On 10/09/2021 22:35, Mark Johnston wrote:
On Fri, Sep 10, 2021 at 10:15:37PM +0300, Andriy Gapon wrote:
On 10/09/2021 21:51, Andriy Gapon wrote:
I observe a problem with the code that can be seen here:
https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/blob/master/src/modules/rtp/sap.c#L142
The code uses ioctl(FIONREAD) to check the size of available data in a socket.
Does / should this work?
Then the code calls recvmsg() on the socket with single vector with iov_len
equal to the size obtained earlier.
But the return value from recvmsg() is smaller than the iov_len value.
In my test I see 215 vs expected 263 (so, the difference is 48).
Does this ring a bell to anyone?
I see this on a month old 14.0-CURRENT arm64.
From a quick look at soreceive_dgram() and some dtrace-ing, it seems that each
time recvmsg() is called soreceive_dgram() gets an mbuf chain where the first
mbuf is MT_SONAME (8), the second one is MT_CONTROL (14) and only the third one
is MT_DATA.
Could it be that data in the first two mbuf-s (especially the MT_CONTROL one) is
reported by FIONREAD? Or, in other words, accounted in sb_acc?
But then it's not actually returned, of course, in recvmsg() ?
Indeed, I suspect that this is the problem. Note that for
kevent(EVFILT_READ) we subtract the number of control message bytes from
the returned value, see filt_soread(). I wonder if FIONREAD should do
the same thing.
Thank you for the suggestion.
I think that it is a reasonable expectation that FIONREAD returns a number of
bytes that can be actually read.
I'll look at filt_soread().
Thank you!
--
Andriy Gapon
