[PATCH v2 3/3] sideband: add defense against packets missing a band designator

From: "Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: Jonathan Tan <jonathantanmy@google.com>,
	Jeff King <peff@peff.net>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Johannes Schindelin <johannes.schindelin@gmx.de>,
	Johannes Schindelin <johannes.schindelin@gmx.de>
Subject: [PATCH v2 3/3] sideband: add defense against packets missing a band designator
Date: Mon, 19 Oct 2020 19:35:42 +0000	[thread overview]
Message-ID: <c61e560451c4d7f101a23acec69117ddac563330.1603136143.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.753.v2.git.1603136142.gitgitgadget@gmail.com>

From: Johannes Schindelin <johannes.schindelin@gmx.de>

While there is currently no instance of code producing this type of
packet, if the `demultiplex_sideband()` would receive a packet whose
payload is not only empty but even misses the band designator, it would
mistake it for a flush packet.

Let's defend against such a bug in the future.

Note: `demultiplex_sideband()` will treat empty flush, delim, eof and
response-end packets all alike: it will treat them as flush packets.
Since empty packets by definition are missing a band designator, this is
the best that function can do. Therefore, it would make little sense to
pass the `status` on to `demultiplex_sideband()`. Besides, fbd76cd450
(sideband: reverse its dependency on pkt-line, 2019-01-16) went out of
its way to _stop_ the code inside `demultiplex_sideband()` from relying
on anything in `pkt-line.h`; that includes the data type `enum
packet_read_status` that we would need if we were to pass `status` as a
parameter to that function.

Based on a suggestion by Jeff King.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 pkt-line.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/pkt-line.c b/pkt-line.c
index 657a702927..f72048f623 100644
--- a/pkt-line.c
+++ b/pkt-line.c
@@ -461,8 +461,10 @@ int recv_sideband(const char *me, int in_stream, int out)
 	enum sideband_type sideband_type;
 
 	while (1) {
-		len = packet_read(in_stream, NULL, NULL, buf, LARGE_PACKET_MAX,
-				  0);
+		int status = packet_read_with_status(in_stream, NULL, NULL, buf,
+						     LARGE_PACKET_MAX, &len, 0);
+		if (!len && status == PACKET_READ_NORMAL)
+			BUG("missing band designator");
 		if (!demultiplex_sideband(me, buf, len, 0, &scratch,
 					  &sideband_type))
 			continue;
@@ -520,6 +522,8 @@ enum packet_read_status packet_reader_read(struct packet_reader *reader)
 							 reader->options);
 		if (!reader->use_sideband)
 			break;
+		if (!reader->pktlen && reader->status == PACKET_READ_NORMAL)
+			BUG("missing band designator");
 		if (demultiplex_sideband(reader->me, reader->buffer,
 					 reader->pktlen, 1, &scratch,
 					 &sideband_type))
-- 
gitgitgadget
