gdb/fork-child.c

   1 /* Fork a Unix child process, and set up to debug it, for GDB.
   2
   3    Copyright (C) 1990-2016 Free Software Foundation, Inc.
   4
   5    Contributed by Cygnus Support.
   6
   7    This file is part of GDB.
   8
   9    This program is free software; you can redistribute it and/or modify
  10    it under the terms of the GNU General Public License as published by
  11    the Free Software Foundation; either version 3 of the License, or
  12    (at your option) any later version.
  13
  14    This program is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  17    GNU General Public License for more details.
  18
  19    You should have received a copy of the GNU General Public License
  20    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  21
  22 #include "defs.h"
  23 #include "inferior.h"
  24 #include "terminal.h"
  25 #include "target.h"
  26 #include "gdb_wait.h"
  27 #include "gdb_vfork.h"
  28 #include "gdbcore.h"
  29 #include "gdbthread.h"
  30 #include "command.h" /* for dont_repeat () */
  31 #include "gdbcmd.h"
  32 #include "solib.h"
  33 #include "filestuff.h"
  34 #include "top.h"
  35 #include "signals-state-save-restore.h"
  36 #include <signal.h>
  37
  38 /* This just gets used as a default if we can't find SHELL.  */
  39 #define SHELL_FILE "/bin/sh"
  40
  41 extern char **environ;
  42
  43 static char *exec_wrapper;
  44
  45 /* Break up SCRATCH into an argument vector suitable for passing to
  46    execvp and store it in ARGV.  E.g., on "run a b c d" this routine
  47    would get as input the string "a b c d", and as output it would
  48    fill in ARGV with the four arguments "a", "b", "c", "d".  */
  49
  50 static void
  51 breakup_args (char *scratch, char **argv)
  52 {
  53   char *cp = scratch, *tmp;
  54
  55   for (;;)
  56     {
  57       /* Scan past leading separators */
  58       while (*cp == ' ' || *cp == '\t' || *cp == '\n')
  59         cp++;
  60
  61       /* Break if at end of string.  */
  62       if (*cp == '\0')
  63         break;
  64
  65       /* Take an arg.  */
  66       *argv++ = cp;
  67
  68       /* Scan for next arg separator.  */
  69       tmp = strchr (cp, ' ');
  70       if (tmp == NULL)
  71         tmp = strchr (cp, '\t');
  72       if (tmp == NULL)
  73         tmp = strchr (cp, '\n');
  74
  75       /* No separators => end of string => break.  */
  76       if (tmp == NULL)
  77         break;
  78       cp = tmp;
  79
  80       /* Replace the separator with a terminator.  */
  81       *cp++ = '\0';
  82     }
  83
  84   /* Null-terminate the vector.  */
  85   *argv = NULL;
  86 }
  87
  88 /* When executing a command under the given shell, return non-zero if
  89    the '!' character should be escaped when embedded in a quoted
  90    command-line argument.  */
  91
  92 static int
  93 escape_bang_in_quoted_argument (const char *shell_file)
  94 {
  95   const int shell_file_len = strlen (shell_file);
  96
  97   /* Bang should be escaped only in C Shells.  For now, simply check
  98      that the shell name ends with 'csh', which covers at least csh
  99      and tcsh.  This should be good enough for now.  */
 100
 101   if (shell_file_len < 3)
 102     return 0;
 103
 104   if (shell_file[shell_file_len - 3] == 'c'
 105       && shell_file[shell_file_len - 2] == 's'
 106       && shell_file[shell_file_len - 1] == 'h')
 107     return 1;
 108
 109   return 0;
 110 }
 111
 112 /* Start an inferior Unix child process and sets inferior_ptid to its
 113    pid.  EXEC_FILE is the file to run.  ALLARGS is a string containing
 114    the arguments to the program.  ENV is the environment vector to
 115    pass.  SHELL_FILE is the shell file, or NULL if we should pick
 116    one.  EXEC_FUN is the exec(2) function to use, or NULL for the default
 117    one.  */
 118
 119 /* This function is NOT reentrant.  Some of the variables have been
 120    made static to ensure that they survive the vfork call.  */
 121
 122 int
 123 fork_inferior (char *exec_file_arg, char *allargs, char **env,
 124                void (*traceme_fun) (void), void (*init_trace_fun) (int),
 125                void (*pre_trace_fun) (void), char *shell_file_arg,
 126                void (*exec_fun)(const char *file, char * const *argv,
 127                                 char * const *env))
 128 {
 129   int pid;
 130   static char default_shell_file[] = SHELL_FILE;
 131   /* Set debug_fork then attach to the child while it sleeps, to debug.  */
 132   static int debug_fork = 0;
 133   /* This is set to the result of setpgrp, which if vforked, will be visible
 134      to you in the parent process.  It's only used by humans for debugging.  */
 135   static int debug_setpgrp = 657473;
 136   static char *shell_file;
 137   static char *exec_file;
 138   char **save_our_env;
 139   int shell = 0;
 140   static char **argv;
 141   const char *inferior_io_terminal = get_inferior_io_terminal ();
 142   struct inferior *inf;
 143   int i;
 144   int save_errno;
 145   struct ui *save_ui;
 146
 147   /* If no exec file handed to us, get it from the exec-file command
 148      -- with a good, common error message if none is specified.  */
 149   exec_file = exec_file_arg;
 150   if (exec_file == 0)
 151     exec_file = get_exec_file (1);
 152
 153   /* 'startup_with_shell' is declared in inferior.h and bound to the
 154      "set startup-with-shell" option.  If 0, we'll just do a
 155      fork/exec, no shell, so don't bother figuring out what shell.  */
 156   shell_file = shell_file_arg;
 157   if (startup_with_shell)
 158     {
 159       /* Figure out what shell to start up the user program under.  */
 160       if (shell_file == NULL)
 161         shell_file = getenv ("SHELL");
 162       if (shell_file == NULL)
 163         shell_file = default_shell_file;
 164       shell = 1;
 165     }
 166
 167   if (!shell)
 168     {
 169       /* We're going to call execvp.  Create argument vector.
 170          Calculate an upper bound on the length of the vector by
 171          assuming that every other character is a separate
 172          argument.  */
 173       int argc = (strlen (allargs) + 1) / 2 + 2;
 174
 175       argv = XALLOCAVEC (char *, argc);
 176       argv[0] = exec_file;
 177       breakup_args (allargs, &argv[1]);
 178     }
 179   else
 180     {
 181       /* We're going to call a shell.  */
 182       char *shell_command;
 183       int len;
 184       char *p;
 185       int need_to_quote;
 186       const int escape_bang = escape_bang_in_quoted_argument (shell_file);
 187
 188       /* Multiplying the length of exec_file by 4 is to account for the
 189          fact that it may expand when quoted; it is a worst-case number
 190          based on every character being '.  */
 191       len = 5 + 4 * strlen (exec_file) + 1 + strlen (allargs) + 1 + /*slop */ 12;
 192       if (exec_wrapper)
 193         len += strlen (exec_wrapper) + 1;
 194
 195       shell_command = (char *) alloca (len);
 196       shell_command[0] = '\0';
 197
 198       strcat (shell_command, "exec ");
 199
 200       /* Add any exec wrapper.  That may be a program name with arguments, so
 201          the user must handle quoting.  */
 202       if (exec_wrapper)
 203         {
 204           strcat (shell_command, exec_wrapper);
 205           strcat (shell_command, " ");
 206         }
 207
 208       /* Now add exec_file, quoting as necessary.  */
 209
 210       /* Quoting in this style is said to work with all shells.  But
 211          csh on IRIX 4.0.1 can't deal with it.  So we only quote it if
 212          we need to.  */
 213       p = exec_file;
 214       while (1)
 215         {
 216           switch (*p)
 217             {
 218             case '\'':
 219             case '!':
 220             case '"':
 221             case '(':
 222             case ')':
 223             case '$':
 224             case '&':
 225             case ';':
 226             case '<':
 227             case '>':
 228             case ' ':
 229             case '\n':
 230             case '\t':
 231               need_to_quote = 1;
 232               goto end_scan;
 233
 234             case '\0':
 235               need_to_quote = 0;
 236               goto end_scan;
 237
 238             default:
 239               break;
 240             }
 241           ++p;
 242         }
 243     end_scan:
 244       if (need_to_quote)
 245         {
 246           strcat (shell_command, "'");
 247           for (p = exec_file; *p != '\0'; ++p)
 248             {
 249               if (*p == '\'')
 250                 strcat (shell_command, "'\\''");
 251               else if (*p == '!' && escape_bang)
 252                 strcat (shell_command, "\\!");
 253               else
 254                 strncat (shell_command, p, 1);
 255             }
 256           strcat (shell_command, "'");
 257         }
 258       else
 259         strcat (shell_command, exec_file);
 260
 261       strcat (shell_command, " ");
 262       strcat (shell_command, allargs);
 263
 264       /* If we decided above to start up with a shell, we exec the
 265          shell, "-c" says to interpret the next arg as a shell command
 266          to execute, and this command is "exec <target-program>
 267          <args>".  */
 268       argv = (char **) alloca (4 * sizeof (char *));
 269       argv[0] = shell_file;
 270       argv[1] = "-c";
 271       argv[2] = shell_command;
 272       argv[3] = (char *) 0;
 273     }
 274
 275   /* Retain a copy of our environment variables, since the child will
 276      replace the value of environ and if we're vforked, we have to
 277      restore it.  */
 278   save_our_env = environ;
 279
 280   /* Likewise the current UI.  */
 281   save_ui = current_ui;
 282
 283   /* Tell the terminal handling subsystem what tty we plan to run on;
 284      it will just record the information for later.  */
 285   new_tty_prefork (inferior_io_terminal);
 286
 287   /* It is generally good practice to flush any possible pending stdio
 288      output prior to doing a fork, to avoid the possibility of both
 289      the parent and child flushing the same data after the fork.  */
 290   gdb_flush (main_ui->m_gdb_stdout);
 291   gdb_flush (main_ui->m_gdb_stderr);
 292
 293   /* If there's any initialization of the target layers that must
 294      happen to prepare to handle the child we're about fork, do it
 295      now...  */
 296   if (pre_trace_fun != NULL)
 297     (*pre_trace_fun) ();
 298
 299   /* Create the child process.  Since the child process is going to
 300      exec(3) shortly afterwards, try to reduce the overhead by
 301      calling vfork(2).  However, if PRE_TRACE_FUN is non-null, it's
 302      likely that this optimization won't work since there's too much
 303      work to do between the vfork(2) and the exec(3).  This is known
 304      to be the case on ttrace(2)-based HP-UX, where some handshaking
 305      between parent and child needs to happen between fork(2) and
 306      exec(2).  However, since the parent is suspended in the vforked
 307      state, this doesn't work.  Also note that the vfork(2) call might
 308      actually be a call to fork(2) due to the fact that autoconf will
 309      ``#define vfork fork'' on certain platforms.  */
 310   if (pre_trace_fun || debug_fork)
 311     pid = fork ();
 312   else
 313     pid = vfork ();
 314
 315   if (pid < 0)
 316     perror_with_name (("vfork"));
 317
 318   if (pid == 0)
 319     {
 320       /* Switch to the main UI, so that gdb_std{in/out/err} in the
 321          child are mapped to std{in/out/err}.  This makes it possible
 322          to use fprintf_unfiltered/warning/error/etc. in the child
 323          from here on.  */
 324       current_ui = main_ui;
 325
 326       /* Close all file descriptors except those that gdb inherited
 327          (usually 0/1/2), so they don't leak to the inferior.  Note
 328          that this closes the file descriptors of all secondary
 329          UIs.  */
 330       close_most_fds ();
 331
 332       if (debug_fork)
 333         sleep (debug_fork);
 334
 335       /* Create a new session for the inferior process, if necessary.
 336          It will also place the inferior in a separate process group.  */
 337       if (create_tty_session () <= 0)
 338         {
 339           /* No session was created, but we still want to run the inferior
 340              in a separate process group.  */
 341           debug_setpgrp = gdb_setpgid ();
 342           if (debug_setpgrp == -1)
 343             perror (_("setpgrp failed in child"));
 344         }
 345
 346       /* Ask the tty subsystem to switch to the one we specified
 347          earlier (or to share the current terminal, if none was
 348          specified).  */
 349       new_tty ();
 350
 351       /* Changing the signal handlers for the inferior after
 352          a vfork can also change them for the superior, so we don't mess
 353          with signals here.  See comments in
 354          initialize_signals for how we get the right signal handlers
 355          for the inferior.  */
 356
 357       /* "Trace me, Dr. Memory!"  */
 358       (*traceme_fun) ();
 359
 360       /* The call above set this process (the "child") as debuggable
 361         by the original gdb process (the "parent").  Since processes
 362         (unlike people) can have only one parent, if you are debugging
 363         gdb itself (and your debugger is thus _already_ the
 364         controller/parent for this child), code from here on out is
 365         undebuggable.  Indeed, you probably got an error message
 366         saying "not parent".  Sorry; you'll have to use print
 367         statements!  */
 368
 369       restore_original_signals_state ();
 370
 371       /* There is no execlpe call, so we have to set the environment
 372          for our child in the global variable.  If we've vforked, this
 373          clobbers the parent, but environ is restored a few lines down
 374          in the parent.  By the way, yes we do need to look down the
 375          path to find $SHELL.  Rich Pixley says so, and I agree.  */
 376       environ = env;
 377
 378       if (exec_fun != NULL)
 379         (*exec_fun) (argv[0], argv, env);
 380       else
 381         execvp (argv[0], argv);
 382
 383       /* If we get here, it's an error.  */
 384       save_errno = errno;
 385       fprintf_unfiltered (gdb_stderr, "Cannot exec %s", argv[0]);
 386       for (i = 1; argv[i] != NULL; i++)
 387         fprintf_unfiltered (gdb_stderr, " %s", argv[i]);
 388       fprintf_unfiltered (gdb_stderr, ".\n");
 389       fprintf_unfiltered (gdb_stderr, "Error: %s\n",
 390                           safe_strerror (save_errno));
 391       gdb_flush (gdb_stderr);
 392       _exit (0177);
 393     }
 394
 395   /* Restore our environment in case a vforked child clob'd it.  */
 396   environ = save_our_env;
 397
 398   /* Likewise the current UI.  */
 399   current_ui = save_ui;
 400
 401   if (!have_inferiors ())
 402     init_thread_list ();
 403
 404   inf = current_inferior ();
 405
 406   inferior_appeared (inf, pid);
 407
 408   /* Needed for wait_for_inferior stuff below.  */
 409   inferior_ptid = pid_to_ptid (pid);
 410
 411   new_tty_postfork ();
 412
 413   /* We have something that executes now.  We'll be running through
 414      the shell at this point, but the pid shouldn't change.  Targets
 415      supporting MT should fill this task's ptid with more data as soon
 416      as they can.  */
 417   add_thread_silent (inferior_ptid);
 418
 419   /* Now that we have a child process, make it our target, and
 420      initialize anything target-vector-specific that needs
 421      initializing.  */
 422   if (init_trace_fun)
 423     (*init_trace_fun) (pid);
 424
 425   /* We are now in the child process of interest, having exec'd the
 426      correct program, and are poised at the first instruction of the
 427      new program.  */
 428   return pid;
 429 }
 430
 431 /* Accept NTRAPS traps from the inferior.  */
 432
 433 void
 434 startup_inferior (int ntraps)
 435 {
 436   int pending_execs = ntraps;
 437   int terminal_initted = 0;
 438   ptid_t resume_ptid;
 439
 440   if (startup_with_shell)
 441     {
 442       /* One trap extra for exec'ing the shell.  */
 443       pending_execs++;
 444     }
 445
 446   if (target_supports_multi_process ())
 447     resume_ptid = pid_to_ptid (ptid_get_pid (inferior_ptid));
 448   else
 449     resume_ptid = minus_one_ptid;
 450
 451   /* The process was started by the fork that created it, but it will
 452      have stopped one instruction after execing the shell.  Here we
 453      must get it up to actual execution of the real program.  */
 454
 455   if (exec_wrapper)
 456     pending_execs++;
 457
 458   while (1)
 459     {
 460       enum gdb_signal resume_signal = GDB_SIGNAL_0;
 461       ptid_t event_ptid;
 462
 463       struct target_waitstatus ws;
 464       memset (&ws, 0, sizeof (ws));
 465       event_ptid = target_wait (resume_ptid, &ws, 0);
 466
 467       if (ws.kind == TARGET_WAITKIND_IGNORE)
 468         /* The inferior didn't really stop, keep waiting.  */
 469         continue;
 470
 471       switch (ws.kind)
 472         {
 473           case TARGET_WAITKIND_SPURIOUS:
 474           case TARGET_WAITKIND_LOADED:
 475           case TARGET_WAITKIND_FORKED:
 476           case TARGET_WAITKIND_VFORKED:
 477           case TARGET_WAITKIND_SYSCALL_ENTRY:
 478           case TARGET_WAITKIND_SYSCALL_RETURN:
 479             /* Ignore gracefully during startup of the inferior.  */
 480             switch_to_thread (event_ptid);
 481             break;
 482
 483           case TARGET_WAITKIND_SIGNALLED:
 484             target_terminal_ours ();
 485             target_mourn_inferior ();
 486             error (_("During startup program terminated with signal %s, %s."),
 487                    gdb_signal_to_name (ws.value.sig),
 488                    gdb_signal_to_string (ws.value.sig));
 489             return;
 490
 491           case TARGET_WAITKIND_EXITED:
 492             target_terminal_ours ();
 493             target_mourn_inferior ();
 494             if (ws.value.integer)
 495               error (_("During startup program exited with code %d."),
 496                      ws.value.integer);
 497             else
 498               error (_("During startup program exited normally."));
 499             return;
 500
 501           case TARGET_WAITKIND_EXECD:
 502             /* Handle EXEC signals as if they were SIGTRAP signals.  */
 503             xfree (ws.value.execd_pathname);
 504             resume_signal = GDB_SIGNAL_TRAP;
 505             switch_to_thread (event_ptid);
 506             break;
 507
 508           case TARGET_WAITKIND_STOPPED:
 509             resume_signal = ws.value.sig;
 510             switch_to_thread (event_ptid);
 511             break;
 512         }
 513
 514       if (resume_signal != GDB_SIGNAL_TRAP)
 515         {
 516           /* Let shell child handle its own signals in its own way.  */
 517           target_continue (resume_ptid, resume_signal);
 518         }
 519       else
 520         {
 521           /* We handle SIGTRAP, however; it means child did an exec.  */
 522           if (!terminal_initted)
 523             {
 524               /* Now that the child has exec'd we know it has already
 525                  set its process group.  On POSIX systems, tcsetpgrp
 526                  will fail with EPERM if we try it before the child's
 527                  setpgid.  */
 528
 529               /* Set up the "saved terminal modes" of the inferior
 530                  based on what modes we are starting it with.  */
 531               target_terminal_init ();
 532
 533               /* Install inferior's terminal modes.  */
 534               target_terminal_inferior ();
 535
 536               terminal_initted = 1;
 537             }
 538
 539           if (--pending_execs == 0)
 540             break;
 541
 542           /* Just make it go on.  */
 543           target_continue_no_signal (resume_ptid);
 544         }
 545     }
 546
 547   /* Mark all threads non-executing.  */
 548   set_executing (resume_ptid, 0);
 549 }
 550
 551 /* Implement the "unset exec-wrapper" command.  */
 552
 553 static void
 554 unset_exec_wrapper_command (char *args, int from_tty)
 555 {
 556   xfree (exec_wrapper);
 557   exec_wrapper = NULL;
 558 }
 559
 560 static void
 561 show_startup_with_shell (struct ui_file *file, int from_tty,
 562                          struct cmd_list_element *c, const char *value)
 563 {
 564   fprintf_filtered (file,
 565                     _("Use of shell to start subprocesses is %s.\n"),
 566                     value);
 567 }
 568
 569 /* Provide a prototype to silence -Wmissing-prototypes.  */
 570 extern initialize_file_ftype _initialize_fork_child;
 571
 572 void
 573 _initialize_fork_child (void)
 574 {
 575   add_setshow_filename_cmd ("exec-wrapper", class_run, &exec_wrapper, _("\
 576 Set a wrapper for running programs.\n\
 577 The wrapper prepares the system and environment for the new program."),
 578                             _("\
 579 Show the wrapper for running programs."), NULL,
 580                             NULL, NULL,
 581                             &setlist, &showlist);
 582
 583   add_cmd ("exec-wrapper", class_run, unset_exec_wrapper_command,
 584            _("Disable use of an execution wrapper."),
 585            &unsetlist);
 586
 587   add_setshow_boolean_cmd ("startup-with-shell", class_support,
 588                            &startup_with_shell, _("\
 589 Set use of shell to start subprocesses.  The default is on."), _("\
 590 Show use of shell to start subprocesses."), NULL,
 591                            NULL,
 592                            show_startup_with_shell,
 593                            &setlist, &showlist);
 594 }