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0fdd1e2c DG |
1 | /* |
2 | * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca> | |
3 | * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> | |
4 | * | |
d14d33bf AM |
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. | |
0fdd1e2c | 8 | * |
d14d33bf AM |
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. | |
0fdd1e2c | 13 | * |
d14d33bf AM |
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. | |
0fdd1e2c DG |
17 | */ |
18 | ||
6c1c0768 | 19 | #define _LGPL_SOURCE |
0fdd1e2c DG |
20 | #include <fcntl.h> |
21 | #include <limits.h> | |
22 | #include <sys/mman.h> | |
23 | #include <sys/stat.h> | |
24 | #include <sys/types.h> | |
25 | #include <sys/wait.h> | |
26 | #include <unistd.h> | |
27 | #include <urcu.h> | |
28 | ||
db758600 | 29 | #include <common/error.h> |
0fdd1e2c DG |
30 | |
31 | #include "shm.h" | |
32 | ||
33 | /* | |
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 | |
39 | * starts. | |
40 | */ | |
41 | static int get_wait_shm(char *shm_path, size_t mmap_size, int global) | |
42 | { | |
43 | int wait_shm_fd, ret; | |
0fdd1e2c DG |
44 | mode_t mode; |
45 | ||
0525e9ae DG |
46 | assert(shm_path); |
47 | ||
0fdd1e2c DG |
48 | /* Default permissions */ |
49 | mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP; | |
50 | ||
7972d619 DG |
51 | /* |
52 | * Change owner of the shm path. | |
53 | */ | |
0fdd1e2c | 54 | if (global) { |
0fdd1e2c | 55 | /* |
7972d619 DG |
56 | * If global session daemon, any application can |
57 | * register. Make it initially writeable so applications | |
58 | * registering concurrently can do ftruncate() by | |
59 | * themselves. | |
0fdd1e2c | 60 | */ |
7972d619 | 61 | mode |= S_IROTH | S_IWOTH; |
0fdd1e2c DG |
62 | } |
63 | ||
64 | /* | |
7d051034 DG |
65 | * We're alone in a child process, so we can modify the process-wide |
66 | * umask. | |
0fdd1e2c | 67 | */ |
7d051034 | 68 | umask(~mode); |
0fdd1e2c | 69 | |
7d051034 DG |
70 | /* |
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. | |
cf86ff2c JG |
73 | * |
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. | |
77 | * | |
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. | |
81 | * | |
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 | |
87 | * fail with EACCES. | |
88 | * | |
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 | |
95 | * a new named shm. | |
7d051034 DG |
96 | */ |
97 | wait_shm_fd = shm_open(shm_path, O_RDWR | O_CREAT, mode); | |
98 | if (wait_shm_fd < 0) { | |
cf86ff2c JG |
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); | |
106 | } | |
107 | if (wait_shm_fd < 0) { | |
108 | PERROR("Failed to open wait shm at %s", shm_path); | |
109 | goto error; | |
110 | } | |
7d051034 | 111 | } |
0fdd1e2c | 112 | |
7d051034 DG |
113 | ret = ftruncate(wait_shm_fd, mmap_size); |
114 | if (ret < 0) { | |
df0f840b | 115 | PERROR("ftruncate wait shm"); |
7d051034 DG |
116 | exit(EXIT_FAILURE); |
117 | } | |
0fdd1e2c | 118 | |
409a0c56 | 119 | #ifndef __FreeBSD__ |
7972d619 DG |
120 | if (global) { |
121 | ret = fchown(wait_shm_fd, 0, 0); | |
122 | if (ret < 0) { | |
123 | PERROR("fchown"); | |
124 | exit(EXIT_FAILURE); | |
125 | } | |
126 | /* | |
127 | * If global session daemon, any application can | |
128 | * register so the shm needs to be set in read-only mode | |
129 | * for others. | |
130 | */ | |
131 | mode &= ~S_IWOTH; | |
132 | ret = fchmod(wait_shm_fd, mode); | |
133 | if (ret < 0) { | |
134 | PERROR("fchmod"); | |
135 | exit(EXIT_FAILURE); | |
136 | } | |
137 | } else { | |
138 | ret = fchown(wait_shm_fd, getuid(), getgid()); | |
139 | if (ret < 0) { | |
140 | PERROR("fchown"); | |
141 | exit(EXIT_FAILURE); | |
142 | } | |
0fdd1e2c | 143 | } |
409a0c56 | 144 | #else |
7972d619 | 145 | #warning "FreeBSD does not support setting file mode on shm FD." |
409a0c56 | 146 | #endif |
0fdd1e2c | 147 | |
0fdd1e2c DG |
148 | DBG("Got the wait shm fd %d", wait_shm_fd); |
149 | ||
150 | return wait_shm_fd; | |
151 | ||
152 | error: | |
153 | DBG("Failing to get the wait shm fd"); | |
154 | ||
155 | return -1; | |
156 | } | |
157 | ||
158 | /* | |
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. | |
162 | * | |
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. | |
165 | */ | |
166 | char *shm_ust_get_mmap(char *shm_path, int global) | |
167 | { | |
6c699394 | 168 | size_t mmap_size; |
0fdd1e2c DG |
169 | int wait_shm_fd, ret; |
170 | char *wait_shm_mmap; | |
6c699394 | 171 | long sys_page_size; |
0fdd1e2c | 172 | |
0525e9ae DG |
173 | assert(shm_path); |
174 | ||
6c699394 DG |
175 | sys_page_size = sysconf(_SC_PAGE_SIZE); |
176 | if (sys_page_size < 0) { | |
177 | PERROR("sysconf PAGE_SIZE"); | |
178 | goto error; | |
179 | } | |
180 | mmap_size = sys_page_size; | |
181 | ||
0fdd1e2c DG |
182 | wait_shm_fd = get_wait_shm(shm_path, mmap_size, global); |
183 | if (wait_shm_fd < 0) { | |
184 | goto error; | |
185 | } | |
186 | ||
187 | wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ, | |
188 | MAP_SHARED, wait_shm_fd, 0); | |
7d051034 | 189 | |
0fdd1e2c DG |
190 | /* close shm fd immediately after taking the mmap reference */ |
191 | ret = close(wait_shm_fd); | |
192 | if (ret) { | |
df0f840b | 193 | PERROR("Error closing fd"); |
0fdd1e2c DG |
194 | } |
195 | ||
196 | if (wait_shm_mmap == MAP_FAILED) { | |
197 | DBG("mmap error (can be caused by race with ust)."); | |
198 | goto error; | |
199 | } | |
200 | ||
201 | return wait_shm_mmap; | |
202 | ||
203 | error: | |
204 | return NULL; | |
205 | } |