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"
  13 //config:       default y
  14 //config:       depends on FEATURE_IPC_SYSLOG
  15 //config:       help
  16 //config:         If you enabled Circular Buffer support, you almost
  17 //config:         certainly want to enable this feature as well. This
  18 //config:         utility will allow you to read the messages that are
  19 //config:         stored in the syslogd circular buffer.
  20 //config:
  21 //config:config FEATURE_LOGREAD_REDUCED_LOCKING
  22 //config:       bool "Double buffering"
  23 //config:       default y
  24 //config:       depends on LOGREAD
  25 //config:       help
  26 //config:         'logread' ouput to slow serial terminals can have
  27 //config:         side effects on syslog because of the semaphore.
  28 //config:         This option make logread to double buffer copy
  29 //config:         from circular buffer, minimizing semaphore
  30 //config:         contention at some minor memory expense.
  31 //config:
  32
  33 //applet:IF_LOGREAD(APPLET(logread, BB_DIR_SBIN, BB_SUID_DROP))
  34
  35 //kbuild:lib-$(CONFIG_LOGREAD) += logread.o
  36
  37 //usage:#define logread_trivial_usage
  38 //usage:       "[-fF]"
  39 //usage:#define logread_full_usage "\n\n"
  40 //usage:       "Show messages in syslogd's circular buffer\n"
  41 //usage:     "\n        -f      Output data as log grows"
  42 //usage:     "\n        -F      Same as -f, but dump buffer first"
  43
  44 #include "libbb.h"
  45 #include "common_bufsiz.h"
  46 #include <sys/ipc.h>
  47 #include <sys/sem.h>
  48 #include <sys/shm.h>
  49
  50 #define DEBUG 0
  51
  52 /* our shared key (syslogd.c and logread.c must be in sync) */
  53 enum { KEY_ID = 0x414e4547 }; /* "GENA" */
  54
  55 struct shbuf_ds {
  56         int32_t size;           // size of data - 1
  57         int32_t tail;           // end of message list
  58         char data[1];           // messages
  59 };
  60
  61 static const struct sembuf init_sem[3] = {
  62         {0, -1, IPC_NOWAIT | SEM_UNDO},
  63         {1, 0}, {0, +1, SEM_UNDO}
  64 };
  65
  66 struct globals {
  67         struct sembuf SMrup[1]; // {0, -1, IPC_NOWAIT | SEM_UNDO},
  68         struct sembuf SMrdn[2]; // {1, 0}, {0, +1, SEM_UNDO}
  69         struct shbuf_ds *shbuf;
  70 } FIX_ALIASING;
  71 #define G (*(struct globals*)bb_common_bufsiz1)
  72 #define SMrup (G.SMrup)
  73 #define SMrdn (G.SMrdn)
  74 #define shbuf (G.shbuf)
  75 #define INIT_G() do { \
  76         setup_common_bufsiz(); \
  77         memcpy(SMrup, init_sem, sizeof(init_sem)); \
  78 } while (0)
  79
  80 #if 0
  81 static void error_exit(const char *str) NORETURN;
  82 static void error_exit(const char *str)
  83 {
  84         /* Release all acquired resources */
  85         shmdt(shbuf);
  86         bb_perror_msg_and_die(str);
  87 }
  88 #else
  89 /* On Linux, shmdt is not mandatory on exit */
  90 # define error_exit(str) bb_perror_msg_and_die(str)
  91 #endif
  92
  93 /*
  94  * sem_up - up()'s a semaphore.
  95  */
  96 static void sem_up(int semid)
  97 {
  98         if (semop(semid, SMrup, 1) == -1)
  99                 error_exit("semop[SMrup]");
 100 }
 101
 102 static void interrupted(int sig)
 103 {
 104         /* shmdt(shbuf); - on Linux, shmdt is not mandatory on exit */
 105         kill_myself_with_sig(sig);
 106 }
 107
 108 int logread_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 109 int logread_main(int argc UNUSED_PARAM, char **argv)
 110 {
 111         unsigned cur;
 112         int log_semid; /* ipc semaphore id */
 113         int log_shmid; /* ipc shared memory id */
 114         int follow = getopt32(argv, "fF");
 115
 116         INIT_G();
 117
 118         log_shmid = shmget(KEY_ID, 0, 0);
 119         if (log_shmid == -1)
 120                 bb_perror_msg_and_die("can't %s syslogd buffer", "find");
 121
 122         /* Attach shared memory to our char* */
 123         shbuf = shmat(log_shmid, NULL, SHM_RDONLY);
 124         if (shbuf == NULL)
 125                 bb_perror_msg_and_die("can't %s syslogd buffer", "access");
 126
 127         log_semid = semget(KEY_ID, 0, 0);
 128         if (log_semid == -1)
 129                 error_exit("can't get access to semaphores for syslogd buffer");
 130
 131         bb_signals(BB_FATAL_SIGS, interrupted);
 132
 133         /* Suppose atomic memory read */
 134         /* Max possible value for tail is shbuf->size - 1 */
 135         cur = shbuf->tail;
 136
 137         /* Loop for -f or -F, one pass otherwise */
 138         do {
 139                 unsigned shbuf_size;
 140                 unsigned shbuf_tail;
 141                 const char *shbuf_data;
 142 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
 143                 int i;
 144                 int len_first_part;
 145                 int len_total = len_total; /* for gcc */
 146                 char *copy = copy; /* for gcc */
 147 #endif
 148                 if (semop(log_semid, SMrdn, 2) == -1)
 149                         error_exit("semop[SMrdn]");
 150
 151                 /* Copy the info, helps gcc to realize that it doesn't change */
 152                 shbuf_size = shbuf->size;
 153                 shbuf_tail = shbuf->tail;
 154                 shbuf_data = shbuf->data; /* pointer! */
 155
 156                 if (DEBUG)
 157                         printf("cur:%u tail:%u size:%u\n",
 158                                         cur, shbuf_tail, shbuf_size);
 159
 160                 if (!(follow & 1)) { /* not -f */
 161                         /* if -F, "convert" it to -f, so that we dont
 162                          * dump the entire buffer on each iteration
 163                          */
 164                         follow >>= 1;
 165
 166                         /* advance to oldest complete message */
 167                         /* find NUL */
 168                         cur += strlen(shbuf_data + cur);
 169                         if (cur >= shbuf_size) { /* last byte in buffer? */
 170                                 cur = strnlen(shbuf_data, shbuf_tail);
 171                                 if (cur == shbuf_tail)
 172                                         goto unlock; /* no complete messages */
 173                         }
 174                         /* advance to first byte of the message */
 175                         cur++;
 176                         if (cur >= shbuf_size) /* last byte in buffer? */
 177                                 cur = 0;
 178                 } else { /* -f */
 179                         if (cur == shbuf_tail) {
 180                                 sem_up(log_semid);
 181                                 fflush_all();
 182                                 sleep(1); /* TODO: replace me with a sleep_on */
 183                                 continue;
 184                         }
 185                 }
 186
 187                 /* Read from cur to tail */
 188 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
 189                 len_first_part = len_total = shbuf_tail - cur;
 190                 if (len_total < 0) {
 191                         /* message wraps: */
 192                         /* [SECOND PART.........FIRST PART] */
 193                         /*  ^data      ^tail    ^cur      ^size */
 194                         len_total += shbuf_size;
 195                 }
 196                 copy = xmalloc(len_total + 1);
 197                 if (len_first_part < 0) {
 198                         /* message wraps (see above) */
 199                         len_first_part = shbuf_size - cur;
 200                         memcpy(copy + len_first_part, shbuf_data, shbuf_tail);
 201                 }
 202                 memcpy(copy, shbuf_data + cur, len_first_part);
 203                 copy[len_total] = '\0';
 204                 cur = shbuf_tail;
 205 #else
 206                 while (cur != shbuf_tail) {
 207                         fputs(shbuf_data + cur, stdout);
 208                         cur += strlen(shbuf_data + cur) + 1;
 209                         if (cur >= shbuf_size)
 210                                 cur = 0;
 211                 }
 212 #endif
 213  unlock:
 214                 /* release the lock on the log chain */
 215                 sem_up(log_semid);
 216
 217 #if ENABLE_FEATURE_LOGREAD_REDUCED_LOCKING
 218                 for (i = 0; i < len_total; i += strlen(copy + i) + 1) {
 219                         fputs(copy + i, stdout);
 220                 }
 221                 free(copy);
 222 #endif
 223                 fflush_all();
 224         } while (follow);
 225
 226         /* shmdt(shbuf); - on Linux, shmdt is not mandatory on exit */
 227
 228         fflush_stdout_and_exit(EXIT_SUCCESS);
 229 }