gdb/nat/fork-inferior.c

   1 /* Fork a Unix child process, and set up to debug it, for GDB and GDBserver.
   2
   3    Copyright (C) 1990-2017 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 #include "common-defs.h"
  21 #include "fork-inferior.h"
  22 #include "target/waitstatus.h"
  23 #include "filestuff.h"
  24 #include "target/target.h"
  25 #include "common-inferior.h"
  26 #include "common-gdbthread.h"
  27 #include "signals-state-save-restore.h"
  28 #include <vector>
  29
  30 extern char **environ;
  31
  32 /* Default shell file to be used if 'startup-with-shell' is set but
  33    $SHELL is not.  */
  34 #define SHELL_FILE "/bin/sh"
  35
  36 /* Build the argument vector for execv(3).  */
  37
  38 class execv_argv
  39 {
  40 public:
  41   /* EXEC_FILE is the file to run.  ALLARGS is a string containing the
  42      arguments to the program.  If starting with a shell, SHELL_FILE
  43      is the shell to run.  Otherwise, SHELL_FILE is NULL.  */
  44   execv_argv (const char *exec_file, const std::string &allargs,
  45               const char *shell_file);
  46
  47   /* Return a pointer to the built argv, in the type expected by
  48      execv.  The result is (only) valid for as long as this execv_argv
  49      object is live.  We return a "char **" because that's the type
  50      that the execv functions expect.  Note that it is guaranteed that
  51      the execv functions do not modify the argv[] array nor the
  52      strings to which the array point.  */
  53   char **argv ()
  54   {
  55     return const_cast<char **> (&m_argv[0]);
  56   }
  57
  58 private:
  59   /* Disable copying.  */
  60   execv_argv (const execv_argv &) = delete;
  61   void operator= (const execv_argv &) = delete;
  62
  63   /* Helper methods for constructing the argument vector.  */
  64
  65   /* Used when building an argv for a straight execv call, without
  66      going via the shell.  */
  67   void init_for_no_shell (const char *exec_file,
  68                           const std::string &allargs);
  69
  70   /* Used when building an argv for execing a shell that execs the
  71      child program.  */
  72   void init_for_shell (const char *exec_file,
  73                        const std::string &allargs,
  74                        const char *shell_file);
  75
  76   /* The argument vector built.  Holds non-owning pointers.  Elements
  77      either point to the strings passed to the execv_argv ctor, or
  78      inside M_STORAGE.  */
  79   std::vector<const char *> m_argv;
  80
  81   /* Storage.  In the no-shell case, this contains a copy of the
  82      arguments passed to the ctor, split by '\0'.  In the shell case,
  83      this contains the quoted shell command.  I.e., SHELL_COMMAND in
  84      {"$SHELL" "-c", SHELL_COMMAND, NULL}.  */
  85   std::string m_storage;
  86 };
  87
  88 /* Create argument vector for straight call to execvp.  Breaks up
  89    ALLARGS into an argument vector suitable for passing to execvp and
  90    stores it in M_ARGV.  E.g., on "run a b c d" this routine would get
  91    as input the string "a b c d", and as output it would fill in
  92    M_ARGV with the four arguments "a", "b", "c", "d".  Each argument
  93    in M_ARGV points to a substring of a copy of ALLARGS stored in
  94    M_STORAGE.  */
  95
  96 void
  97 execv_argv::init_for_no_shell (const char *exec_file,
  98                                const std::string &allargs)
  99 {
 100
 101   /* Save/work with a copy stored in our storage.  The pointers pushed
 102      to M_ARGV point directly into M_STORAGE, which is modified in
 103      place with the necessary NULL terminators.  This avoids N heap
 104      allocations and string dups when 1 is sufficient.  */
 105   std::string &args_copy = m_storage = allargs;
 106
 107   m_argv.push_back (exec_file);
 108
 109   for (size_t cur_pos = 0; cur_pos < args_copy.size ();)
 110     {
 111       /* Skip whitespace-like chars.  */
 112       std::size_t pos = args_copy.find_first_not_of (" \t\n", cur_pos);
 113
 114       if (pos != std::string::npos)
 115         cur_pos = pos;
 116
 117       /* Find the position of the next separator.  */
 118       std::size_t next_sep = args_copy.find_first_of (" \t\n", cur_pos);
 119
 120       if (next_sep == std::string::npos)
 121         {
 122           /* No separator found, which means this is the last
 123              argument.  */
 124           next_sep = args_copy.size ();
 125         }
 126       else
 127         {
 128           /* Replace the separator with a terminator.  */
 129           args_copy[next_sep++] = '\0';
 130         }
 131
 132       m_argv.push_back (&args_copy[cur_pos]);
 133
 134       cur_pos = next_sep;
 135     }
 136
 137   /* NULL-terminate the vector.  */
 138   m_argv.push_back (NULL);
 139 }
 140
 141 /* When executing a command under the given shell, return true if the
 142    '!' character should be escaped when embedded in a quoted
 143    command-line argument.  */
 144
 145 static bool
 146 escape_bang_in_quoted_argument (const char *shell_file)
 147 {
 148   size_t shell_file_len = strlen (shell_file);
 149
 150   /* Bang should be escaped only in C Shells.  For now, simply check
 151      that the shell name ends with 'csh', which covers at least csh
 152      and tcsh.  This should be good enough for now.  */
 153
 154   if (shell_file_len < 3)
 155     return false;
 156
 157   if (shell_file[shell_file_len - 3] == 'c'
 158       && shell_file[shell_file_len - 2] == 's'
 159       && shell_file[shell_file_len - 1] == 'h')
 160     return true;
 161
 162   return false;
 163 }
 164
 165 /* See declaration.  */
 166
 167 execv_argv::execv_argv (const char *exec_file,
 168                         const std::string &allargs,
 169                         const char *shell_file)
 170 {
 171   if (shell_file == NULL)
 172     init_for_no_shell (exec_file, allargs);
 173   else
 174     init_for_shell (exec_file, allargs, shell_file);
 175 }
 176
 177 /* See declaration.  */
 178
 179 void
 180 execv_argv::init_for_shell (const char *exec_file,
 181                             const std::string &allargs,
 182                             const char *shell_file)
 183 {
 184   const char *exec_wrapper = get_exec_wrapper ();
 185
 186   /* We're going to call a shell.  */
 187   bool escape_bang = escape_bang_in_quoted_argument (shell_file);
 188
 189   /* We need to build a new shell command string, and make argv point
 190      to it.  So build it in the storage.  */
 191   std::string &shell_command = m_storage;
 192
 193   shell_command = "exec ";
 194
 195   /* Add any exec wrapper.  That may be a program name with arguments,
 196      so the user must handle quoting.  */
 197   if (exec_wrapper != NULL)
 198     {
 199       shell_command += exec_wrapper;
 200       shell_command += ' ';
 201     }
 202
 203   /* Now add exec_file, quoting as necessary.  */
 204
 205   /* Quoting in this style is said to work with all shells.  But csh
 206      on IRIX 4.0.1 can't deal with it.  So we only quote it if we need
 207      to.  */
 208   bool need_to_quote;
 209   const char *p = exec_file;
 210   while (1)
 211     {
 212       switch (*p)
 213         {
 214         case '\'':
 215         case '!':
 216         case '"':
 217         case '(':
 218         case ')':
 219         case '$':
 220         case '&':
 221         case ';':
 222         case '<':
 223         case '>':
 224         case ' ':
 225         case '\n':
 226         case '\t':
 227           need_to_quote = true;
 228           goto end_scan;
 229
 230         case '\0':
 231           need_to_quote = false;
 232           goto end_scan;
 233
 234         default:
 235           break;
 236         }
 237       ++p;
 238     }
 239  end_scan:
 240   if (need_to_quote)
 241     {
 242       shell_command += '\'';
 243       for (p = exec_file; *p != '\0'; ++p)
 244         {
 245           if (*p == '\'')
 246             shell_command += "'\\''";
 247           else if (*p == '!' && escape_bang)
 248             shell_command += "\\!";
 249           else
 250             shell_command += *p;
 251         }
 252       shell_command += '\'';
 253     }
 254   else
 255     shell_command += exec_file;
 256
 257   shell_command += ' ' + allargs;
 258
 259   /* If we decided above to start up with a shell, we exec the shell.
 260      "-c" says to interpret the next arg as a shell command to
 261      execute, and this command is "exec <target-program> <args>".  */
 262   m_argv.reserve (4);
 263   m_argv.push_back (shell_file);
 264   m_argv.push_back ("-c");
 265   m_argv.push_back (shell_command.c_str ());
 266   m_argv.push_back (NULL);
 267 }
 268
 269 /* Return the shell that must be used to startup the inferior.  The
 270    first attempt is the environment variable SHELL; if it is not set,
 271    then we default to SHELL_FILE.  */
 272
 273 static const char *
 274 get_startup_shell ()
 275 {
 276   static const char *ret;
 277
 278   ret = getenv ("SHELL");
 279   if (ret == NULL)
 280     ret = SHELL_FILE;
 281
 282   return ret;
 283 }
 284
 285 /* See nat/fork-inferior.h.  */
 286
 287 pid_t
 288 fork_inferior (const char *exec_file_arg, const std::string &allargs,
 289                char **env, void (*traceme_fun) (),
 290                void (*init_trace_fun) (int), void (*pre_trace_fun) (),
 291                const char *shell_file_arg,
 292                void (*exec_fun)(const char *file, char * const *argv,
 293                                 char * const *env))
 294 {
 295   pid_t pid;
 296   /* Set debug_fork then attach to the child while it sleeps, to debug.  */
 297   int debug_fork = 0;
 298   const char *shell_file;
 299   const char *exec_file;
 300   char **save_our_env;
 301   int i;
 302   int save_errno;
 303
 304   /* If no exec file handed to us, get it from the exec-file command
 305      -- with a good, common error message if none is specified.  */
 306   if (exec_file_arg == NULL)
 307     exec_file = get_exec_file (1);
 308   else
 309     exec_file = exec_file_arg;
 310
 311   /* 'startup_with_shell' is declared in inferior.h and bound to the
 312      "set startup-with-shell" option.  If 0, we'll just do a
 313      fork/exec, no shell, so don't bother figuring out what shell.  */
 314   if (startup_with_shell)
 315     {
 316       shell_file = shell_file_arg;
 317
 318       /* Figure out what shell to start up the user program under.  */
 319       if (shell_file == NULL)
 320         shell_file = get_startup_shell ();
 321
 322       gdb_assert (shell_file != NULL);
 323     }
 324   else
 325     shell_file = NULL;
 326
 327   /* Build the argument vector.  */
 328   execv_argv child_argv (exec_file, allargs, shell_file);
 329
 330   /* Retain a copy of our environment variables, since the child will
 331      replace the value of environ and if we're vforked, we have to
 332      restore it.  */
 333   save_our_env = environ;
 334
 335   /* Perform any necessary actions regarding to TTY before the
 336      fork/vfork call.  */
 337   prefork_hook (allargs.c_str ());
 338
 339   /* It is generally good practice to flush any possible pending stdio
 340      output prior to doing a fork, to avoid the possibility of both
 341      the parent and child flushing the same data after the fork.  */
 342   gdb_flush_out_err ();
 343
 344   /* If there's any initialization of the target layers that must
 345      happen to prepare to handle the child we're about fork, do it
 346      now...  */
 347   if (pre_trace_fun != NULL)
 348     (*pre_trace_fun) ();
 349
 350   /* Create the child process.  Since the child process is going to
 351      exec(3) shortly afterwards, try to reduce the overhead by
 352      calling vfork(2).  However, if PRE_TRACE_FUN is non-null, it's
 353      likely that this optimization won't work since there's too much
 354      work to do between the vfork(2) and the exec(3).  This is known
 355      to be the case on ttrace(2)-based HP-UX, where some handshaking
 356      between parent and child needs to happen between fork(2) and
 357      exec(2).  However, since the parent is suspended in the vforked
 358      state, this doesn't work.  Also note that the vfork(2) call might
 359      actually be a call to fork(2) due to the fact that autoconf will
 360      ``#define vfork fork'' on certain platforms.  */
 361 #if !(defined(__UCLIBC__) && defined(HAS_NOMMU))
 362   if (pre_trace_fun || debug_fork)
 363     pid = fork ();
 364   else
 365 #endif
 366     pid = vfork ();
 367
 368   if (pid < 0)
 369     perror_with_name (("vfork"));
 370
 371   if (pid == 0)
 372     {
 373       /* Close all file descriptors except those that gdb inherited
 374          (usually 0/1/2), so they don't leak to the inferior.  Note
 375          that this closes the file descriptors of all secondary
 376          UIs.  */
 377       close_most_fds ();
 378
 379       if (debug_fork)
 380         sleep (debug_fork);
 381
 382       /* Execute any necessary post-fork actions before we exec.  */
 383       postfork_child_hook ();
 384
 385       /* Changing the signal handlers for the inferior after
 386          a vfork can also change them for the superior, so we don't mess
 387          with signals here.  See comments in
 388          initialize_signals for how we get the right signal handlers
 389          for the inferior.  */
 390
 391       /* "Trace me, Dr. Memory!"  */
 392       (*traceme_fun) ();
 393
 394       /* The call above set this process (the "child") as debuggable
 395         by the original gdb process (the "parent").  Since processes
 396         (unlike people) can have only one parent, if you are debugging
 397         gdb itself (and your debugger is thus _already_ the
 398         controller/parent for this child), code from here on out is
 399         undebuggable.  Indeed, you probably got an error message
 400         saying "not parent".  Sorry; you'll have to use print
 401         statements!  */
 402
 403       restore_original_signals_state ();
 404
 405       /* There is no execlpe call, so we have to set the environment
 406          for our child in the global variable.  If we've vforked, this
 407          clobbers the parent, but environ is restored a few lines down
 408          in the parent.  By the way, yes we do need to look down the
 409          path to find $SHELL.  Rich Pixley says so, and I agree.  */
 410       environ = env;
 411
 412       char **argv = child_argv.argv ();
 413
 414       if (exec_fun != NULL)
 415         (*exec_fun) (argv[0], &argv[0], env);
 416       else
 417         execvp (argv[0], &argv[0]);
 418
 419       /* If we get here, it's an error.  */
 420       save_errno = errno;
 421       warning ("Cannot exec %s", argv[0]);
 422
 423       for (i = 1; argv[i] != NULL; i++)
 424         warning (" %s", argv[i]);
 425
 426       warning ("Error: %s\n", safe_strerror (save_errno));
 427
 428       _exit (0177);
 429     }
 430
 431   /* Restore our environment in case a vforked child clob'd it.  */
 432   environ = save_our_env;
 433
 434   postfork_hook (pid);
 435
 436   /* Now that we have a child process, make it our target, and
 437      initialize anything target-vector-specific that needs
 438      initializing.  */
 439   if (init_trace_fun)
 440     (*init_trace_fun) (pid);
 441
 442   /* We are now in the child process of interest, having exec'd the
 443      correct program, and are poised at the first instruction of the
 444      new program.  */
 445   return pid;
 446 }
 447
 448 /* See nat/fork-inferior.h.  */
 449
 450 ptid_t
 451 startup_inferior (pid_t pid, int ntraps,
 452                   struct target_waitstatus *last_waitstatus,
 453                   ptid_t *last_ptid)
 454 {
 455   int pending_execs = ntraps;
 456   int terminal_initted = 0;
 457   ptid_t resume_ptid;
 458
 459   if (startup_with_shell)
 460     {
 461       /* One trap extra for exec'ing the shell.  */
 462       pending_execs++;
 463     }
 464
 465   if (target_supports_multi_process ())
 466     resume_ptid = pid_to_ptid (pid);
 467   else
 468     resume_ptid = minus_one_ptid;
 469
 470   /* The process was started by the fork that created it, but it will
 471      have stopped one instruction after execing the shell.  Here we
 472      must get it up to actual execution of the real program.  */
 473   if (get_exec_wrapper () != NULL)
 474     pending_execs++;
 475
 476   while (1)
 477     {
 478       enum gdb_signal resume_signal = GDB_SIGNAL_0;
 479       ptid_t event_ptid;
 480
 481       struct target_waitstatus ws;
 482       memset (&ws, 0, sizeof (ws));
 483       event_ptid = target_wait (resume_ptid, &ws, 0);
 484
 485       if (last_waitstatus != NULL)
 486         *last_waitstatus = ws;
 487       if (last_ptid != NULL)
 488         *last_ptid = event_ptid;
 489
 490       if (ws.kind == TARGET_WAITKIND_IGNORE)
 491         /* The inferior didn't really stop, keep waiting.  */
 492         continue;
 493
 494       switch (ws.kind)
 495         {
 496           case TARGET_WAITKIND_SPURIOUS:
 497           case TARGET_WAITKIND_LOADED:
 498           case TARGET_WAITKIND_FORKED:
 499           case TARGET_WAITKIND_VFORKED:
 500           case TARGET_WAITKIND_SYSCALL_ENTRY:
 501           case TARGET_WAITKIND_SYSCALL_RETURN:
 502             /* Ignore gracefully during startup of the inferior.  */
 503             switch_to_thread (event_ptid);
 504             break;
 505
 506           case TARGET_WAITKIND_SIGNALLED:
 507             target_terminal_ours ();
 508             target_mourn_inferior (event_ptid);
 509             error (_("During startup program terminated with signal %s, %s."),
 510                    gdb_signal_to_name (ws.value.sig),
 511                    gdb_signal_to_string (ws.value.sig));
 512             return resume_ptid;
 513
 514           case TARGET_WAITKIND_EXITED:
 515             target_terminal_ours ();
 516             target_mourn_inferior (event_ptid);
 517             if (ws.value.integer)
 518               error (_("During startup program exited with code %d."),
 519                      ws.value.integer);
 520             else
 521               error (_("During startup program exited normally."));
 522             return resume_ptid;
 523
 524           case TARGET_WAITKIND_EXECD:
 525             /* Handle EXEC signals as if they were SIGTRAP signals.  */
 526             xfree (ws.value.execd_pathname);
 527             resume_signal = GDB_SIGNAL_TRAP;
 528             switch_to_thread (event_ptid);
 529             break;
 530
 531           case TARGET_WAITKIND_STOPPED:
 532             resume_signal = ws.value.sig;
 533             switch_to_thread (event_ptid);
 534             break;
 535         }
 536
 537       if (resume_signal != GDB_SIGNAL_TRAP)
 538         {
 539           /* Let shell child handle its own signals in its own way.  */
 540           target_continue (resume_ptid, resume_signal);
 541         }
 542       else
 543         {
 544           /* We handle SIGTRAP, however; it means child did an exec.  */
 545           if (!terminal_initted)
 546             {
 547               /* Now that the child has exec'd we know it has already
 548                  set its process group.  On POSIX systems, tcsetpgrp
 549                  will fail with EPERM if we try it before the child's
 550                  setpgid.  */
 551
 552               /* Set up the "saved terminal modes" of the inferior
 553                  based on what modes we are starting it with.  */
 554               target_terminal_init ();
 555
 556               /* Install inferior's terminal modes.  */
 557               target_terminal_inferior ();
 558
 559               terminal_initted = 1;
 560             }
 561
 562           if (--pending_execs == 0)
 563             break;
 564
 565           /* Just make it go on.  */
 566           target_continue_no_signal (resume_ptid);
 567         }
 568     }
 569
 570   return resume_ptid;
 571 }
 572
 573 /* See nat/fork-inferior.h.  */
 574
 575 void
 576 trace_start_error (const char *fmt, ...)
 577 {
 578   va_list ap;
 579
 580   va_start (ap, fmt);
 581   warning ("Could not trace the inferior process.\nError: ");
 582   vwarning (fmt, ap);
 583   va_end (ap);
 584
 585   gdb_flush_out_err ();
 586   _exit (0177);
 587 }
 588
 589 /* See nat/fork-inferior.h.  */
 590
 591 void
 592 trace_start_error_with_name (const char *string)
 593 {
 594   trace_start_error ("%s: %s", string, safe_strerror (errno));
 595 }