Fix potential pointer overflow in xlogreader.c.

While checking if a record could fit in the circular WAL decoding
buffer, the coding from commit 3f1ce97 used arithmetic that could
overflow.  64 bit systems were unaffected for various technical reasons,
which probably explains the lack of problem reports.  Likewise for 32
bit systems running known 32 bit kernels.  The systems at risk of
problems appear to be 32 bit processes running on 64 bit kernels, with
unlucky placement in memory.

Per complaint from GCC -fsanitize=undefined -m32, while testing
variations of 039_end_of_wal.pl.

Back-patch to 15.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGKH0oRPOX7DhiQ_b51sM8HqcPp2J3WA-Oen%3DdXog%2BAGGQ%40mail.gmail.com

Author: macdice

src/backend/access/transam/xlogreader.c

@@ -457,18 +457,37 @@ XLogReadRecordAlloc(XLogReaderState *state, size_t xl_tot_len, bool allow_oversi
 	if (state->decode_buffer_tail >= state->decode_buffer_head)
 	{
 		/* Empty, or tail is to the right of head. */
-		if (state->decode_buffer_tail + required_space <=
-			state->decode_buffer + state->decode_buffer_size)
+		if (required_space <=
+			state->decode_buffer_size -
+			(state->decode_buffer_tail - state->decode_buffer))
 		{
-			/* There is space between tail and end. */
+			/*-
+			 * There is space between tail and end.
+			 *
+			 * +-----+--------------------+-----+
+			 * |     |////////////////////|here!|
+			 * +-----+--------------------+-----+
+			 *       ^                    ^
+			 *       |                    |
+			 *       h                    t
+			 */
 			decoded = (DecodedXLogRecord *) state->decode_buffer_tail;
 			decoded->oversized = false;
 			return decoded;
 		}
-		else if (state->decode_buffer + required_space <
-				 state->decode_buffer_head)
+		else if (required_space <
+				 state->decode_buffer_head - state->decode_buffer)
 		{
-			/* There is space between start and head. */
+			/*-
+			 * There is space between start and head.
+			 *
+			 * +-----+--------------------+-----+
+			 * |here!|////////////////////|     |
+			 * +-----+--------------------+-----+
+			 *       ^                    ^
+			 *       |                    |
+			 *       h                    t
+			 */
 			decoded = (DecodedXLogRecord *) state->decode_buffer;
 			decoded->oversized = false;
 			return decoded;
@@ -477,10 +496,19 @@ XLogReadRecordAlloc(XLogReaderState *state, size_t xl_tot_len, bool allow_oversi
 	else
 	{
 		/* Tail is to the left of head. */
-		if (state->decode_buffer_tail + required_space <
-			state->decode_buffer_head)
+		if (required_space <
+			state->decode_buffer_head - state->decode_buffer_tail)
 		{
-			/* There is space between tail and head. */
+			/*-
+			 * There is space between tail and head.
+			 *
+			 * +-----+--------------------+-----+
+			 * |/////|here!               |/////|
+			 * +-----+--------------------+-----+
+			 *       ^                    ^
+			 *       |                    |
+			 *       t                    h
+			 */
 			decoded = (DecodedXLogRecord *) state->decode_buffer_tail;
 			decoded->oversized = false;
 			return decoded;