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

2 files changed, 81 insertions, 87 deletions

diff --git a/sysklogd/logread.c b/sysklogd/logread.c
index ac354b5c3..597e285a5 100644
--- a/sysklogd/logread.c
+++ b/sysklogd/logread.c
@@ -19,11 +19,10 @@
 enum { KEY_ID = 0x414e4547 }; /* "GENA" */
 
 static 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
-} *buf;                         // shared memory pointer
+        char data[1];           // messages
+} *shbuf;
 
 // Semaphore operation structures
 static struct sembuf SMrup[1] = {{0, -1, IPC_NOWAIT | SEM_UNDO}}; // set SMrup
@@ -34,7 +33,7 @@ static void error_exit(const char *str) ATTRIBUTE_NORETURN;
 static void error_exit(const char *str)
 {
         //release all acquired resources
-        shmdt(buf);
+        shmdt(shbuf);
         bb_perror_msg_and_die(str);
 }
 
@@ -50,7 +49,7 @@ static void sem_up(int semid)
 static void interrupted(int sig ATTRIBUTE_UNUSED)
 {
         signal(SIGINT, SIG_IGN);
-        shmdt(buf);
+        shmdt(shbuf);
         exit(0);
 }
 
@@ -66,79 +65,105 @@ int logread_main(int argc, char **argv)
         if (log_shmid == -1)
                 bb_perror_msg_and_die("can't find syslogd buffer");
 
-        // Attach shared memory to our char*
-        buf = shmat(log_shmid, NULL, SHM_RDONLY);
-        if (buf == NULL)
+        /* Attach shared memory to our char* */
+        shbuf = shmat(log_shmid, NULL, SHM_RDONLY);
+        if (shbuf == NULL)
                 bb_perror_msg_and_die("can't access syslogd buffer");
 
         log_semid = semget(KEY_ID, 0, 0);
         if (log_semid == -1)
                 error_exit("can't get access to semaphores for syslogd buffer");
 
-        // attempt to redefine ^C signal
         signal(SIGINT, interrupted);
 
-        // Suppose atomic memory move
-        cur = follow ? buf->tail : buf->head;
+        /* Suppose atomic memory read */
+        /* Max possible value for tail is shbuf->size - 1 */
+        cur = shbuf->tail;
 
+        /* Loop for logread -f, one pass if there was no -f */
         do {
+                unsigned shbuf_size;
+                unsigned shbuf_tail;
+                const char *shbuf_data;
 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
-                char *buf_data;
-                int log_len, j;
+                int i;
+                int len_first_part;
+                int len_total = len_total; /* for gcc */
+                char *copy = copy; /* for gcc */
 #endif
                 if (semop(log_semid, SMrdn, 2) == -1)
                         error_exit("semop[SMrdn]");
 
-                if (DEBUG)
-                        printf("head:%i cur:%d tail:%i size:%i\n",
-                                        buf->head, cur, buf->tail, buf->size);
+                /* Copy the info, helps gcc to realize that it doesn't change */
+                shbuf_size = shbuf->size;
+                shbuf_tail = shbuf->tail;
+                shbuf_data = shbuf->data; /* pointer! */
 
-                if (buf->head == buf->tail || cur == buf->tail) {
-                        if (follow) {
+                if (DEBUG)
+                        printf("cur:%d tail:%i size:%i\n",
+                                        cur, shbuf_tail, shbuf_size);
+
+                if (!follow) {
+                        /* advance to oldest complete message */
+                        /* find NUL */
+                        cur += strlen(shbuf_data + cur);
+                        if (cur >= shbuf_size) { /* last byte in buffer? */
+                                cur = strnlen(shbuf_data, shbuf_tail);
+                                if (cur == shbuf_tail)
+                                        goto unlock; /* no complete messages */
+                        }
+                        /* advance to first byte of the message */
+                        cur++;
+                        if (cur >= shbuf_size) /* last byte in buffer? */
+                                cur = 0;
+                } else { /* logread -f */
+                        if (cur == shbuf_tail) {
                                 sem_up(log_semid);
                                 fflush(stdout);
                                 sleep(1); /* TODO: replace me with a sleep_on */
                                 continue;
                         }
-                        puts("<empty syslog>");
                 }
 
-                // Read Memory
+                /* Read from cur to tail */
 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
-                log_len = buf->tail - cur;
-                if (log_len < 0)
-                        log_len += buf->size;
-                buf_data = xmalloc(log_len);
-
-                if (buf->tail >= cur)
-                        j = log_len;
-                else
-                        j = buf->size - cur;
-                memcpy(buf_data, buf->data + cur, j);
-
-                if (buf->tail < cur)
-                        memcpy(buf_data + buf->size - cur, buf->data, buf->tail);
-                cur = buf->tail;
+                len_first_part = len_total = shbuf_tail - cur;
+                if (len_total < 0) {
+                        /* message wraps: */
+                        /* [SECOND PART.........FIRST PART] */
+                        /*  ^data      ^tail    ^cur      ^size */
+                        len_total += shbuf_size;
+                }
+                copy = xmalloc(len_total + 1);
+                if (len_first_part < 0) {
+                        /* message wraps (see above) */
+                        len_first_part = shbuf_size - cur;
+                        memcpy(copy + len_first_part, shbuf_data, shbuf_tail);
+                }
+                memcpy(copy, shbuf_data + cur, len_first_part);
+                copy[len_total] = '\0';
+                cur = shbuf_tail;
 #else
-                while (cur != buf->tail) {
-                        fputs(buf->data + cur, stdout);
-                        cur += strlen(buf->data + cur) + 1;
-                        if (cur >= buf->size)
+                while (cur != shbuf_tail) {
+                        fputs(shbuf_data + cur, stdout);
+                        cur += strlen(shbuf_data + cur) + 1;
+                        if (cur >= shbuf_size)
                                 cur = 0;
                 }
 #endif
-                // release the lock on the log chain
+ unlock:
+                /* release the lock on the log chain */
                 sem_up(log_semid);
 
 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
-                for (j = 0; j < log_len; j += strlen(buf_data+j) + 1) {
-                        fputs(buf_data + j, stdout);
+                for (i = 0; i < len_total; i += strlen(copy + i) + 1) {
+                        fputs(copy + i, stdout);
                 }
-                free(buf_data);
+                free(copy);
 #endif
         } while (follow);
 
-        shmdt(buf);
+        shmdt(shbuf);
 
         fflush_stdout_and_exit(EXIT_SUCCESS);
 }
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
         }