2 * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
3 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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.
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.
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.
24 #include <sys/types.h>
29 #include <common/error.h>
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
41 static int get_wait_shm(char *shm_path
, size_t mmap_size
, int global
)
48 /* Default permissions */
49 mode
= S_IRUSR
| S_IWUSR
| S_IRGRP
| S_IWGRP
;
52 * Change owner of the shm path.
56 * If global session daemon, any application can
57 * register. Make it initially writeable so applications
58 * registering concurrently can do ftruncate() by
61 mode
|= S_IROTH
| S_IWOTH
;
65 * We're alone in a child process, so we can modify the process-wide
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.
74 * A sysctl, fs.protected_regular may prevent the session daemon from
75 * opening a previously created shm when the O_CREAT flag is provided.
76 * Systemd enables this ABI-breaking change by default since v241.
78 * First, attempt to use the create-or-open semantic that is
79 * desired here. If this fails with EACCES, work around this broken
80 * behaviour and attempt to open the shm without the O_CREAT flag.
82 * The two attempts are made in this order since applications are
83 * expected to race with the session daemon to create this shm.
84 * Attempting an shm_open() without the O_CREAT flag first could fail
85 * because the file doesn't exist. It could then be created by an
86 * application, which would cause a second try with the O_CREAT flag to
89 * Note that this introduces a new failure mode where a user could
90 * launch an application (creating the shm) and unlink the shm while
91 * the session daemon is launching, causing the second attempt
92 * to fail. This is not recovered-from as unlinking the shm will
93 * prevent userspace tracing from succeeding anyhow: the sessiond would
94 * use a now-unlinked shm, while the next application would create
97 wait_shm_fd
= shm_open(shm_path
, O_RDWR
| O_CREAT
, mode
);
98 if (wait_shm_fd
< 0) {
99 if (errno
== EACCES
) {
100 /* Work around sysctl fs.protected_regular. */
101 DBG("shm_open of %s returned EACCES, this may be caused "
102 "by the fs.protected_regular sysctl. "
103 "Attempting to open the shm without "
104 "creating it.", shm_path
);
105 wait_shm_fd
= shm_open(shm_path
, O_RDWR
, mode
);
107 if (wait_shm_fd
< 0) {
108 PERROR("Failed to open wait shm at %s", shm_path
);
113 ret
= ftruncate(wait_shm_fd
, mmap_size
);
115 PERROR("ftruncate wait shm");
121 ret
= fchown(wait_shm_fd
, 0, 0);
127 * If global session daemon, any application can
128 * register so the shm needs to be set in read-only mode
132 ret
= fchmod(wait_shm_fd
, mode
);
138 ret
= fchown(wait_shm_fd
, getuid(), getgid());
145 #warning "FreeBSD does not support setting file mode on shm FD."
148 DBG("Got the wait shm fd %d", wait_shm_fd
);
153 DBG("Failing to get the wait shm fd");
159 * Return the wait shm mmap for UST application notification. The global
160 * variable is used to indicate if the the session daemon is global
161 * (root:tracing) or running with an unprivileged user.
163 * This returned value is used by futex_wait_update() in futex.c to WAKE all
164 * waiters which are UST application waiting for a session daemon.
166 char *shm_ust_get_mmap(char *shm_path
, int global
)
169 int wait_shm_fd
, ret
;
175 sys_page_size
= sysconf(_SC_PAGE_SIZE
);
176 if (sys_page_size
< 0) {
177 PERROR("sysconf PAGE_SIZE");
180 mmap_size
= sys_page_size
;
182 wait_shm_fd
= get_wait_shm(shm_path
, mmap_size
, global
);
183 if (wait_shm_fd
< 0) {
187 wait_shm_mmap
= mmap(NULL
, mmap_size
, PROT_WRITE
| PROT_READ
,
188 MAP_SHARED
, wait_shm_fd
, 0);
190 /* close shm fd immediately after taking the mmap reference */
191 ret
= close(wait_shm_fd
);
193 PERROR("Error closing fd");
196 if (wait_shm_mmap
== MAP_FAILED
) {
197 DBG("mmap error (can be caused by race with ust).");
201 return wait_shm_mmap
;
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