2 * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca>
3 * Copyright (C) 2011 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
5 * SPDX-License-Identifier: GPL-2.0-only
14 #include <sys/types.h>
19 #include <common/error.h>
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
31 static int get_wait_shm(char *shm_path
, size_t mmap_size
, int global
)
38 /* Default permissions */
39 mode
= S_IRUSR
| S_IWUSR
| S_IRGRP
| S_IWGRP
;
42 * Change owner of the shm path.
46 * If global session daemon, any application can
47 * register. Make it initially writeable so applications
48 * registering concurrently can do ftruncate() by
51 mode
|= S_IROTH
| S_IWOTH
;
55 * We're alone in a child process, so we can modify the process-wide
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.
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.
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.
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
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
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
);
97 if (wait_shm_fd
< 0) {
98 PERROR("Failed to open wait shm at %s", shm_path
);
103 ret
= ftruncate(wait_shm_fd
, mmap_size
);
105 PERROR("ftruncate wait shm");
110 ret
= fchown(wait_shm_fd
, 0, 0);
116 * If global session daemon, any application can
117 * register so the shm needs to be set in read-only mode
121 ret
= fchmod(wait_shm_fd
, mode
);
127 ret
= fchown(wait_shm_fd
, getuid(), getgid());
134 DBG("Got the wait shm fd %d", wait_shm_fd
);
139 DBG("Failing to get the wait shm fd");
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.
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.
152 char *shm_ust_get_mmap(char *shm_path
, int global
)
155 int wait_shm_fd
, ret
;
161 sys_page_size
= sysconf(_SC_PAGE_SIZE
);
162 if (sys_page_size
< 0) {
163 PERROR("sysconf PAGE_SIZE");
166 mmap_size
= sys_page_size
;
168 wait_shm_fd
= get_wait_shm(shm_path
, mmap_size
, global
);
169 if (wait_shm_fd
< 0) {
173 wait_shm_mmap
= mmap(NULL
, mmap_size
, PROT_WRITE
| PROT_READ
,
174 MAP_SHARED
, wait_shm_fd
, 0);
176 /* close shm fd immediately after taking the mmap reference */
177 ret
= close(wait_shm_fd
);
179 PERROR("Error closing fd");
182 if (wait_shm_mmap
== MAP_FAILED
) {
183 DBG("mmap error (can be caused by race with ust).");
187 return wait_shm_mmap
;
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