rculfhash: check for callers from RCU read-side C.S.
[lttng-tools.git] / src / bin / lttng-sessiond / shm.c
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1/*
2 * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
3 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
4 *
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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.
0fdd1e2c 8 *
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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.
0fdd1e2c 13 *
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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.
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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
db758600 29#include <common/error.h>
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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 */
41static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
42{
43 int wait_shm_fd, ret;
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44 mode_t mode;
45
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46 assert(shm_path);
47
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48 /* Default permissions */
49 mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
50
51 /* Change owner of the shm path */
52 if (global) {
53 ret = chown(shm_path, 0, 0);
54 if (ret < 0) {
55 if (errno != ENOENT) {
df0f840b 56 PERROR("chown wait shm");
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57 goto error;
58 }
59 }
60
61 /*
62 * If global session daemon, any application can register so the shm
63 * needs to be set in read-only mode for others.
64 */
65 mode |= S_IROTH;
66 } else {
67 ret = chown(shm_path, getuid(), getgid());
68 if (ret < 0) {
69 if (errno != ENOENT) {
df0f840b 70 PERROR("chown wait shm");
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71 goto error;
72 }
73 }
74 }
75
76 /*
77 * Set permissions to the shm even if we did not create the shm.
78 */
79 ret = chmod(shm_path, mode);
80 if (ret < 0) {
81 if (errno != ENOENT) {
df0f840b 82 PERROR("chmod wait shm");
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83 goto error;
84 }
85 }
86
87 /*
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88 * We're alone in a child process, so we can modify the process-wide
89 * umask.
0fdd1e2c 90 */
7d051034 91 umask(~mode);
0fdd1e2c 92
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93 /*
94 * Try creating shm (or get rw access). We don't do an exclusive open,
95 * because we allow other processes to create+ftruncate it concurrently.
96 */
97 wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
98 if (wait_shm_fd < 0) {
df0f840b 99 PERROR("shm_open wait shm");
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100 goto error;
101 }
0fdd1e2c 102
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103 ret = ftruncate(wait_shm_fd, mmap_size);
104 if (ret < 0) {
df0f840b 105 PERROR("ftruncate wait shm");
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106 exit(EXIT_FAILURE);
107 }
0fdd1e2c 108
409a0c56 109#ifndef __FreeBSD__
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110 ret = fchmod(wait_shm_fd, mode);
111 if (ret < 0) {
df0f840b 112 PERROR("fchmod");
0fdd1e2c 113 exit(EXIT_FAILURE);
0fdd1e2c 114 }
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115#else
116#warning "FreeBSD does not support setting file mode on shm FD. Remember that for secure use, lttng-sessiond should be started before applications linked on lttng-ust."
117#endif
0fdd1e2c 118
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119 DBG("Got the wait shm fd %d", wait_shm_fd);
120
121 return wait_shm_fd;
122
123error:
124 DBG("Failing to get the wait shm fd");
125
126 return -1;
127}
128
129/*
130 * Return the wait shm mmap for UST application notification. The global
131 * variable is used to indicate if the the session daemon is global
132 * (root:tracing) or running with an unprivileged user.
133 *
134 * This returned value is used by futex_wait_update() in futex.c to WAKE all
135 * waiters which are UST application waiting for a session daemon.
136 */
137char *shm_ust_get_mmap(char *shm_path, int global)
138{
139 size_t mmap_size = sysconf(_SC_PAGE_SIZE);
140 int wait_shm_fd, ret;
141 char *wait_shm_mmap;
142
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143 assert(shm_path);
144
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145 wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
146 if (wait_shm_fd < 0) {
147 goto error;
148 }
149
150 wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
151 MAP_SHARED, wait_shm_fd, 0);
7d051034 152
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153 /* close shm fd immediately after taking the mmap reference */
154 ret = close(wait_shm_fd);
155 if (ret) {
df0f840b 156 PERROR("Error closing fd");
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157 }
158
159 if (wait_shm_mmap == MAP_FAILED) {
160 DBG("mmap error (can be caused by race with ust).");
161 goto error;
162 }
163
164 return wait_shm_mmap;
165
166error:
167 return NULL;
168}
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