syslogd,logread: get rid of head pointer, fix logread bug in the process

function                                             old     new   delta
logread_main                                         450     462     +12
syslogd_main                                        1246    1252      +6
shbuf                                                  -       4      +4
buf                                                   34      30      -4
packed_usage                                       22729   22724      -5
log_locally                                          957     767    -190
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/3 up/down: 22/-199)          Total: -177 bytes
   text    data     bss     dec     hex filename
 773886    1116   11316  786318   bff8e busybox_old
 773714    1116   11316  786146   bfee2 busybox_unstripped

1 files changed, 12 insertions, 43 deletions

diff --git a/sysklogd/syslogd.c b/sysklogd/syslogd.c
index 0ed16bc..6ecbd16 100644
--- a/sysklogd/syslogd.c
+++ b/sysklogd/syslogd.c
@@ -46,8 +46,7 @@ enum { MAX_READ = 256 };
 
 /* Semaphore operation structures */
 struct shbuf_ds {
-	int32_t size;   /* size of data written */
-	int32_t head;   /* start of message list */
+	int32_t size;   /* size of data - 1 */
 	int32_t tail;   /* end of message list */
 	char data[1];   /* data/messages */
 };
@@ -212,8 +211,9 @@ static void ipcsyslog_init(void)
 		bb_perror_msg_and_die("shmat");
 	}
 
-	G.shbuf->size = G.shm_size - offsetof(struct shbuf_ds, data);
-	G.shbuf->head = G.shbuf->tail = 0;
+	memset(G.shbuf, 0, G.shm_size);
+	G.shbuf->size = G.shm_size - offsetof(struct shbuf_ds, data) - 1;
+	/*G.shbuf->tail = 0;*/
 
 	// we'll trust the OS to set initial semval to 0 (let's hope)
 	G.s_semid = semget(KEY_ID, 2, IPC_CREAT | IPC_EXCL | 1023);
@@ -231,7 +231,6 @@ static void ipcsyslog_init(void)
 static void log_to_shmem(const char *msg, int len)
 {
 	int old_tail, new_tail;
-	char *c;
 
 	if (semop(G.s_semid, G.SMwdn, 3) == -1) {
 		bb_perror_msg_and_die("SMwdn");
@@ -240,49 +239,20 @@ static void log_to_shmem(const char *msg, int len)
 	/* Circular Buffer Algorithm:
 	 * --------------------------
 	 * tail == position where to store next syslog message.
-	 * head == position of next message to retrieve ("print").
-	 * if head == tail, there is no "unprinted" messages left.
-	 * head is typically advanced by separate "reader" program,
-	 * but if there isn't one, we have to do it ourself.
-	 * messages are NUL-separated.
+	 * tail's max value is (shbuf->size - 1)
+	 * Last byte of buffer is never used and remains NUL.
 	 */
 	len++; /* length with NUL included */
  again:
 	old_tail = G.shbuf->tail;
 	new_tail = old_tail + len;
 	if (new_tail < G.shbuf->size) {
-		/* No need to move head if shbuf->head <= old_tail,
-		 * else... */
-		if (old_tail < G.shbuf->head && G.shbuf->head <= new_tail) {
-			/* ...need to move head forward */
-			c = memchr(G.shbuf->data + new_tail, '\0',
-					   G.shbuf->size - new_tail);
-			if (!c) /* no NUL ahead of us, wrap around */
-				c = memchr(G.shbuf->data, '\0', old_tail);
-			if (!c) { /* still nothing? point to this msg... */
-				G.shbuf->head = old_tail;
-			} else {
-				/* convert pointer to offset + skip NUL */
-				G.shbuf->head = c - G.shbuf->data + 1;
-			}
-		}
 		/* store message, set new tail */
 		memcpy(G.shbuf->data + old_tail, msg, len);
 		G.shbuf->tail = new_tail;
 	} else {
-		/* we need to break up the message and wrap it around */
 		/* k == available buffer space ahead of old tail */
-		int k = G.shbuf->size - old_tail - 1;
-		if (G.shbuf->head > old_tail) {
-			/* we are going to overwrite head, need to
-			 * move it out of the way */
-			c = memchr(G.shbuf->data, '\0', old_tail);
-			if (!c) { /* nothing? point to this msg... */
-				G.shbuf->head = old_tail;
-			} else { /* convert pointer to offset + skip NUL */
-				G.shbuf->head = c - G.shbuf->data + 1;
-			}
-		}
+		int k = G.shbuf->size - old_tail;
 		/* copy what fits to the end of buffer, and repeat */
 		memcpy(G.shbuf->data + old_tail, msg, k);
 		msg += k;
@@ -294,7 +264,7 @@ static void log_to_shmem(const char *msg, int len)
 		bb_perror_msg_and_die("SMwup");
 	}
 	if (DEBUG)
-		printf("head:%d tail:%d\n", G.shbuf->head, G.shbuf->tail);
+		printf("tail:%d\n", G.shbuf->tail);
 }
 #else
 void ipcsyslog_cleanup(void);
@@ -339,11 +309,10 @@ static void log_locally(char *msg)
 		}
 #if ENABLE_FEATURE_ROTATE_LOGFILE
 		{
-		struct stat statf;
-
-		G.isRegular = (fstat(G.logFD, &statf) == 0 && (statf.st_mode & S_IFREG));
-		/* bug (mostly harmless): can wrap around if file > 4gb */
-		G.curFileSize = statf.st_size;
+			struct stat statf;
+			G.isRegular = (fstat(G.logFD, &statf) == 0 && S_ISREG(statf.st_mode));
+			/* bug (mostly harmless): can wrap around if file > 4gb */
+			G.curFileSize = statf.st_size;
 		}
 #endif
 	}