[PATCH 2/2] tap, pasta: Handle short writes to /dev/tap

[David Gibson <david@gibson.dropbear.id.au>]

tap_send_frames_pasta() sends frames to the namespace by sending them to
our the /dev/tap device.  If that write() returns an error, we already
handle it.  However we don't handle the case where the write() returns
short, meaning we haven't successfully transmitted the whole frame.

I don't know if this can ever happen with the kernel tap device, but we
should at least report the case so we don't get a cryptic failure.  For
the purposes of the return value for tap_send_frames_pasta() we treat this
case as though it was an error (on the grounds that a partial frame is no
use to the namespace).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tap.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tap.c b/tap.c
index 00622e7..4f11000 100644
--- a/tap.c
+++ b/tap.c
@@ -321,7 +321,9 @@ static size_t tap_send_frames_pasta(const struct ctx *c,
 	size_t i;
 
 	for (i = 0; i < n; i++) {
-		if (write(c->fd_tap, iov[i].iov_base, iov[i].iov_len) < 0) {
+		ssize_t rc = write(c->fd_tap, iov[i].iov_base, iov[i].iov_len);
+
+		if (rc < 0) {
 			debug("tap write: %s", strerror(errno));
 
 			switch (errno) {
@@ -336,6 +338,10 @@ static size_t tap_send_frames_pasta(const struct ctx *c,
 			default:
 				die("Write error on tap device, exiting");
 			}
+		} else if ((size_t)rc < iov[i].iov_len) {
+			debug("short write on tuntap: %zd/%zu",
+			      rc, iov[i].iov_len);
+			break;
 		}
 	}
 
-- 
@@ -321,7 +321,9 @@ static size_t tap_send_frames_pasta(const struct ctx *c,
 	size_t i;
 
 	for (i = 0; i < n; i++) {
-		if (write(c->fd_tap, iov[i].iov_base, iov[i].iov_len) < 0) {
+		ssize_t rc = write(c->fd_tap, iov[i].iov_base, iov[i].iov_len);
+
+		if (rc < 0) {
 			debug("tap write: %s", strerror(errno));
 
 			switch (errno) {
@@ -336,6 +338,10 @@ static size_t tap_send_frames_pasta(const struct ctx *c,
 			default:
 				die("Write error on tap device, exiting");
 			}
+		} else if ((size_t)rc < iov[i].iov_len) {
+			debug("short write on tuntap: %zd/%zu",
+			      rc, iov[i].iov_len);
+			break;
 		}
 	}
 
-- 
2.41.0