Fix: futex wait: handle spurious futex wakeups
[lttng-tools.git] / src / common / shm.cpp
... / ...
CommitLineData
1/*
2 * Copyright (C) 2011 EfficiOS Inc.
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.hpp>
20
21#include "shm.hpp"
22
23/*
24 * We deal with the shm_open vs ftruncate race (happening when the sessiond owns
25 * the shm and does not let everybody modify it, to ensure safety against
26 * shm_unlink) by simply letting the mmap fail and retrying after a few
27 * seconds. For global shm, everybody has rw access to it until the sessiond
28 * starts.
29 */
30static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
31{
32 int wait_shm_fd, ret;
33 mode_t mode, old_mode;
34
35 LTTNG_ASSERT(shm_path);
36
37 /* Default permissions */
38 mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
39
40 /*
41 * Change owner of the shm path.
42 */
43 if (global) {
44 /*
45 * If global session daemon, any application can
46 * register. Make it initially writeable so applications
47 * registering concurrently can do ftruncate() by
48 * themselves.
49 */
50 mode |= S_IROTH | S_IWOTH;
51 }
52
53 old_mode = umask(~mode);
54
55 /*
56 * Try creating shm (or get rw access). We don't do an exclusive open,
57 * because we allow other processes to create+ftruncate it concurrently.
58 *
59 * A sysctl, fs.protected_regular may prevent the session daemon from
60 * opening a previously created shm when the O_CREAT flag is provided.
61 * Systemd enables this ABI-breaking change by default since v241.
62 *
63 * First, attempt to use the create-or-open semantic that is
64 * desired here. If this fails with EACCES, work around this broken
65 * behaviour and attempt to open the shm without the O_CREAT flag.
66 *
67 * The two attempts are made in this order since applications are
68 * expected to race with the session daemon to create this shm.
69 * Attempting an shm_open() without the O_CREAT flag first could fail
70 * because the file doesn't exist. It could then be created by an
71 * application, which would cause a second try with the O_CREAT flag to
72 * fail with EACCES.
73 *
74 * Note that this introduces a new failure mode where a user could
75 * launch an application (creating the shm) and unlink the shm while
76 * the session daemon is launching, causing the second attempt
77 * to fail. This is not recovered-from as unlinking the shm will
78 * prevent userspace tracing from succeeding anyhow: the sessiond would
79 * use a now-unlinked shm, while the next application would create
80 * a new named shm.
81 */
82 wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode);
83 if (wait_shm_fd < 0) {
84 if (errno == EACCES) {
85 /* Work around sysctl fs.protected_regular. */
86 DBG("shm_open of %s returned EACCES, this may be caused "
87 "by the fs.protected_regular sysctl. "
88 "Attempting to open the shm without "
89 "creating it.", shm_path);
90 wait_shm_fd = shm_open(shm_path, O_RDWR, mode);
91 }
92 if (wait_shm_fd < 0) {
93 PERROR("Failed to open \"wait\" shared memory object: path = '%s'", shm_path);
94 goto error;
95 }
96 }
97
98 ret = ftruncate(wait_shm_fd, mmap_size);
99 if (ret < 0) {
100 PERROR("Failed to truncate \"wait\" shared memory object: fd = %d, size = %zu",
101 wait_shm_fd, mmap_size);
102 goto error;
103 }
104
105 if (global) {
106 ret = fchown(wait_shm_fd, 0, 0);
107 if (ret < 0) {
108 PERROR("Failed to set ownership of \"wait\" shared memory object: fd = %d, owner = 0, group = 0",
109 wait_shm_fd);
110 goto error;
111 }
112 /*
113 * If global session daemon, any application can
114 * register so the shm needs to be set in read-only mode
115 * for others.
116 */
117 mode &= ~S_IWOTH;
118 ret = fchmod(wait_shm_fd, mode);
119 if (ret < 0) {
120 PERROR("Failed to set the mode of the \"wait\" shared memory object: fd = %d, mode = %d",
121 wait_shm_fd, mode);
122 goto error;
123 }
124 } else {
125 ret = fchown(wait_shm_fd, getuid(), getgid());
126 if (ret < 0) {
127 PERROR("Failed to set ownership of \"wait\" shared memory object: fd = %d, owner = %d, group = %d",
128 wait_shm_fd, getuid(), getgid());
129 goto error;
130 }
131 }
132
133 DBG("Wait shared memory file descriptor created successfully: path = '%s', mmap_size = %zu, global = %s, fd = %d",
134 shm_path, mmap_size, global ? "true" : "false",
135 wait_shm_fd);
136
137end:
138 (void) umask(old_mode);
139 return wait_shm_fd;
140
141error:
142 DBG("Failing to get the wait shm fd");
143 if (wait_shm_fd >= 0) {
144 if (close(wait_shm_fd)) {
145 PERROR("Failed to close wait shm file descriptor during error handling");
146 }
147 }
148
149 wait_shm_fd = -1;
150 goto end;
151}
152
153/*
154 * Return the wait shm mmap for UST application notification. The global
155 * variable is used to indicate if the the session daemon is global
156 * (root:tracing) or running with an unprivileged user.
157 *
158 * This returned value is used by futex_wait_update() in futex.c to WAKE all
159 * waiters which are UST application waiting for a session daemon.
160 */
161char *shm_ust_get_mmap(char *shm_path, int global)
162{
163 size_t mmap_size;
164 int wait_shm_fd, ret;
165 char *wait_shm_mmap;
166 long sys_page_size;
167
168 LTTNG_ASSERT(shm_path);
169
170 sys_page_size = sysconf(_SC_PAGE_SIZE);
171 if (sys_page_size < 0) {
172 PERROR("Failed to get PAGE_SIZE of system");
173 goto error;
174 }
175 mmap_size = sys_page_size;
176
177 wait_shm_fd = get_wait_shm(shm_path, mmap_size, global);
178 if (wait_shm_fd < 0) {
179 goto error;
180 }
181
182 wait_shm_mmap = (char *) mmap(NULL, mmap_size, PROT_WRITE | PROT_READ,
183 MAP_SHARED, wait_shm_fd, 0);
184
185 /* close shm fd immediately after taking the mmap reference */
186 ret = close(wait_shm_fd);
187 if (ret) {
188 PERROR("Failed to close \"wait\" shared memory object file descriptor: fd = %d",
189 wait_shm_fd);
190 }
191
192 if (wait_shm_mmap == MAP_FAILED) {
193 DBG("Failed to mmap the \"wait\" shareed memory object (can be caused by race with ust): path = '%s', global = %s",
194 shm_path, global ? "true" : "false");
195 goto error;
196 }
197
198 return wait_shm_mmap;
199
200error:
201 return NULL;
202}
203
204/*
205 * shm_create_anonymous is never called concurrently within a process.
206 */
207int shm_create_anonymous(const char *owner_name)
208{
209 char tmp_name[NAME_MAX];
210 int shmfd, ret;
211
212 ret = snprintf(tmp_name, NAME_MAX, "/shm-%s-%d", owner_name, getpid());
213 if (ret < 0) {
214 PERROR("Failed to format shm path: owner_name = '%s', pid = %d",
215 owner_name, getpid());
216 return -1;
217 }
218
219 /*
220 * Allocate shm, and immediately unlink its shm oject, keeping only the
221 * file descriptor as a reference to the object.
222 */
223 shmfd = shm_open(tmp_name, O_CREAT | O_EXCL | O_RDWR, 0700);
224 if (shmfd < 0) {
225 PERROR("Failed to open shared memory object: path = '%s'", tmp_name);
226 goto error_shm_open;
227 }
228
229 ret = shm_unlink(tmp_name);
230 if (ret < 0 && errno != ENOENT) {
231 PERROR("Failed to unlink shared memory object: path = '%s'",
232 tmp_name);
233 goto error_shm_release;
234 }
235
236 return shmfd;
237
238error_shm_release:
239 ret = close(shmfd);
240 if (ret) {
241 PERROR("Failed to close shared memory object file descriptor: fd = %d, path = '%s'",
242 shmfd, tmp_name);
243 }
244error_shm_open:
245 return -1;
246}
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