rtp: fix 'size mismatch' on BSD style operating systems

On FreeBSD (and probably other BSDs as well), the FIONREAD ioctl
on UDP sockets does not return the size of the next datagram (like
it does on Linux), but returns the size of the output buffer: this
count contain multiple datagrams and also contains the headers.

We fixed this by taking the result of the FIONREAD as lower bound
for the size, adding an upper bound and then removing the check
that the sizes should be exactly the same.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/718>

2 files changed, 30 insertions, 10 deletions

diff --git a/src/modules/rtp/rtp-native.c b/src/modules/rtp/rtp-native.c
index 86760a627..d1a3911d7 100644
--- a/src/modules/rtp/rtp-native.c
+++ b/src/modules/rtp/rtp-native.c
@@ -200,7 +200,6 @@ int pa_rtp_recv(pa_rtp_context *c, pa_memchunk *chunk, pa_mempool *pool, uint32_
     uint32_t ssrc;
     uint8_t payload;
     unsigned cc;
-    ssize_t r;
     uint8_t aux[1024];
     bool found_tstamp = false;
 
@@ -209,6 +208,15 @@ int pa_rtp_recv(pa_rtp_context *c, pa_memchunk *chunk, pa_mempool *pool, uint32_
 
     pa_memchunk_reset(chunk);
 
+    /* FIONREAD works on both BSD and Linux, but they do something different:
+     * - on Linux it returns the amount of bytes in the next datagram
+     * - on BSDs it returns the total amount of bytes in the output buffer; this can be
+     *   more than one datagram and includes headers
+     *
+     * So the result will be a lower bound of how many bytes are needed, but might not be
+     * the exact size of the buffer size needed.
+     */
+
     if (ioctl(c->fd, FIONREAD, &size) < 0) {
         pa_log_warn("FIONREAD failed: %s", pa_cstrerror(errno));
         goto fail;
@@ -238,6 +246,9 @@ int pa_rtp_recv(pa_rtp_context *c, pa_memchunk *chunk, pa_mempool *pool, uint32_
         size = 1;
     }
 
+    /* Since size is a lower bound, also constrain it to an upper bound */
+    size = PA_MIN(size, 1<<16);
+
     if (c->recv_buf_size < (size_t) size) {
         do
             c->recv_buf_size *= 2;
@@ -259,12 +270,11 @@ int pa_rtp_recv(pa_rtp_context *c, pa_memchunk *chunk, pa_mempool *pool, uint32_
     m.msg_controllen = sizeof(aux);
     m.msg_flags = 0;
 
-    r = recvmsg(c->fd, &m, 0);
-
-    if (r != size) {
-        if (r < 0 && errno != EAGAIN && errno != EINTR)
-            pa_log_warn("recvmsg() failed: %s", r < 0 ? pa_cstrerror(errno) : "size mismatch");
+    size = recvmsg(c->fd, &m, 0);
 
+    if (size < 0) {
+        if (errno != EAGAIN && errno != EINTR)
+            pa_log_warn("recvmsg() failed: %s", pa_cstrerror(errno));
         goto fail;
     }
 
diff --git a/src/modules/rtp/sap.c b/src/modules/rtp/sap.c
index 8614022f8..00a648057 100644
--- a/src/modules/rtp/sap.c
+++ b/src/modules/rtp/sap.c
@@ -146,15 +146,25 @@ int pa_sap_recv(pa_sap_context *c, bool *goodbye) {
     char *buf = NULL, *e;
     uint32_t header;
     unsigned six, ac, k;
-    ssize_t r;
-
+ 
     pa_assert(c);
     pa_assert(goodbye);
 
+    /* FIONREAD works on both BSD and Linux, but they do something different:
+     * - on Linux it returns the amount of bytes in the next datagram
+     * - on BSDs it returns the total amount of bytes in the output buffer; this can be
+     *   more than one datagram and includes headers
+     *
+     * So the result will be a lower bound of how many bytes are needed, but might not be
+     * the exact size of the buffer size needed.
+     */
+
     if (ioctl(c->fd, FIONREAD, &size) < 0) {
         pa_log_warn("FIONREAD failed: %s", pa_cstrerror(errno));
         goto fail;
     }
+    /* Since size is a lower bound, also constrain it to an upper bound */
+    size = PA_MIN(size, 1<<16);
 
     buf = pa_xnew(char, (unsigned) size+1);
     buf[size] = 0;
@@ -170,8 +180,8 @@ int pa_sap_recv(pa_sap_context *c, bool *goodbye) {
     m.msg_controllen = 0;
     m.msg_flags = 0;
 
-    if ((r = recvmsg(c->fd, &m, 0)) != size) {
-        pa_log_warn("recvmsg() failed: %s", r < 0 ? pa_cstrerror(errno) : "size mismatch");
+    if ((size = recvmsg(c->fd, &m, 0)) < 0) {
+        pa_log_warn("recvmsg() failed: %s", pa_cstrerror(errno));
         goto fail;
     }