sysklogd/logread.c

   1 /* vi: set sw=4 ts=4: */
   2 /*
   3  * circular buffer syslog implementation for busybox
   4  *
   5  * Copyright (C) 2000 by Gennady Feldman <gfeldman@gena01.com>
   6  *
   7  * Maintainer: Gennady Feldman <gfeldman@gena01.com> as of Mar 12, 2001
   8  *
   9  * Licensed under GPLv2 or later, see file LICENSE in this source tree.
  10  */
  11 //config:config LOGREAD
  12 //config:       bool "logread (5 kb)"
  13 //config:       default y
  14 //WRONG: it should be compilable without SYSLOG=y:
  15 //WRONG:        depends on FEATURE_IPC_SYSLOG
  16 //config:       help
  17 //config:       If you enabled Circular Buffer support, you almost
  18 //config:       certainly want to enable this feature as well. This
  19 //config:       utility will allow you to read the messages that are
  20 //config:       stored in the syslogd circular buffer.
  21 //config:
  22 //config:config FEATURE_LOGREAD_REDUCED_LOCKING
  23 //config:       bool "Double buffering"
  24 //config:       default y
  25 //config:       depends on LOGREAD
  26 //config:       help
  27 //config:       'logread' output to slow serial terminals can have
  28 //config:       side effects on syslog because of the semaphore.
  29 //config:       This option make logread to double buffer copy
  30 //config:       from circular buffer, minimizing semaphore
  31 //config:       contention at some minor memory expense.
  32 //config:
  33
  34 //applet:IF_LOGREAD(APPLET(logread, BB_DIR_SBIN, BB_SUID_DROP))
  35
  36 //kbuild:lib-$(CONFIG_LOGREAD) += logread.o
  37
  38 //usage:#define logread_trivial_usage
  39 //usage:       "[-fF]"
  40 //usage:#define logread_full_usage "\n\n"
  41 //usage:       "Show messages in syslogd's circular buffer\n"
  42 //usage:     "\n        -f      Output data as log grows"
  43 //usage:     "\n        -F      Same as -f, but dump buffer first"
  44
  45 #include "libbb.h"
  46 #include "common_bufsiz.h"
  47 #include <sys/ipc.h>
  48 #include <sys/sem.h>
  49 #include <sys/shm.h>
  50
  51 #define DEBUG 0
  52
  53 /* our shared key (syslogd.c and logread.c must be in sync) */
  54 enum { KEY_ID = 0x414e4547 }; /* "GENA" */
  55
  56 struct shbuf_ds {
  57         int32_t size;           // size of data - 1
  58         int32_t tail;           // end of message list
  59         char data[1];           // messages
  60 };
  61
  62 static const struct sembuf init_sem[3] = {
  63         {0, -1, IPC_NOWAIT | SEM_UNDO},
  64         {1, 0}, {0, +1, SEM_UNDO}
  65 };
  66
  67 struct globals {
  68         struct sembuf SMrup[1]; // {0, -1, IPC_NOWAIT | SEM_UNDO},
  69         struct sembuf SMrdn[2]; // {1, 0}, {0, +1, SEM_UNDO}
  70         struct shbuf_ds *shbuf;
  71 } FIX_ALIASING;
  72 #define G (*(struct globals*)bb_common_bufsiz1)
  73 #define SMrup (G.SMrup)
  74 #define SMrdn (G.SMrdn)
  75 #define shbuf (G.shbuf)
  76 #define INIT_G() do { \
  77         setup_common_bufsiz(); \
  78         memcpy(SMrup, init_sem, sizeof(init_sem)); \
  79 } while (0)
  80
  81 #if 0
  82 static void error_exit(const char *str) NORETURN;
  83 static void error_exit(const char *str)
  84 {
  85         /* Release all acquired resources */
  86         shmdt(shbuf);
  87         bb_perror_msg_and_die(str);
  88 }
  89 #else
  90 /* On Linux, shmdt is not mandatory on exit */
  91 # define error_exit(str) bb_simple_perror_msg_and_die(str)
  92 #endif
  93
  94 /*
  95  * sem_up - up()'s a semaphore.
  96  */
  97 static void sem_up(int semid)
  98 {
  99         if (semop(semid, SMrup, 1) == -1)
 100                 error_exit("semop[SMrup]");
 101 }
 102
 103 static void interrupted(int sig)
 104 {
 105         /* shmdt(shbuf); - on Linux, shmdt is not mandatory on exit */
 106         kill_myself_with_sig(sig);
 107 }
 108
 109 int logread_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 110 int logread_main(int argc UNUSED_PARAM, char **argv)
 111 {
 112         unsigned cur;
 113         int log_semid; /* ipc semaphore id */
 114         int log_shmid; /* ipc shared memory id */
 115         int follow = getopt32(argv, "fF");
 116
 117         INIT_G();
 118
 119         log_shmid = shmget(KEY_ID, 0, 0);
 120         if (log_shmid == -1)
 121                 bb_perror_msg_and_die("can't %s syslogd buffer", "find");
 122
 123         /* Attach shared memory to our char* */
 124         shbuf = shmat(log_shmid, NULL, SHM_RDONLY);
 125         if (shbuf == NULL)
 126                 bb_perror_msg_and_die("can't %s syslogd buffer", "access");
 127
 128         log_semid = semget(KEY_ID, 0, 0);
 129         if (log_semid == -1)
 130                 error_exit("can't get access to semaphores for syslogd buffer");
 131
 132         bb_signals(BB_FATAL_SIGS, interrupted);
 133
 134         /* Suppose atomic memory read */
 135         /* Max possible value for tail is shbuf->size - 1 */
 136         cur = shbuf->tail;
 137
 138         /* Loop for -f or -F, one pass otherwise */
 139         do {
 140                 unsigned shbuf_size;
 141                 unsigned shbuf_tail;
 142                 const char *shbuf_data;
 143 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
 144                 int i;
 145                 int len_first_part;
 146                 int len_total = len_total; /* for gcc */
 147                 char *copy = copy; /* for gcc */
 148 #endif
 149                 if (semop(log_semid, SMrdn, 2) == -1)
 150                         error_exit("semop[SMrdn]");
 151
 152                 /* Copy the info, helps gcc to realize that it doesn't change */
 153                 shbuf_size = shbuf->size;
 154                 shbuf_tail = shbuf->tail;
 155                 shbuf_data = shbuf->data; /* pointer! */
 156
 157                 if (DEBUG)
 158                         printf("cur:%u tail:%u size:%u\n",
 159                                         cur, shbuf_tail, shbuf_size);
 160
 161                 if (!(follow & 1)) { /* not -f */
 162                         /* if -F, "convert" it to -f, so that we don't
 163                          * dump the entire buffer on each iteration
 164                          */
 165                         follow >>= 1;
 166
 167                         /* advance to oldest complete message */
 168                         /* find NUL */
 169                         cur += strlen(shbuf_data + cur);
 170                         if (cur >= shbuf_size) { /* last byte in buffer? */
 171                                 cur = strnlen(shbuf_data, shbuf_tail);
 172                                 if (cur == shbuf_tail)
 173                                         goto unlock; /* no complete messages */
 174                         }
 175                         /* advance to first byte of the message */
 176                         cur++;
 177                         if (cur >= shbuf_size) /* last byte in buffer? */
 178                                 cur = 0;
 179                 } else { /* -f */
 180                         if (cur == shbuf_tail) {
 181                                 sem_up(log_semid);
 182                                 fflush_all();
 183                                 sleep1(); /* TODO: replace me with a sleep_on */
 184                                 continue;
 185                         }
 186                 }
 187
 188                 /* Read from cur to tail */
 189 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
 190                 len_first_part = len_total = shbuf_tail - cur;
 191                 if (len_total < 0) {
 192                         /* message wraps: */
 193                         /* [SECOND PART.........FIRST PART] */
 194                         /*  ^data      ^tail    ^cur      ^size */
 195                         len_total += shbuf_size;
 196                 }
 197                 copy = xmalloc(len_total + 1);
 198                 if (len_first_part < 0) {
 199                         /* message wraps (see above) */
 200                         len_first_part = shbuf_size - cur;
 201                         memcpy(copy + len_first_part, shbuf_data, shbuf_tail);
 202                 }
 203                 memcpy(copy, shbuf_data + cur, len_first_part);
 204                 copy[len_total] = '\0';
 205                 cur = shbuf_tail;
 206 #else
 207                 while (cur != shbuf_tail) {
 208                         fputs_stdout(shbuf_data + cur);
 209                         cur += strlen(shbuf_data + cur) + 1;
 210                         if (cur >= shbuf_size)
 211                                 cur = 0;
 212                 }
 213 #endif
 214  unlock:
 215                 /* release the lock on the log chain */
 216                 sem_up(log_semid);
 217
 218 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
 219                 for (i = 0; i < len_total; i += strlen(copy + i) + 1) {
 220                         fputs_stdout(copy + i);
 221                 }
 222                 free(copy);
 223 #endif
 224                 fflush_all();
 225         } while (follow);
 226
 227         /* shmdt(shbuf); - on Linux, shmdt is not mandatory on exit */
 228
 229         fflush_stdout_and_exit_SUCCESS();
 230 }