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
   6  * it under the terms of the GNU General Public License, version 2 only,
   7  * as published by the Free Software Foundation.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License along
  15  * with this program; if not, write to the Free Software Foundation, Inc.,
  16  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  17  */
  18
  19 #define _LGPL_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         assert(shm_path);
  47
  48         /* Default permissions */
  49         mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
  50
  51         /*
  52          * Change owner of the shm path.
  53          */
  54         if (global) {
  55                 /*
  56                  * If global session daemon, any application can
  57                  * register. Make it initially writeable so applications
  58                  * registering concurrently can do ftruncate() by
  59                  * themselves.
  60                  */
  61                 mode |= S_IROTH | S_IWOTH;
  62         }
  63
  64         /*
  65          * We're alone in a child process, so we can modify the process-wide
  66          * umask.
  67          */
  68         umask(~mode);
  69
  70         /*
  71          * Try creating shm (or get rw access). We don't do an exclusive open,
  72          * because we allow other processes to create+ftruncate it concurrently.
  73          */
  74         wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
  75         if (wait_shm_fd < 0) {
  76                 PERROR("shm_open wait shm");
  77                 goto error;
  78         }
  79
  80         ret = ftruncate(wait_shm_fd, mmap_size);
  81         if (ret < 0) {
  82                 PERROR("ftruncate wait shm");
  83                 exit(EXIT_FAILURE);
  84         }
  85
  86 #ifndef __FreeBSD__
  87         if (global) {
  88                 ret = fchown(wait_shm_fd, 0, 0);
  89                 if (ret < 0) {
  90                         PERROR("fchown");
  91                         exit(EXIT_FAILURE);
  92                 }
  93                 /*
  94                  * If global session daemon, any application can
  95                  * register so the shm needs to be set in read-only mode
  96                  * for others.
  97                  */
  98                 mode &= ~S_IWOTH;
  99                 ret = fchmod(wait_shm_fd, mode);
 100                 if (ret < 0) {
 101                         PERROR("fchmod");
 102                         exit(EXIT_FAILURE);
 103                 }
 104         } else {
 105                 ret = fchown(wait_shm_fd, getuid(), getgid());
 106                 if (ret < 0) {
 107                         PERROR("fchown");
 108                         exit(EXIT_FAILURE);
 109                 }
 110         }
 111 #else
 112 #warning "FreeBSD does not support setting file mode on shm FD."
 113 #endif
 114
 115         DBG("Got the wait shm fd %d", wait_shm_fd);
 116
 117         return wait_shm_fd;
 118
 119 error:
 120         DBG("Failing to get the wait shm fd");
 121
 122         return -1;
 123 }
 124
 125 /*
 126  * Return the wait shm mmap for UST application notification. The global
 127  * variable is used to indicate if the the session daemon is global
 128  * (root:tracing) or running with an unprivileged user.
 129  *
 130  * This returned value is used by futex_wait_update() in futex.c to WAKE all
 131  * waiters which are UST application waiting for a session daemon.
 132  */
 133 char *shm_ust_get_mmap(char *shm_path, int global)
 134 {
 135         size_t mmap_size;
 136         int wait_shm_fd, ret;
 137         char *wait_shm_mmap;
 138         long sys_page_size;
 139
 140         assert(shm_path);
 141
 142         sys_page_size = sysconf(_SC_PAGE_SIZE);
 143         if (sys_page_size < 0) {
 144                 PERROR("sysconf PAGE_SIZE");
 145                 goto error;
 146         }
 147         mmap_size = sys_page_size;
 148
 149         wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
 150         if (wait_shm_fd < 0) {
 151                 goto error;
 152         }
 153
 154         wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
 155                         MAP_SHARED, wait_shm_fd, 0);
 156
 157         /* close shm fd immediately after taking the mmap reference */
 158         ret = close(wait_shm_fd);
 159         if (ret) {
 160                 PERROR("Error closing fd");
 161         }
 162
 163         if (wait_shm_mmap == MAP_FAILED) {
 164                 DBG("mmap error (can be caused by race with ust).");
 165                 goto error;
 166         }
 167
 168         return wait_shm_mmap;
 169
 170 error:
 171         return NULL;
 172 }