src/shared/pager.c

   1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
   2
   3 #include <errno.h>
   4 #include <stddef.h>
   5 #include <stdint.h>
   6 #include <stdio.h>
   7 #include <stdlib.h>
   8 #include <sys/prctl.h>
   9 #include <unistd.h>
  10
  11 #include "sd-login.h"
  12
  13 #include "copy.h"
  14 #include "env-util.h"
  15 #include "fd-util.h"
  16 #include "fileio.h"
  17 #include "io-util.h"
  18 #include "locale-util.h"
  19 #include "log.h"
  20 #include "macro.h"
  21 #include "pager.h"
  22 #include "process-util.h"
  23 #include "rlimit-util.h"
  24 #include "signal-util.h"
  25 #include "string-util.h"
  26 #include "strv.h"
  27 #include "terminal-util.h"
  28 #include "util.h"
  29
  30 static pid_t pager_pid = 0;
  31
  32 static int stored_stdout = -1;
  33 static int stored_stderr = -1;
  34 static bool stdout_redirected = false;
  35 static bool stderr_redirected = false;
  36
  37 _noreturn_ static void pager_fallback(void) {
  38         int r;
  39
  40         r = copy_bytes(STDIN_FILENO, STDOUT_FILENO, (uint64_t) -1, 0);
  41         if (r < 0) {
  42                 log_error_errno(r, "Internal pager failed: %m");
  43                 _exit(EXIT_FAILURE);
  44         }
  45
  46         _exit(EXIT_SUCCESS);
  47 }
  48
  49 static int no_quit_on_interrupt(int exe_name_fd, const char *less_opts) {
  50         _cleanup_fclose_ FILE *file = NULL;
  51         _cleanup_free_ char *line = NULL;
  52         int r;
  53
  54         assert(exe_name_fd >= 0);
  55         assert(less_opts);
  56
  57         /* This takes ownership of exe_name_fd */
  58         file = fdopen(exe_name_fd, "r");
  59         if (!file) {
  60                 safe_close(exe_name_fd);
  61                 return log_error_errno(errno, "Failed to create FILE object: %m");
  62         }
  63
  64         /* Find the last line */
  65         for (;;) {
  66                 _cleanup_free_ char *t = NULL;
  67
  68                 r = read_line(file, LONG_LINE_MAX, &t);
  69                 if (r < 0)
  70                         return log_error_errno(r, "Failed to read from socket: %m");
  71                 if (r == 0)
  72                         break;
  73
  74                 free_and_replace(line, t);
  75         }
  76
  77         /* We only treat "less" specially.
  78          * Return true whenever option K is *not* set. */
  79         r = streq_ptr(line, "less") && !strchr(less_opts, 'K');
  80
  81         log_debug("Pager executable is \"%s\", options \"%s\", quit_on_interrupt: %s",
  82                   strnull(line), less_opts, yes_no(!r));
  83         return r;
  84 }
  85
  86 int pager_open(PagerFlags flags) {
  87         _cleanup_close_pair_ int fd[2] = { -1, -1 }, exe_name_pipe[2] = { -1, -1 };
  88         _cleanup_strv_free_ char **pager_args = NULL;
  89         const char *pager, *less_opts;
  90         int r;
  91
  92         if (flags & PAGER_DISABLE)
  93                 return 0;
  94
  95         if (pager_pid > 0)
  96                 return 1;
  97
  98         if (terminal_is_dumb())
  99                 return 0;
 100
 101         if (!is_main_thread())
 102                 return log_error_errno(SYNTHETIC_ERRNO(EPERM), "Pager invoked from wrong thread.");
 103
 104         pager = getenv("SYSTEMD_PAGER");
 105         if (!pager)
 106                 pager = getenv("PAGER");
 107
 108         if (pager) {
 109                 pager_args = strv_split(pager, WHITESPACE);
 110                 if (!pager_args)
 111                         return log_oom();
 112
 113                 /* If the pager is explicitly turned off, honour it */
 114                 if (strv_isempty(pager_args) || strv_equal(pager_args, STRV_MAKE("cat")))
 115                         return 0;
 116         }
 117
 118         /* Determine and cache number of columns/lines before we spawn the pager so that we get the value from the
 119          * actual tty */
 120         (void) columns();
 121         (void) lines();
 122
 123         if (pipe2(fd, O_CLOEXEC) < 0)
 124                 return log_error_errno(errno, "Failed to create pager pipe: %m");
 125
 126         /* This is a pipe to feed the name of the executed pager binary into the parent */
 127         if (pipe2(exe_name_pipe, O_CLOEXEC) < 0)
 128                 return log_error_errno(errno, "Failed to create exe_name pipe: %m");
 129
 130         /* Initialize a good set of less options */
 131         less_opts = getenv("SYSTEMD_LESS");
 132         if (!less_opts)
 133                 less_opts = "FRSXMK";
 134         if (flags & PAGER_JUMP_TO_END)
 135                 less_opts = strjoina(less_opts, " +G");
 136
 137         /* We set SIGINT as PR_DEATHSIG signal here, to match the "K" parameter we set in $LESS, which enables SIGINT behaviour. */
 138         r = safe_fork("(pager)", FORK_RESET_SIGNALS|FORK_DEATHSIG_SIGINT|FORK_RLIMIT_NOFILE_SAFE|FORK_LOG, &pager_pid);
 139         if (r < 0)
 140                 return r;
 141         if (r == 0) {
 142                 const char *less_charset, *exe;
 143
 144                 /* In the child start the pager */
 145
 146                 if (dup2(fd[0], STDIN_FILENO) < 0) {
 147                         log_error_errno(errno, "Failed to duplicate file descriptor to STDIN: %m");
 148                         _exit(EXIT_FAILURE);
 149                 }
 150
 151                 safe_close_pair(fd);
 152
 153                 if (setenv("LESS", less_opts, 1) < 0) {
 154                         log_error_errno(errno, "Failed to set environment variable LESS: %m");
 155                         _exit(EXIT_FAILURE);
 156                 }
 157
 158                 /* Initialize a good charset for less. This is particularly important if we output UTF-8
 159                  * characters. */
 160                 less_charset = getenv("SYSTEMD_LESSCHARSET");
 161                 if (!less_charset && is_locale_utf8())
 162                         less_charset = "utf-8";
 163                 if (less_charset &&
 164                     setenv("LESSCHARSET", less_charset, 1) < 0) {
 165                         log_error_errno(errno, "Failed to set environment variable LESSCHARSET: %m");
 166                         _exit(EXIT_FAILURE);
 167                 }
 168
 169                 /* People might invoke us from sudo, don't needlessly allow less to be a way to shell out
 170                  * privileged stuff. If the user set $SYSTEMD_PAGERSECURE, trust their configuration of the
 171                  * pager. If they didn't, use secure mode when under euid is changed. If $SYSTEMD_PAGERSECURE
 172                  * wasn't explicitly set, and we autodetect the need for secure mode, only use the pager we
 173                  * know to be good. */
 174                 int use_secure_mode = getenv_bool_secure("SYSTEMD_PAGERSECURE");
 175                 bool trust_pager = use_secure_mode >= 0;
 176                 if (use_secure_mode == -ENXIO) {
 177                         uid_t uid;
 178
 179                         r = sd_pid_get_owner_uid(0, &uid);
 180                         if (r < 0)
 181                                 log_debug_errno(r, "sd_pid_get_owner_uid() failed, enabling pager secure mode: %m");
 182
 183                         use_secure_mode = r < 0 || uid != geteuid();
 184
 185                 } else if (use_secure_mode < 0) {
 186                         log_warning_errno(use_secure_mode, "Unable to parse $SYSTEMD_PAGERSECURE, assuming true: %m");
 187                         use_secure_mode = true;
 188                 }
 189
 190                 /* We generally always set variables used by less, even if we end up using a different pager.
 191                  * They shouldn't hurt in any case, and ideally other pagers would look at them too. */
 192                 r = set_unset_env("LESSSECURE", use_secure_mode ? "1" : NULL, true);
 193                 if (r < 0) {
 194                         log_error_errno(r, "Failed to adjust environment variable LESSSECURE: %m");
 195                         _exit(EXIT_FAILURE);
 196                 }
 197
 198                 if (trust_pager && pager_args) { /* The pager config might be set globally, and we cannot
 199                                                   * know if the user adjusted it to be appropriate for the
 200                                                   * secure mode. Thus, start the pager specified through
 201                                                   * envvars only when $SYSTEMD_PAGERSECURE was explicitly set
 202                                                   * as well. */
 203                         r = loop_write(exe_name_pipe[1], pager_args[0], strlen(pager_args[0]) + 1, false);
 204                         if (r < 0) {
 205                                 log_error_errno(r, "Failed to write pager name to socket: %m");
 206                                 _exit(EXIT_FAILURE);
 207                         }
 208
 209                         execvp(pager_args[0], pager_args);
 210                         log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_WARNING, errno,
 211                                        "Failed to execute '%s', using fallback pagers: %m", pager_args[0]);
 212                 }
 213
 214                 /* Debian's alternatives command for pagers is called 'pager'. Note that we do not call
 215                  * sensible-pagers here, since that is just a shell script that implements a logic that is
 216                  * similar to this one anyway, but is Debian-specific. */
 217                 FOREACH_STRING(exe, "pager", "less", "more") {
 218                         /* Only less implements secure mode right now. */
 219                         if (use_secure_mode && !streq(exe, "less"))
 220                                 continue;
 221
 222                         r = loop_write(exe_name_pipe[1], exe, strlen(exe) + 1, false);
 223                         if (r  < 0) {
 224                                 log_error_errno(r, "Failed to write pager name to socket: %m");
 225                                 _exit(EXIT_FAILURE);
 226                         }
 227                         execlp(exe, exe, NULL);
 228                         log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_WARNING, errno,
 229                                        "Failed to execute '%s', using next fallback pager: %m", exe);
 230                 }
 231
 232                 /* Our builtin is also very secure. */
 233                 r = loop_write(exe_name_pipe[1], "(built-in)", strlen("(built-in)") + 1, false);
 234                 if (r < 0) {
 235                         log_error_errno(r, "Failed to write pager name to socket: %m");
 236                         _exit(EXIT_FAILURE);
 237                 }
 238                 /* Close pipe to signal the parent to start sending data */
 239                 safe_close_pair(exe_name_pipe);
 240                 pager_fallback();
 241                 /* not reached */
 242         }
 243
 244         /* Return in the parent */
 245         stored_stdout = fcntl(STDOUT_FILENO, F_DUPFD_CLOEXEC, 3);
 246         if (dup2(fd[1], STDOUT_FILENO) < 0) {
 247                 stored_stdout = safe_close(stored_stdout);
 248                 return log_error_errno(errno, "Failed to duplicate pager pipe: %m");
 249         }
 250         stdout_redirected = true;
 251
 252         stored_stderr = fcntl(STDERR_FILENO, F_DUPFD_CLOEXEC, 3);
 253         if (dup2(fd[1], STDERR_FILENO) < 0) {
 254                 stored_stderr = safe_close(stored_stderr);
 255                 return log_error_errno(errno, "Failed to duplicate pager pipe: %m");
 256         }
 257         stderr_redirected = true;
 258
 259         exe_name_pipe[1] = safe_close(exe_name_pipe[1]);
 260
 261         r = no_quit_on_interrupt(TAKE_FD(exe_name_pipe[0]), less_opts);
 262         if (r < 0)
 263                 return r;
 264         if (r > 0)
 265                 (void) ignore_signals(SIGINT, -1);
 266
 267         return 1;
 268 }
 269
 270 void pager_close(void) {
 271
 272         if (pager_pid <= 0)
 273                 return;
 274
 275         /* Inform pager that we are done */
 276         (void) fflush(stdout);
 277         if (stdout_redirected)
 278                 if (stored_stdout < 0 || dup2(stored_stdout, STDOUT_FILENO) < 0)
 279                         (void) close(STDOUT_FILENO);
 280         stored_stdout = safe_close(stored_stdout);
 281         (void) fflush(stderr);
 282         if (stderr_redirected)
 283                 if (stored_stderr < 0 || dup2(stored_stderr, STDERR_FILENO) < 0)
 284                         (void) close(STDERR_FILENO);
 285         stored_stderr = safe_close(stored_stderr);
 286         stdout_redirected = stderr_redirected = false;
 287
 288         (void) kill(pager_pid, SIGCONT);
 289         (void) wait_for_terminate(pager_pid, NULL);
 290         pager_pid = 0;
 291 }
 292
 293 bool pager_have(void) {
 294         return pager_pid > 0;
 295 }
 296
 297 int show_man_page(const char *desc, bool null_stdio) {
 298         const char *args[4] = { "man", NULL, NULL, NULL };
 299         char *e = NULL;
 300         pid_t pid;
 301         size_t k;
 302         int r;
 303
 304         k = strlen(desc);
 305
 306         if (desc[k-1] == ')')
 307                 e = strrchr(desc, '(');
 308
 309         if (e) {
 310                 char *page = NULL, *section = NULL;
 311
 312                 page = strndupa(desc, e - desc);
 313                 section = strndupa(e + 1, desc + k - e - 2);
 314
 315                 args[1] = section;
 316                 args[2] = page;
 317         } else
 318                 args[1] = desc;
 319
 320         r = safe_fork("(man)", FORK_RESET_SIGNALS|FORK_DEATHSIG|(null_stdio ? FORK_NULL_STDIO : 0)|FORK_RLIMIT_NOFILE_SAFE|FORK_LOG, &pid);
 321         if (r < 0)
 322                 return r;
 323         if (r == 0) {
 324                 /* Child */
 325                 execvp(args[0], (char**) args);
 326                 log_error_errno(errno, "Failed to execute man: %m");
 327                 _exit(EXIT_FAILURE);
 328         }
 329
 330         return wait_for_terminate_and_check(NULL, pid, 0);
 331 }