common: Add index allocator for error counters
[lttng-tools.git] / src / bin / lttng-sessiond / 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 shm at %s", shm_path);
99 goto error;
100 }
101 }
102
103 ret = ftruncate(wait_shm_fd, mmap_size);
104 if (ret < 0) {
105 PERROR("ftruncate wait shm");
106 exit(EXIT_FAILURE);
107 }
108
109 if (global) {
110 ret = fchown(wait_shm_fd, 0, 0);
111 if (ret < 0) {
112 PERROR("fchown");
113 exit(EXIT_FAILURE);
114 }
115 /*
116 * If global session daemon, any application can
117 * register so the shm needs to be set in read-only mode
118 * for others.
119 */
120 mode &= ~S_IWOTH;
121 ret = fchmod(wait_shm_fd, mode);
122 if (ret < 0) {
123 PERROR("fchmod");
124 exit(EXIT_FAILURE);
125 }
126 } else {
127 ret = fchown(wait_shm_fd, getuid(), getgid());
128 if (ret < 0) {
129 PERROR("fchown");
130 exit(EXIT_FAILURE);
131 }
132 }
133
134 DBG("Got the wait shm fd %d", wait_shm_fd);
135
136 return wait_shm_fd;
137
138 error:
139 DBG("Failing to get the wait shm fd");
140
141 return -1;
142 }
143
144 /*
145 * Return the wait shm mmap for UST application notification. The global
146 * variable is used to indicate if the the session daemon is global
147 * (root:tracing) or running with an unprivileged user.
148 *
149 * This returned value is used by futex_wait_update() in futex.c to WAKE all
150 * waiters which are UST application waiting for a session daemon.
151 */
152 char *shm_ust_get_mmap(char *shm_path, int global)
153 {
154 size_t mmap_size;
155 int wait_shm_fd, ret;
156 char *wait_shm_mmap;
157 long sys_page_size;
158
159 assert(shm_path);
160
161 sys_page_size = sysconf(_SC_PAGE_SIZE);
162 if (sys_page_size < 0) {
163 PERROR("sysconf PAGE_SIZE");
164 goto error;
165 }
166 mmap_size = sys_page_size;
167
168 wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
169 if (wait_shm_fd < 0) {
170 goto error;
171 }
172
173 wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
174 MAP_SHARED, wait_shm_fd, 0);
175
176 /* close shm fd immediately after taking the mmap reference */
177 ret = close(wait_shm_fd);
178 if (ret) {
179 PERROR("Error closing fd");
180 }
181
182 if (wait_shm_mmap == MAP_FAILED) {
183 DBG("mmap error (can be caused by race with ust).");
184 goto error;
185 }
186
187 return wait_shm_mmap;
188
189 error:
190 return NULL;
191 }
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