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 0ed16bc9e..6ecbd16f0 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
         }