[Vm-dev] Re: VMMaker-eem.163 fixes old SoundPlugin bug

[Eliot Miranda]

Hi All,

    it turns out I jumped the gun on this.  We have been hunting a buffer
overrun bug and I mistakenly thought this was it.  In fact our bug was in
one of the media codec support libraries we use from a third-party.

I still think the code is confusing, and it looks to me as if it is broken
on Mac OS and correct, but confusing, on win32.  Anyone who wants to review
this /please/ do.  Here's the original code:

SoundPlugin>>primitiveSoundRecordSamplesInto: buf startingAt:
startWordIndex
"Record a buffer's worth of 16-bit sound samples."
| bufSizeInBytes samplesRecorded |
self primitive: 'primitiveSoundRecordSamples'
parameters: #(WordArray SmallInteger ).

interpreterProxy failed ifFalse:
 [bufSizeInBytes := (interpreterProxy slotSizeOf: buf cPtrAsOop) * 4.
interpreterProxy success: (startWordIndex >= 1 and: [startWordIndex - 1 * 2
< bufSizeInBytes])].

interpreterProxy failed ifFalse:
 [samplesRecorded := self cCode: 'snd_RecordSamplesIntoAtLength((int)buf,
startWordIndex - 1, bufSizeInBytes)'].
^ samplesRecorded asPositiveIntegerObj

My objection in the above is that we do the bounds check above but leave it
up to the support code snd_RecordSamplesIntoAtLength to derive the start
pointer buf + ((startWordIndex - 1) * 2).  IMO these two belong together.

Now in the Mac code there is no code to subtract (startWordIndex - 1) * 2
from bufSizeInBytes, and so without my fix there could be a buffer overrun:

int snd_RecordSamplesIntoAtLength(int buf, int startSliceIndex, int
bufferSizeInBytes) {
    /* if data is available, copy as many sample slices as possible into the
       given buffer starting at the given slice index. do not write past the
       end of the buffer, which is buf + bufferSizeInBytes. return the
number
       of slices (not bytes) copied. a slice is one 16-bit sample in mono
       or two 16-bit samples in stereo. */
    int bytesPerSlice = (recordBuffer1.brokenOSXWasMono) ? 2 :
((recordBuffer1.stereo) ? 4 : 2);
    char *nextBuf = (char *) buf + (startSliceIndex * bytesPerSlice);
    char *bufEnd = (char *) buf + bufferSizeInBytes;
    char *src, *srcEnd;
    RecordBuffer recBuf = nil;
    int bytesCopied;

On win32 there is code to do the subtraction.  e.g. looking at the Direct
Sound side of the code:

int dx_snd_RecordSamplesIntoAtLength(int buf, int startSliceIndex, int
bufferSizeInBytes) {
  /* if data is available, copy as many sample slices as possible into the
     given buffer starting at the given slice index. do not write past the
     end of the buffer, which is buf + bufferSizeInBytes. return the number
     of slices (not bytes) copied. a slice is one 16-bit sample in mono
     or two 16-bit samples in stereo. */
  int bytesPerSlice = (waveInFormat.nChannels * 2);
  int bytesCopied;
  char *srcPtr, *srcPtr2, *dstPtr;
  HRESULT hRes;
  DWORD srcLen, srcLen2;

...
  /* Figure out how much data we want to copy (don't overflow either the
source or destination buffers. */
  bytesCopied = bufferSizeInBytes - (startSliceIndex * bytesPerSlice);
  if(bytesCopied > recBufferAvailable)
    bytesCopied = recBufferAvailable;

  /* Lock the portion of the DirectSound buffer that we will copy data from.
*/
  hRes = IDirectSoundCaptureBuffer_Lock(lpdRecBuffer,
                    recBufferReadPosition,
                    bytesCopied,
                    (void*)&srcPtr, &srcLen,
                    (void*)&srcPtr2, &srcLen2,
                    0);

So this is an example of a really confusing API.  If instead the API were
snd_RecordSamplesIntoAtLength(int buf, int bufferSizeInBytes)
and the primitive read

SoundPlugin>>primitiveSoundRecordSamplesInto: buf startingAt:
startWordIndex
"Record a buffer's worth of 16-bit sound samples."
| bufSizeInBytes samplesRecorded startByteIndex |
self primitive: 'primitiveSoundRecordSamples'
parameters: #(WordArray SmallInteger ).

interpreterProxy failed ifFalse:
 [bufSizeInBytes := (interpreterProxy slotSizeOf: buf cPtrAsOop) * 4.
startByteIndex := startWordIndex - 1 * .
interpreterProxy success: (startWordIndex >= 1 and: [startByteIndex <
bufSizeInBytes])].

interpreterProxy failed ifFalse:
 [samplesRecorded := self cCode: 'snd_RecordSamplesIntoAtLength((int)buf
+ startByteIndex, bufSizeInBytes - startByteIndex'].
^ samplesRecorded asPositiveIntegerObj

there would be much less chance for (as the Fat Controller says) confusion
and delay.


cheers
Eliot
On Mon, Mar 22, 2010 at 5:18 PM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

> Use of primitiveSoundRecordSamples with a start index other than 1 could
> corrupt the heap if enough samples were recorded.
>
> Name: VMMaker-eem.163
> Author: eem
> Time: 22 March 2010, 5:15:57 pm
> UUID: 2fa86dda-e18c-48cd-bcac-856504aba8dc
> Ancestors: VMMaker-ar.162
>
> SoundPlugin: fix bounds bug in primitiveSoundRecordSamples.  The
> call to snd_RecordSamplesIntoAtLength neglected to subtract the
> start index from the size of the buffer to be written into.
>
> best
> Eliot
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/vm-dev/attachments/20100329/85a6deab/attachment-0001.htm