| 1 | /* |
| 2 | * Copyright (C) 2011 David Goulet <david.goulet@polymtl.ca> |
| 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.h> |
| 20 | |
| 21 | #include "shm.h" |
| 22 | |
| 23 | /* |
| 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 |
| 29 | * starts. |
| 30 | */ |
| 31 | static int get_wait_shm(char *shm_path, size_t mmap_size, int global) |
| 32 | { |
| 33 | int wait_shm_fd, ret; |
| 34 | mode_t mode; |
| 35 | |
| 36 | assert(shm_path); |
| 37 | |
| 38 | /* Default permissions */ |
| 39 | mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP; |
| 40 | |
| 41 | /* |
| 42 | * Change owner of the shm path. |
| 43 | */ |
| 44 | if (global) { |
| 45 | /* |
| 46 | * If global session daemon, any application can |
| 47 | * register. Make it initially writeable so applications |
| 48 | * registering concurrently can do ftruncate() by |
| 49 | * themselves. |
| 50 | */ |
| 51 | mode |= S_IROTH | S_IWOTH; |
| 52 | } |
| 53 | |
| 54 | /* |
| 55 | * We're alone in a child process, so we can modify the process-wide |
| 56 | * umask. |
| 57 | */ |
| 58 | umask(~mode); |
| 59 | |
| 60 | /* |
| 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. |
| 63 | * |
| 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. |
| 67 | * |
| 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. |
| 71 | * |
| 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 |
| 77 | * fail with EACCES. |
| 78 | * |
| 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 |
| 85 | * a new named shm. |
| 86 | */ |
| 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); |
| 96 | } |
| 97 | if (wait_shm_fd < 0) { |
| 98 | PERROR("Failed to open wait shm at %s", shm_path); |
| 99 | goto error; |
| 100 | } |
| 101 | } |
| 102 | |
| 103 | ret = ftruncate(wait_shm_fd, mmap_size); |
| 104 | if (ret < 0) { |
| 105 | PERROR("ftruncate wait shm"); |
| 106 | exit(EXIT_FAILURE); |
| 107 | } |
| 108 | |
| 109 | #ifndef __FreeBSD__ |
| 110 | if (global) { |
| 111 | ret = fchown(wait_shm_fd, 0, 0); |
| 112 | if (ret < 0) { |
| 113 | PERROR("fchown"); |
| 114 | exit(EXIT_FAILURE); |
| 115 | } |
| 116 | /* |
| 117 | * If global session daemon, any application can |
| 118 | * register so the shm needs to be set in read-only mode |
| 119 | * for others. |
| 120 | */ |
| 121 | mode &= ~S_IWOTH; |
| 122 | ret = fchmod(wait_shm_fd, mode); |
| 123 | if (ret < 0) { |
| 124 | PERROR("fchmod"); |
| 125 | exit(EXIT_FAILURE); |
| 126 | } |
| 127 | } else { |
| 128 | ret = fchown(wait_shm_fd, getuid(), getgid()); |
| 129 | if (ret < 0) { |
| 130 | PERROR("fchown"); |
| 131 | exit(EXIT_FAILURE); |
| 132 | } |
| 133 | } |
| 134 | #else |
| 135 | #warning "FreeBSD does not support setting file mode on shm FD." |
| 136 | #endif |
| 137 | |
| 138 | DBG("Got the wait shm fd %d", wait_shm_fd); |
| 139 | |
| 140 | return wait_shm_fd; |
| 141 | |
| 142 | error: |
| 143 | DBG("Failing to get the wait shm fd"); |
| 144 | |
| 145 | return -1; |
| 146 | } |
| 147 | |
| 148 | /* |
| 149 | * Return the wait shm mmap for UST application notification. The global |
| 150 | * variable is used to indicate if the the session daemon is global |
| 151 | * (root:tracing) or running with an unprivileged user. |
| 152 | * |
| 153 | * This returned value is used by futex_wait_update() in futex.c to WAKE all |
| 154 | * waiters which are UST application waiting for a session daemon. |
| 155 | */ |
| 156 | char *shm_ust_get_mmap(char *shm_path, int global) |
| 157 | { |
| 158 | size_t mmap_size; |
| 159 | int wait_shm_fd, ret; |
| 160 | char *wait_shm_mmap; |
| 161 | long sys_page_size; |
| 162 | |
| 163 | assert(shm_path); |
| 164 | |
| 165 | sys_page_size = sysconf(_SC_PAGE_SIZE); |
| 166 | if (sys_page_size < 0) { |
| 167 | PERROR("sysconf PAGE_SIZE"); |
| 168 | goto error; |
| 169 | } |
| 170 | mmap_size = sys_page_size; |
| 171 | |
| 172 | wait_shm_fd = get_wait_shm(shm_path, mmap_size, global); |
| 173 | if (wait_shm_fd < 0) { |
| 174 | goto error; |
| 175 | } |
| 176 | |
| 177 | wait_shm_mmap = mmap(NULL, mmap_size, PROT_WRITE | PROT_READ, |
| 178 | MAP_SHARED, wait_shm_fd, 0); |
| 179 | |
| 180 | /* close shm fd immediately after taking the mmap reference */ |
| 181 | ret = close(wait_shm_fd); |
| 182 | if (ret) { |
| 183 | PERROR("Error closing fd"); |
| 184 | } |
| 185 | |
| 186 | if (wait_shm_mmap == MAP_FAILED) { |
| 187 | DBG("mmap error (can be caused by race with ust)."); |
| 188 | goto error; |
| 189 | } |
| 190 | |
| 191 | return wait_shm_mmap; |
| 192 | |
| 193 | error: |
| 194 | return NULL; |
| 195 | } |