linux: test whether reading from the event FD succeeds.

It *should* never succeed, but 1) you never know and 2) that gets newer
versions of GCC, and Coverity, to stop whining that we're not checking
the return value of read().

(We can't check of the write() to the event FD without adding a new API
that's like pcap_breakloop() but that 1) returns a success/error
indication and 2) provides an error message.)

1 files changed, 38 insertions, 1 deletions

diff --git a/pcap-linux.c b/pcap-linux.c
index 6e1d89b6..546d0369 100644
--- a/pcap-linux.c
+++ b/pcap-linux.c
@@ -3890,9 +3890,46 @@ static int pcap_wait_for_frames_mmap(pcap_t *handle)
 			 * Now check the event device.
 			 */
 			if (pollinfo[1].revents & POLLIN) {
+				ssize_t nread;
 				uint64_t value;
-				(void)read(handlep->poll_breakloop_fd, &value,
+
+				/*
+				 * This should never fail, but, just
+				 * in case....
+				 */
+				nread = read(handlep->poll_breakloop_fd, &value,
 				    sizeof(value));
+				if (nread == -1) {
+					pcap_fmt_errmsg_for_errno(handle->errbuf,
+					    PCAP_ERRBUF_SIZE,
+					    errno,
+					    "Error reading from event FD");
+					return PCAP_ERROR;
+				}
+
+				/*
+				 * According to the Linux read(2) man
+				 * page, read() will transfer at most
+				 * 2^31-1 bytes, so the return value is
+				 * either -1 or a value between 0
+				 * and 2^31-1, so it's non-negative.
+				 *
+				 * Cast it to size_t to squelch
+				 * warnings from the compiler; add this
+				 * comment to squelch warnings from
+				 * humans reading the code. :-)
+				 *
+				 * Don't treat an EOF as an error, but
+				 * *do* treat a short read as an error;
+				 * that "shouldn't happen", but....
+				 */
+				if (nread != 0 &&
+				    (size_t)nread < sizeof(value)) {
+					snprintf(handle->errbuf, PCAP_ERRBUF_SIZE,
+					    "Short read from event FD: expected %zu, got %zd",
+					    sizeof(value), nread);
+					return PCAP_ERROR;
+				}
 
 				/*
 				 * This event gets signaled by a