| 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 | */ |
| 30 | static 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 | |
| 137 | end: |
| 138 | (void) umask(old_mode); |
| 139 | return wait_shm_fd; |
| 140 | |
| 141 | error: |
| 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 | */ |
| 161 | char *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 | |
| 200 | error: |
| 201 | return NULL; |
| 202 | } |
| 203 | |
| 204 | /* |
| 205 | * shm_create_anonymous is never called concurrently within a process. |
| 206 | */ |
| 207 | int 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 | |
| 238 | error_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 | } |
| 244 | error_shm_open: |
| 245 | return -1; |
| 246 | } |