src/bin/lttng-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 <common/error.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         mode_t mode;
  45
  46         /* Default permissions */
  47         mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
  48
  49         /* Change owner of the shm path */
  50         if (global) {
  51                 ret = chown(shm_path, 0, 0);
  52                 if (ret < 0) {
  53                         if (errno != ENOENT) {
  54                                 perror("chown wait shm");
  55                                 goto error;
  56                         }
  57                 }
  58
  59                 /*
  60                  * If global session daemon, any application can register so the shm
  61                  * needs to be set in read-only mode for others.
  62                  */
  63                 mode |= S_IROTH;
  64         } else {
  65                 ret = chown(shm_path, getuid(), getgid());
  66                 if (ret < 0) {
  67                         if (errno != ENOENT) {
  68                                 perror("chown wait shm");
  69                                 goto error;
  70                         }
  71                 }
  72         }
  73
  74         /*
  75          * Set permissions to the shm even if we did not create the shm.
  76          */
  77         ret = chmod(shm_path, mode);
  78         if (ret < 0) {
  79                 if (errno != ENOENT) {
  80                         perror("chmod wait shm");
  81                         goto error;
  82                 }
  83         }
  84
  85         /*
  86          * We're alone in a child process, so we can modify the process-wide
  87          * umask.
  88          */
  89         umask(~mode);
  90
  91         /*
  92          * Try creating shm (or get rw access). We don't do an exclusive open,
  93          * because we allow other processes to create+ftruncate it concurrently.
  94          */
  95         wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
  96         if (wait_shm_fd < 0) {
  97                 perror("shm_open wait shm");
  98                 goto error;
  99         }
 100
 101         ret = ftruncate(wait_shm_fd, mmap_size);
 102         if (ret < 0) {
 103                 perror("ftruncate wait shm");
 104                 exit(EXIT_FAILURE);
 105         }
 106
 107         ret = fchmod(wait_shm_fd, mode);
 108         if (ret < 0) {
 109                 perror("fchmod");
 110                 exit(EXIT_FAILURE);
 111         }
 112
 113         DBG("Got the wait shm fd %d", wait_shm_fd);
 114
 115         return wait_shm_fd;
 116
 117 error:
 118         DBG("Failing to get the wait shm fd");
 119
 120         return -1;
 121 }
 122
 123 /*
 124  * Return the wait shm mmap for UST application notification. The global
 125  * variable is used to indicate if the the session daemon is global
 126  * (root:tracing) or running with an unprivileged user.
 127  *
 128  * This returned value is used by futex_wait_update() in futex.c to WAKE all
 129  * waiters which are UST application waiting for a session daemon.
 130  */
 131 char *shm_ust_get_mmap(char *shm_path, int global)
 132 {
 133         size_t mmap_size = sysconf(_SC_PAGE_SIZE);
 134         int wait_shm_fd, ret;
 135         char *wait_shm_mmap;
 136
 137         wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
 138         if (wait_shm_fd < 0) {
 139                 goto error;
 140         }
 141
 142         wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
 143                         MAP_SHARED, wait_shm_fd, 0);
 144
 145         /* close shm fd immediately after taking the mmap reference */
 146         ret = close(wait_shm_fd);
 147         if (ret) {
 148                 perror("Error closing fd");
 149         }
 150
 151         if (wait_shm_mmap == MAP_FAILED) {
 152                 DBG("mmap error (can be caused by race with ust).");
 153                 goto error;
 154         }
 155
 156         return wait_shm_mmap;
 157
 158 error:
 159         return NULL;
 160 }