common: shm.c: improve logging on various error paths
[lttng-tools.git] / src / common / shm.c
1 /*
2 * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
3 * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
4 *
5 * SPDX-License-Identifier: GPL-2.0-only
6 *
7 */
8
9 #define _LGPL_SOURCE
10 #include <fcntl.h>
11 #include <limits.h>
12 #include <sys/mman.h>
13 #include <sys/stat.h>
14 #include <sys/types.h>
15 #include <sys/wait.h>
16 #include <unistd.h>
17 #include <urcu.h>
18
19 #include <common/error.h>
20
21 #include "shm.h"
22
23 /*
24 * Using fork to set umask in the child process (not multi-thread safe). We
25 * deal with the shm_open vs ftruncate race (happening when the sessiond owns
26 * the shm and does not let everybody modify it, to ensure safety against
27 * shm_unlink) by simply letting the mmap fail and retrying after a few
28 * seconds. For global shm, everybody has rw access to it until the sessiond
29 * starts.
30 */
31 static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
32 {
33 int wait_shm_fd, ret;
34 mode_t mode;
35
36 assert(shm_path);
37
38 /* Default permissions */
39 mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
40
41 /*
42 * Change owner of the shm path.
43 */
44 if (global) {
45 /*
46 * If global session daemon, any application can
47 * register. Make it initially writeable so applications
48 * registering concurrently can do ftruncate() by
49 * themselves.
50 */
51 mode |= S_IROTH | S_IWOTH;
52 }
53
54 /*
55 * We're alone in a child process, so we can modify the process-wide
56 * umask.
57 */
58 umask(~mode);
59
60 /*
61 * Try creating shm (or get rw access). We don't do an exclusive open,
62 * because we allow other processes to create+ftruncate it concurrently.
63 *
64 * A sysctl, fs.protected_regular may prevent the session daemon from
65 * opening a previously created shm when the O_CREAT flag is provided.
66 * Systemd enables this ABI-breaking change by default since v241.
67 *
68 * First, attempt to use the create-or-open semantic that is
69 * desired here. If this fails with EACCES, work around this broken
70 * behaviour and attempt to open the shm without the O_CREAT flag.
71 *
72 * The two attempts are made in this order since applications are
73 * expected to race with the session daemon to create this shm.
74 * Attempting an shm_open() without the O_CREAT flag first could fail
75 * because the file doesn't exist. It could then be created by an
76 * application, which would cause a second try with the O_CREAT flag to
77 * fail with EACCES.
78 *
79 * Note that this introduces a new failure mode where a user could
80 * launch an application (creating the shm) and unlink the shm while
81 * the session daemon is launching, causing the second attempt
82 * to fail. This is not recovered-from as unlinking the shm will
83 * prevent userspace tracing from succeeding anyhow: the sessiond would
84 * use a now-unlinked shm, while the next application would create
85 * a new named shm.
86 */
87 wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
88 if (wait_shm_fd < 0) {
89 if (errno == EACCES) {
90 /* Work around sysctl fs.protected_regular. */
91 DBG("shm_open of %s returned EACCES, this may be caused "
92 "by the fs.protected_regular sysctl. "
93 "Attempting to open the shm without "
94 "creating it.", shm_path);
95 wait_shm_fd = shm_open(shm_path, O_RDWR, mode);
96 }
97 if (wait_shm_fd < 0) {
98 PERROR("Failed to open \"wait\" shared memory object: path = '%s'", shm_path);
99 goto error;
100 }
101 }
102
103 ret = ftruncate(wait_shm_fd, mmap_size);
104 if (ret < 0) {
105 PERROR("Failed to truncate \"wait\" shared memory object: fd = %d, size = %zu",
106 wait_shm_fd, mmap_size);
107 exit(EXIT_FAILURE);
108 }
109
110 if (global) {
111 ret = fchown(wait_shm_fd, 0, 0);
112 if (ret < 0) {
113 PERROR("Failed to set ownership of \"wait\" shared memory object: fd = %d, owner = 0, group = 0",
114 wait_shm_fd);
115 exit(EXIT_FAILURE);
116 }
117 /*
118 * If global session daemon, any application can
119 * register so the shm needs to be set in read-only mode
120 * for others.
121 */
122 mode &= ~S_IWOTH;
123 ret = fchmod(wait_shm_fd, mode);
124 if (ret < 0) {
125 PERROR("Failed to set the mode of the \"wait\" shared memory object: fd = %d, mode = %d",
126 wait_shm_fd, mode);
127 exit(EXIT_FAILURE);
128 }
129 } else {
130 ret = fchown(wait_shm_fd, getuid(), getgid());
131 if (ret < 0) {
132 PERROR("Failed to set ownership of \"wait\" shared memory object: fd = %d, owner = %d, group = %d",
133 wait_shm_fd, getuid(), getgid());
134 exit(EXIT_FAILURE);
135 }
136 }
137
138 DBG("Wait shared memory file descriptor created successfully: path = '%s', mmap_size = %zu, global = %s, fd = %d",
139 shm_path, mmap_size, global ? "true" : "false",
140 wait_shm_fd);
141
142 return wait_shm_fd;
143
144 error:
145 DBG("Failed to open shared memory file descriptor: path = '%s', mmap_size = %zu, global = %s",
146 shm_path, mmap_size, global ? "true" : "false");
147
148 return -1;
149 }
150
151 /*
152 * Return the wait shm mmap for UST application notification. The global
153 * variable is used to indicate if the the session daemon is global
154 * (root:tracing) or running with an unprivileged user.
155 *
156 * This returned value is used by futex_wait_update() in futex.c to WAKE all
157 * waiters which are UST application waiting for a session daemon.
158 */
159 char *shm_ust_get_mmap(char *shm_path, int global)
160 {
161 size_t mmap_size;
162 int wait_shm_fd, ret;
163 char *wait_shm_mmap;
164 long sys_page_size;
165
166 assert(shm_path);
167
168 sys_page_size = sysconf(_SC_PAGE_SIZE);
169 if (sys_page_size < 0) {
170 PERROR("Failed to get PAGE_SIZE of system");
171 goto error;
172 }
173 mmap_size = sys_page_size;
174
175 wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
176 if (wait_shm_fd < 0) {
177 goto error;
178 }
179
180 wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
181 MAP_SHARED, wait_shm_fd, 0);
182
183 /* close shm fd immediately after taking the mmap reference */
184 ret = close(wait_shm_fd);
185 if (ret) {
186 PERROR("Failed to close \"wait\" shared memory object file descriptor: fd = %d",
187 wait_shm_fd);
188 }
189
190 if (wait_shm_mmap == MAP_FAILED) {
191 DBG("Failed to mmap the \"wait\" shareed memory object (can be caused by race with ust): path = '%s', global = %s",
192 shm_path, global ? "true" : "false");
193 goto error;
194 }
195
196 return wait_shm_mmap;
197
198 error:
199 return NULL;
200 }
201
202 /*
203 * shm_create_anonymous is never called concurrently within a process.
204 */
205 int shm_create_anonymous(const char *owner_name)
206 {
207 char tmp_name[NAME_MAX];
208 int shmfd, ret;
209
210 ret = snprintf(tmp_name, NAME_MAX, "/shm-%s-%d", owner_name, getpid());
211 if (ret < 0) {
212 PERROR("Failed to format shm path: owner_name = '%s', pid = %d",
213 owner_name, getpid());
214 return -1;
215 }
216
217 /*
218 * Allocate shm, and immediately unlink its shm oject, keeping only the
219 * file descriptor as a reference to the object.
220 */
221 shmfd = shm_open(tmp_name, O_CREAT | O_EXCL | O_RDWR, 0700);
222 if (shmfd < 0) {
223 PERROR("Failed to open shared memory object: path = '%s'", tmp_name);
224 goto error_shm_open;
225 }
226
227 ret = shm_unlink(tmp_name);
228 if (ret < 0 && errno != ENOENT) {
229 PERROR("Failed to unlink shared memory object: path = '%s'",
230 tmp_name);
231 goto error_shm_release;
232 }
233
234 return shmfd;
235
236 error_shm_release:
237 ret = close(shmfd);
238 if (ret) {
239 PERROR("Failed to close shared memory object file descriptor: fd = %d, path = '%s'",
240 shmfd, tmp_name);
241 }
242 error_shm_open:
243 return -1;
244 }
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