv_data;
```

•  The context is retrieved in the function, `videotoolbox_decoder_callback(...)`

```
-    AVCodecContext *avctx = opaque;
-    VTContext *vtctx = avctx->internal->hwaccel_priv_data;
+    VTContext *vtctx = opaque;
```

Having said that, I see that when the `videotoolbox_start` is called,

• `avctx` is not NULL,
• `avctx->internal->hwaccel_priv_data` is NULL

The first time the `videotoolbox_decoder_callback` is called, `avctx->internal->hwaccel_priv_data` now has a value, so before d7f4ad88a `vtctx` has a value.
After the change, since `avctx->internal->hwaccel_priv_data` is captured in `video toolbox_start`, is NULL and `vtctx` is also NULL.

Again, this happens just the first time the callback is called; from the second time, vtctx has a proper value, and everything proceeds as expected.

I'm willing to change the patch if you think there is a better way, but something needs to be done because the library simply crashes in the current state.
From what I see from the original change, reverting is not an option.

Looking forward to hear feedback on this.

Best Regards
Alessandro
On 6 Dec 2022, 7:20 +0200, "zhilizhao(赵志立)" <quinkblack@foxmail.com>, wrote:
>
> > On Dec 5, 2022, at 21:36, Rick Kern <kernrj@gmail.com> wrote:
> >
> > On Sun, Dec 4, 2022 at 12:51 PM Alessandro Di Nepi <
> > alessandro.dinepi@gmail.com> wrote:
> >
> > > On 4 Dec 2022, 17:01 +0200, FFmpeg development discussions and patches <
> > > ffmpeg-devel@ffmpeg.org>, wrote:
> > > > When this happens, does it continue happening, or is it transient? My
> > > main
> > > > concern is log spamming.
> > > Good question: this is just a transient state, so that it won't continue
> > > happening.
> > > To give you some context: when the decoding start, the value of `vtctx` is
> > > captured "too" early so that the first time the callback is called, it's
> > > still NULL.
> > > The next time it will have a proper value.
> > >
> > If the code isn't setting a variable in time, that issue should be fixed.
> > Otherwise the decoder will drop frames.
>
> Yes, null pointer check doesn’t looks like a resolution to a race
> condition. I’m not sure how the race condition happened in the first
> place.
>
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

["zhilizhao(赵志立)"]

> On Dec 7, 2022, at 00:30, Alessandro Di Nepi <alessandro.dinepi@gmail.com> wrote:
> 
> Got you; giving some context here, and you can find all the details in the ticket #10079 (http://trac.ffmpeg.org/ticket/10079).
> 
> The issue has been introduced with the commit d7f4ad88a0df3c1339e142957bf2c40cd056b8ce.
> This patch basically changed:
> 
> • In the function `videotoolbox_start(AVCodecContext *avctx)`,
> 
> ```
> -    decoder_cb.decompressionOutputRefCon   = avctx;
> +    decoder_cb.decompressionOutputRefCon   = avctx->internal->hwaccel_priv_data;
> ```
> 
> •  The context is retrieved in the function, `videotoolbox_decoder_callback(...)`
> 
> ```
> -    AVCodecContext *avctx = opaque;
> -    VTContext *vtctx = avctx->internal->hwaccel_priv_data;
> +    VTContext *vtctx = opaque;
> ```
> 
> Having said that, I see that when the `videotoolbox_start` is called,
> 
> • `avctx` is not NULL,
> • `avctx->internal->hwaccel_priv_data` is NULL

Did you call av_videotoolbox_default_init() or av_videotoolbox_default_init2()?
I think that’s why avctx->internal->hwaccel_priv_data is NULL. That code path is broken.
Does remove the call of av_videotoolbox_default_init()/av_videotoolbox_default_init2() works for you?

> 
> The first time the `videotoolbox_decoder_callback` is called, `avctx->internal->hwaccel_priv_data` now has a value, so before d7f4ad88a `vtctx` has a value.
> After the change, since `avctx->internal->hwaccel_priv_data` is captured in `video toolbox_start`, is NULL and `vtctx` is also NULL.
> 
> Again, this happens just the first time the callback is called; from the second time, vtctx has a proper value, and everything proceeds as expected.
> 
> I'm willing to change the patch if you think there is a better way, but something needs to be done because the library simply crashes in the current state.
> From what I see from the original change, reverting is not an option.
> 
> Looking forward to hear feedback on this.
> 
> Best Regards
> Alessandro
> On 6 Dec 2022, 7:20 +0200, "zhilizhao(赵志立)" <quinkblack@foxmail.com>, wrote:
>> 
>>> On Dec 5, 2022, at 21:36, Rick Kern <kernrj@gmail.com> wrote:
>>> 
>>> On Sun, Dec 4, 2022 at 12:51 PM Alessandro Di Nepi <
>>> alessandro.dinepi@gmail.com> wrote:
>>> 
>>>> On 4 Dec 2022, 17:01 +0200, FFmpeg development discussions and patches <
>>>> ffmpeg-devel@ffmpeg.org>, wrote:
>>>>> When this happens, does it continue happening, or is it transient? My
>>>> main
>>>>> concern is log spamming.
>>>> Good question: this is just a transient state, so that it won't continue
>>>> happening.
>>>> To give you some context: when the decoding start, the value of `vtctx` is
>>>> captured "too" early so that the first time the callback is called, it's
>>>> still NULL.
>>>> The next time it will have a proper value.
>>>> 
>>> If the code isn't setting a variable in time, that issue should be fixed.
>>> Otherwise the decoder will drop frames.
>> 
>> Yes, null pointer check doesn’t looks like a resolution to a race
>> condition. I’m not sure how the race condition happened in the first
>> place.
>> 
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

On 9 Dec 2022, 6:45 +0200, FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>, wrote:
> Did you call av_videotoolbox_default_init() or av_videotoolbox_default_init2()?
Actually yes, I call `av_videotoolbox_default_init(context);`
> I think that’s why avctx->internal->hwaccel_priv_data is NULL. That code path is broken.
> Does remove the call of av_videotoolbox_default_init()/av_videotoolbox_default_init2() works for you?
Definitively yes, if I remove the call to `av_videotoolbox_default_init` the context is fine.

Thanks for sorting this out. I'd like to contribute and have this fixed or documented for other people.
What should be the desired behavior? Calling `av_videotoolbox_default_init` and having the context not NULL?
Or shall we remove the init at all?

Best Regards
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

Ping on this,
Can I contribute somehow either to fix calling `av_videotoolbox_default_init` or at least update the docs?

Best Regards,
Alessandro
On 11 Dec 2022, 11:57 +0200, Alessandro Di Nepi <alessandro.dinepi@gmail.com>, wrote:
> On 9 Dec 2022, 6:45 +0200, FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>, wrote:
> > Did you call av_videotoolbox_default_init() or av_videotoolbox_default_init2()?
> Actually yes, I call `av_videotoolbox_default_init(context);`
> > I think that’s why avctx->internal->hwaccel_priv_data is NULL. That code path is broken.
> > Does remove the call of av_videotoolbox_default_init()/av_videotoolbox_default_init2() works for you?
> Definitively yes, if I remove the call to `av_videotoolbox_default_init` the context is fine.
>
> Thanks for sorting this out. I'd like to contribute and have this fixed or documented for other people.
> What should be the desired behavior? Calling `av_videotoolbox_default_init` and having the context not NULL?
> Or shall we remove the init at all?
>
> Best Regards
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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Zhao Zhili]

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Alessandro Di Nepi
> Sent: 2022年12月15日 22:16
> To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
> Subject: Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback
> 
> Ping on this,
> Can I contribute somehow either to fix calling `av_videotoolbox_default_init` or at least update the docs?

Updating the docs doesn't work. This is a public API, it must be fixed properly: don't drop a feature, do what
it was supposed to do.

You can give a try, but I'm afraid it's a tricky regression.

> 
> Best Regards,
> Alessandro
> On 11 Dec 2022, 11:57 +0200, Alessandro Di Nepi <alessandro.dinepi@gmail.com>, wrote:
> > On 9 Dec 2022, 6:45 +0200, FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>, wrote:
> > > Did you call av_videotoolbox_default_init() or av_videotoolbox_default_init2()?
> > Actually yes, I call `av_videotoolbox_default_init(context);`
> > > I think that’s why avctx->internal->hwaccel_priv_data is NULL. That code path is broken.
> > > Does remove the call of av_videotoolbox_default_init()/av_videotoolbox_default_init2() works for you?
> > Definitively yes, if I remove the call to `av_videotoolbox_default_init` the context is fine.
> >
> > Thanks for sorting this out. I'd like to contribute and have this fixed or documented for other people.
> > What should be the desired behavior? Calling `av_videotoolbox_default_init` and having the context not NULL?
> > Or shall we remove the init at all?
> >
> > Best Regards
> _______________________________________________
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> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> 
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".

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Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Zhao Zhili]

> From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of Alessandro Di Nepi
> Sent: 2022年12月15日 22:16
> To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
> Subject: Re: [FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback
> 
> Ping on this,
> Can I contribute somehow either to fix calling `av_videotoolbox_default_init` or at least update the docs?

Hope this patch set fixed the original usecase.

http://ffmpeg.org/pipermail/ffmpeg-devel/2023-January/305477.html

> 
> Best Regards,
> Alessandro
> On 11 Dec 2022, 11:57 +0200, Alessandro Di Nepi <alessandro.dinepi@gmail.com>, wrote:
> > On 9 Dec 2022, 6:45 +0200, FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>, wrote:
> > > Did you call av_videotoolbox_default_init() or av_videotoolbox_default_init2()?
> > Actually yes, I call `av_videotoolbox_default_init(context);`
> > > I think that’s why avctx->internal->hwaccel_priv_data is NULL. That code path is broken.
> > > Does remove the call of av_videotoolbox_default_init()/av_videotoolbox_default_init2() works for you?
> > Definitively yes, if I remove the call to `av_videotoolbox_default_init` the context is fine.
> >
> > Thanks for sorting this out. I'd like to contribute and have this fixed or documented for other people.
> > What should be the desired behavior? Calling `av_videotoolbox_default_init` and having the context not NULL?
> > Or shall we remove the init at all?
> >
> > Best Regards
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> 
> To unsubscribe, visit link above, or email
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[FFmpeg-devel] [PATCH] lavc/videotoolbox: validate vt context in the decoder callback

[Alessandro Di Nepi]

The commit d7f4ad88a0df3c1339e142957bf2c40cd056b8ce introduced a race
condition where the passed opaque pointer reference might be NULL,
when the decoding process starts.
This patch checks that vtctx has a value before accessing it.

This patch fixes #10079.

Signed-off-by: Alessandro Di Nepi <alessandro.dinepi@gmail.com>
---
 libavcodec/videotoolbox.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libavcodec/videotoolbox.c b/libavcodec/videotoolbox.c
index 1b1be8ddb4..615e2b087a 100644
--- a/libavcodec/videotoolbox.c
+++ b/libavcodec/videotoolbox.c
@@ -692,6 +692,11 @@ static void videotoolbox_decoder_callback(void *opaque,
 {
     VTContext *vtctx = opaque;

+    if (!vtctx) {
+        av_log(NULL, AV_LOG_WARNING, "vt decoder cb: vt context is null");
+        return;
+    }
+
     if (vtctx->frame) {
         CVPixelBufferRelease(vtctx->frame);
         vtctx->frame = NULL;
--
2.37.1 (Apple Git-137.1)

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