ltt-sessiond/shm.c

   1 /*
   2  * Copyright (C)  2011 - David Goulet <david.goulet@polymtl.ca>
   3  *                       Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   4  *
   5  * This program is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License as published by the Free
   7  * Software Foundation; only version 2 of the License.
   8  *
   9  * This program is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
  12  * more details.
  13  *
  14  * You should have received a copy of the GNU General Public License along with
  15  * this program; if not, write to the Free Software Foundation, Inc., 59 Temple
  16  * Place - Suite 330, Boston, MA  02111-1307, USA.
  17  */
  18
  19 #define _GNU_SOURCE
  20 #include <fcntl.h>
  21 #include <limits.h>
  22 #include <sys/mman.h>
  23 #include <sys/stat.h>
  24 #include <sys/types.h>
  25 #include <sys/wait.h>
  26 #include <unistd.h>
  27 #include <urcu.h>
  28
  29 #include <lttngerr.h>
  30
  31 #include "shm.h"
  32
  33 /*
  34  * Using fork to set umask in the child process (not multi-thread safe). We
  35  * deal with the shm_open vs ftruncate race (happening when the sessiond owns
  36  * the shm and does not let everybody modify it, to ensure safety against
  37  * shm_unlink) by simply letting the mmap fail and retrying after a few
  38  * seconds. For global shm, everybody has rw access to it until the sessiond
  39  * starts.
  40  */
  41 static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
  42 {
  43         int wait_shm_fd, ret;
  44         pid_t pid;
  45         mode_t mode;
  46
  47         /* Default permissions */
  48         mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
  49
  50         /* Change owner of the shm path */
  51         if (global) {
  52                 ret = chown(shm_path, 0, 0);
  53                 if (ret < 0) {
  54                         if (errno != ENOENT) {
  55                                 perror("chown wait shm");
  56                                 goto error;
  57                         }
  58                 }
  59
  60                 /*
  61                  * If global session daemon, any application can register so the shm
  62                  * needs to be set in read-only mode for others.
  63                  */
  64                 mode |= S_IROTH;
  65         } else {
  66                 ret = chown(shm_path, getuid(), getgid());
  67                 if (ret < 0) {
  68                         if (errno != ENOENT) {
  69                                 perror("chown wait shm");
  70                                 goto error;
  71                         }
  72                 }
  73         }
  74
  75         /*
  76          * Set permissions to the shm even if we did not create the shm.
  77          */
  78         ret = chmod(shm_path, mode);
  79         if (ret < 0) {
  80                 if (errno != ENOENT) {
  81                         perror("chmod wait shm");
  82                         goto error;
  83                 }
  84         }
  85
  86         /*
  87          * If the open failed because the file did not exist, try creating it
  88          * ourself.
  89          */
  90         pid = fork();
  91         if (pid > 0) {
  92                 int status;
  93                 /*
  94                  * Parent: wait for child to return, in which case the shared memory
  95                  * map will have been created.
  96                  */
  97                 pid = wait(&status);
  98                 if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
  99                         goto error;
 100                 }
 101
 102                 /*
 103                  * Try to open read-only again after creation.
 104                  */
 105                 wait_shm_fd = shm_open(shm_path, O_RDWR, 0);
 106                 if (wait_shm_fd < 0) {
 107                         /*
 108                          * Real-only open did not work. It's a failure that prohibits using
 109                          * shm.
 110                          */
 111                         ERR("Error opening shm %s", shm_path);
 112                         goto error;
 113                 }
 114                 goto end;
 115         } else if (pid == 0) {
 116                 /*
 117                  * We're alone in a child process, so we can modify the process-wide
 118                  * umask.
 119                  */
 120                 umask(~mode);
 121
 122                 /*
 123                  * Try creating shm (or get rw access). We don't do an exclusive open,
 124                  * because we allow other processes to create+ftruncate it
 125                  * concurrently.
 126                  */
 127                 wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
 128                 if (wait_shm_fd >= 0) {
 129                         ret = ftruncate(wait_shm_fd, mmap_size);
 130                         if (ret < 0) {
 131                                 perror("ftruncate wait shm");
 132                                 exit(EXIT_FAILURE);
 133                         }
 134
 135                         ret = fchmod(wait_shm_fd, mode);
 136                         if (ret < 0) {
 137                                 perror("fchmod");
 138                                 exit(EXIT_FAILURE);
 139                         }
 140                         exit(EXIT_SUCCESS);
 141                 }
 142                 ERR("Error opening shm %s", shm_path);
 143                 exit(EXIT_FAILURE);
 144         } else {
 145                 return -1;
 146         }
 147
 148 end:
 149         DBG("Got the wait shm fd %d", wait_shm_fd);
 150
 151         return wait_shm_fd;
 152
 153 error:
 154         DBG("Failing to get the wait shm fd");
 155
 156         return -1;
 157 }
 158
 159 /*
 160  * Return the wait shm mmap for UST application notification. The global
 161  * variable is used to indicate if the the session daemon is global
 162  * (root:tracing) or running with an unprivileged user.
 163  *
 164  * This returned value is used by futex_wait_update() in futex.c to WAKE all
 165  * waiters which are UST application waiting for a session daemon.
 166  */
 167 char *shm_ust_get_mmap(char *shm_path, int global)
 168 {
 169         size_t mmap_size = sysconf(_SC_PAGE_SIZE);
 170         int wait_shm_fd, ret;
 171         char *wait_shm_mmap;
 172
 173         wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
 174         if (wait_shm_fd < 0) {
 175                 goto error;
 176         }
 177
 178         wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
 179                         MAP_SHARED, wait_shm_fd, 0);
 180         /* close shm fd immediately after taking the mmap reference */
 181         ret = close(wait_shm_fd);
 182         if (ret) {
 183                 perror("Error closing fd");
 184         }
 185
 186         if (wait_shm_mmap == MAP_FAILED) {
 187                 DBG("mmap error (can be caused by race with ust).");
 188                 goto error;
 189         }
 190
 191         return wait_shm_mmap;
 192
 193 error:
 194         return NULL;
 195 }