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) 1990-2018 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 "gdb_tilde_expand.h"
  29 #include <vector>
  30
  31 extern char **environ;
  32
  33 /* Default shell file to be used if 'startup-with-shell' is set but
  34    $SHELL is not.  */
  35 #define SHELL_FILE "/bin/sh"
  36
  37 /* Build the argument vector for execv(3).  */
  38
  39 class execv_argv
  40 {
  41 public:
  42   /* EXEC_FILE is the file to run.  ALLARGS is a string containing the
  43      arguments to the program.  If starting with a shell, SHELL_FILE
  44      is the shell to run.  Otherwise, SHELL_FILE is NULL.  */
  45   execv_argv (const char *exec_file, const std::string &allargs,
  46               const char *shell_file);
  47
  48   /* Return a pointer to the built argv, in the type expected by
  49      execv.  The result is (only) valid for as long as this execv_argv
  50      object is live.  We return a "char **" because that's the type
  51      that the execv functions expect.  Note that it is guaranteed that
  52      the execv functions do not modify the argv[] array nor the
  53      strings to which the array point.  */
  54   char **argv ()
  55   {
  56     return const_cast<char **> (&m_argv[0]);
  57   }
  58
  59 private:
  60   DISABLE_COPY_AND_ASSIGN (execv_argv);
  61
  62   /* Helper methods for constructing the argument vector.  */
  63
  64   /* Used when building an argv for a straight execv call, without
  65      going via the shell.  */
  66   void init_for_no_shell (const char *exec_file,
  67                           const std::string &allargs);
  68
  69   /* Used when building an argv for execing a shell that execs the
  70      child program.  */
  71   void init_for_shell (const char *exec_file,
  72                        const std::string &allargs,
  73                        const char *shell_file);
  74
  75   /* The argument vector built.  Holds non-owning pointers.  Elements
  76      either point to the strings passed to the execv_argv ctor, or
  77      inside M_STORAGE.  */
  78   std::vector<const char *> m_argv;
  79
  80   /* Storage.  In the no-shell case, this contains a copy of the
  81      arguments passed to the ctor, split by '\0'.  In the shell case,
  82      this contains the quoted shell command.  I.e., SHELL_COMMAND in
  83      {"$SHELL" "-c", SHELL_COMMAND, NULL}.  */
  84   std::string m_storage;
  85 };
  86
  87 /* Create argument vector for straight call to execvp.  Breaks up
  88    ALLARGS into an argument vector suitable for passing to execvp and
  89    stores it in M_ARGV.  E.g., on "run a b c d" this routine would get
  90    as input the string "a b c d", and as output it would fill in
  91    M_ARGV with the four arguments "a", "b", "c", "d".  Each argument
  92    in M_ARGV points to a substring of a copy of ALLARGS stored in
  93    M_STORAGE.  */
  94
  95 void
  96 execv_argv::init_for_no_shell (const char *exec_file,
  97                                const std::string &allargs)
  98 {
  99
 100   /* Save/work with a copy stored in our storage.  The pointers pushed
 101      to M_ARGV point directly into M_STORAGE, which is modified in
 102      place with the necessary NULL terminators.  This avoids N heap
 103      allocations and string dups when 1 is sufficient.  */
 104   std::string &args_copy = m_storage = allargs;
 105
 106   m_argv.push_back (exec_file);
 107
 108   for (size_t cur_pos = 0; cur_pos < args_copy.size ();)
 109     {
 110       /* Skip whitespace-like chars.  */
 111       std::size_t pos = args_copy.find_first_not_of (" \t\n", cur_pos);
 112
 113       if (pos != std::string::npos)
 114         cur_pos = pos;
 115
 116       /* Find the position of the next separator.  */
 117       std::size_t next_sep = args_copy.find_first_of (" \t\n", cur_pos);
 118
 119       if (next_sep == std::string::npos)
 120         {
 121           /* No separator found, which means this is the last
 122              argument.  */
 123           next_sep = args_copy.size ();
 124         }
 125       else
 126         {
 127           /* Replace the separator with a terminator.  */
 128           args_copy[next_sep++] = '\0';
 129         }
 130
 131       m_argv.push_back (&args_copy[cur_pos]);
 132
 133       cur_pos = next_sep;
 134     }
 135
 136   /* NULL-terminate the vector.  */
 137   m_argv.push_back (NULL);
 138 }
 139
 140 /* When executing a command under the given shell, return true if the
 141    '!' character should be escaped when embedded in a quoted
 142    command-line argument.  */
 143
 144 static bool
 145 escape_bang_in_quoted_argument (const char *shell_file)
 146 {
 147   size_t shell_file_len = strlen (shell_file);
 148
 149   /* Bang should be escaped only in C Shells.  For now, simply check
 150      that the shell name ends with 'csh', which covers at least csh
 151      and tcsh.  This should be good enough for now.  */
 152
 153   if (shell_file_len < 3)
 154     return false;
 155
 156   if (shell_file[shell_file_len - 3] == 'c'
 157       && shell_file[shell_file_len - 2] == 's'
 158       && shell_file[shell_file_len - 1] == 'h')
 159     return true;
 160
 161   return false;
 162 }
 163
 164 /* See declaration.  */
 165
 166 execv_argv::execv_argv (const char *exec_file,
 167                         const std::string &allargs,
 168                         const char *shell_file)
 169 {
 170   if (shell_file == NULL)
 171     init_for_no_shell (exec_file, allargs);
 172   else
 173     init_for_shell (exec_file, allargs, shell_file);
 174 }
 175
 176 /* See declaration.  */
 177
 178 void
 179 execv_argv::init_for_shell (const char *exec_file,
 180                             const std::string &allargs,
 181                             const char *shell_file)
 182 {
 183   const char *exec_wrapper = get_exec_wrapper ();
 184
 185   /* We're going to call a shell.  */
 186   bool escape_bang = escape_bang_in_quoted_argument (shell_file);
 187
 188   /* We need to build a new shell command string, and make argv point
 189      to it.  So build it in the storage.  */
 190   std::string &shell_command = m_storage;
 191
 192   shell_command = "exec ";
 193
 194   /* Add any exec wrapper.  That may be a program name with arguments,
 195      so the user must handle quoting.  */
 196   if (exec_wrapper != NULL)
 197     {
 198       shell_command += exec_wrapper;
 199       shell_command += ' ';
 200     }
 201
 202   /* Now add exec_file, quoting as necessary.  */
 203
 204   /* Quoting in this style is said to work with all shells.  But csh
 205      on IRIX 4.0.1 can't deal with it.  So we only quote it if we need
 206      to.  */
 207   bool need_to_quote;
 208   const char *p = exec_file;
 209   while (1)
 210     {
 211       switch (*p)
 212         {
 213         case '\'':
 214         case '!':
 215         case '"':
 216         case '(':
 217         case ')':
 218         case '$':
 219         case '&':
 220         case ';':
 221         case '<':
 222         case '>':
 223         case ' ':
 224         case '\n':
 225         case '\t':
 226           need_to_quote = true;
 227           goto end_scan;
 228
 229         case '\0':
 230           need_to_quote = false;
 231           goto end_scan;
 232
 233         default:
 234           break;
 235         }
 236       ++p;
 237     }
 238  end_scan:
 239   if (need_to_quote)
 240     {
 241       shell_command += '\'';
 242       for (p = exec_file; *p != '\0'; ++p)
 243         {
 244           if (*p == '\'')
 245             shell_command += "'\\''";
 246           else if (*p == '!' && escape_bang)
 247             shell_command += "\\!";
 248           else
 249             shell_command += *p;
 250         }
 251       shell_command += '\'';
 252     }
 253   else
 254     shell_command += exec_file;
 255
 256   shell_command += ' ' + allargs;
 257
 258   /* If we decided above to start up with a shell, we exec the shell.
 259      "-c" says to interpret the next arg as a shell command to
 260      execute, and this command is "exec <target-program> <args>".  */
 261   m_argv.reserve (4);
 262   m_argv.push_back (shell_file);
 263   m_argv.push_back ("-c");
 264   m_argv.push_back (shell_command.c_str ());
 265   m_argv.push_back (NULL);
 266 }
 267
 268 /* Return the shell that must be used to startup the inferior.  The
 269    first attempt is the environment variable SHELL; if it is not set,
 270    then we default to SHELL_FILE.  */
 271
 272 static const char *
 273 get_startup_shell ()
 274 {
 275   static const char *ret;
 276
 277   ret = getenv ("SHELL");
 278   if (ret == NULL)
 279     ret = SHELL_FILE;
 280
 281   return ret;
 282 }
 283
 284 /* See nat/fork-inferior.h.  */
 285
 286 pid_t
 287 fork_inferior (const char *exec_file_arg, const std::string &allargs,
 288                char **env, void (*traceme_fun) (),
 289                void (*init_trace_fun) (int), void (*pre_trace_fun) (),
 290                const char *shell_file_arg,
 291                void (*exec_fun)(const char *file, char * const *argv,
 292                                 char * const *env))
 293 {
 294   pid_t pid;
 295   /* Set debug_fork then attach to the child while it sleeps, to debug.  */
 296   int debug_fork = 0;
 297   const char *shell_file;
 298   const char *exec_file;
 299   char **save_our_env;
 300   int i;
 301   int save_errno;
 302   const char *inferior_cwd;
 303   std::string expanded_inferior_cwd;
 304
 305   /* If no exec file handed to us, get it from the exec-file command
 306      -- with a good, common error message if none is specified.  */
 307   if (exec_file_arg == NULL)
 308     exec_file = get_exec_file (1);
 309   else
 310     exec_file = exec_file_arg;
 311
 312   /* 'startup_with_shell' is declared in inferior.h and bound to the
 313      "set startup-with-shell" option.  If 0, we'll just do a
 314      fork/exec, no shell, so don't bother figuring out what shell.  */
 315   if (startup_with_shell)
 316     {
 317       shell_file = shell_file_arg;
 318
 319       /* Figure out what shell to start up the user program under.  */
 320       if (shell_file == NULL)
 321         shell_file = get_startup_shell ();
 322
 323       gdb_assert (shell_file != NULL);
 324     }
 325   else
 326     shell_file = NULL;
 327
 328   /* Build the argument vector.  */
 329   execv_argv child_argv (exec_file, allargs, shell_file);
 330
 331   /* Retain a copy of our environment variables, since the child will
 332      replace the value of environ and if we're vforked, we have to
 333      restore it.  */
 334   save_our_env = environ;
 335
 336   /* Perform any necessary actions regarding to TTY before the
 337      fork/vfork call.  */
 338   prefork_hook (allargs.c_str ());
 339
 340   /* It is generally good practice to flush any possible pending stdio
 341      output prior to doing a fork, to avoid the possibility of both
 342      the parent and child flushing the same data after the fork.  */
 343   gdb_flush_out_err ();
 344
 345   /* Check if the user wants to set a different working directory for
 346      the inferior.  */
 347   inferior_cwd = get_inferior_cwd ();
 348
 349   if (inferior_cwd != NULL)
 350     {
 351       /* Expand before forking because between fork and exec, the child
 352          process may only execute async-signal-safe operations.  */
 353       expanded_inferior_cwd = gdb_tilde_expand (inferior_cwd);
 354       inferior_cwd = expanded_inferior_cwd.c_str ();
 355     }
 356
 357   /* If there's any initialization of the target layers that must
 358      happen to prepare to handle the child we're about fork, do it
 359      now...  */
 360   if (pre_trace_fun != NULL)
 361     (*pre_trace_fun) ();
 362
 363   /* Create the child process.  Since the child process is going to
 364      exec(3) shortly afterwards, try to reduce the overhead by
 365      calling vfork(2).  However, if PRE_TRACE_FUN is non-null, it's
 366      likely that this optimization won't work since there's too much
 367      work to do between the vfork(2) and the exec(3).  This is known
 368      to be the case on ttrace(2)-based HP-UX, where some handshaking
 369      between parent and child needs to happen between fork(2) and
 370      exec(2).  However, since the parent is suspended in the vforked
 371      state, this doesn't work.  Also note that the vfork(2) call might
 372      actually be a call to fork(2) due to the fact that autoconf will
 373      ``#define vfork fork'' on certain platforms.  */
 374 #if !(defined(__UCLIBC__) && defined(HAS_NOMMU))
 375   if (pre_trace_fun || debug_fork)
 376     pid = fork ();
 377   else
 378 #endif
 379     pid = vfork ();
 380
 381   if (pid < 0)
 382     perror_with_name (("vfork"));
 383
 384   if (pid == 0)
 385     {
 386       /* Close all file descriptors except those that gdb inherited
 387          (usually 0/1/2), so they don't leak to the inferior.  Note
 388          that this closes the file descriptors of all secondary
 389          UIs.  */
 390       close_most_fds ();
 391
 392       /* Change to the requested working directory if the user
 393          requested it.  */
 394       if (inferior_cwd != NULL)
 395         {
 396           if (chdir (inferior_cwd) < 0)
 397             trace_start_error_with_name (inferior_cwd);
 398         }
 399
 400       if (debug_fork)
 401         sleep (debug_fork);
 402
 403       /* Execute any necessary post-fork actions before we exec.  */
 404       postfork_child_hook ();
 405
 406       /* Changing the signal handlers for the inferior after
 407          a vfork can also change them for the superior, so we don't mess
 408          with signals here.  See comments in
 409          initialize_signals for how we get the right signal handlers
 410          for the inferior.  */
 411
 412       /* "Trace me, Dr. Memory!"  */
 413       (*traceme_fun) ();
 414
 415       /* The call above set this process (the "child") as debuggable
 416         by the original gdb process (the "parent").  Since processes
 417         (unlike people) can have only one parent, if you are debugging
 418         gdb itself (and your debugger is thus _already_ the
 419         controller/parent for this child), code from here on out is
 420         undebuggable.  Indeed, you probably got an error message
 421         saying "not parent".  Sorry; you'll have to use print
 422         statements!  */
 423
 424       restore_original_signals_state ();
 425
 426       /* There is no execlpe call, so we have to set the environment
 427          for our child in the global variable.  If we've vforked, this
 428          clobbers the parent, but environ is restored a few lines down
 429          in the parent.  By the way, yes we do need to look down the
 430          path to find $SHELL.  Rich Pixley says so, and I agree.  */
 431       environ = env;
 432
 433       char **argv = child_argv.argv ();
 434
 435       if (exec_fun != NULL)
 436         (*exec_fun) (argv[0], &argv[0], env);
 437       else
 438         execvp (argv[0], &argv[0]);
 439
 440       /* If we get here, it's an error.  */
 441       save_errno = errno;
 442       warning ("Cannot exec %s", argv[0]);
 443
 444       for (i = 1; argv[i] != NULL; i++)
 445         warning (" %s", argv[i]);
 446
 447       warning ("Error: %s\n", safe_strerror (save_errno));
 448
 449       _exit (0177);
 450     }
 451
 452   /* Restore our environment in case a vforked child clob'd it.  */
 453   environ = save_our_env;
 454
 455   postfork_hook (pid);
 456
 457   /* Now that we have a child process, make it our target, and
 458      initialize anything target-vector-specific that needs
 459      initializing.  */
 460   if (init_trace_fun)
 461     (*init_trace_fun) (pid);
 462
 463   /* We are now in the child process of interest, having exec'd the
 464      correct program, and are poised at the first instruction of the
 465      new program.  */
 466   return pid;
 467 }
 468
 469 /* See nat/fork-inferior.h.  */
 470
 471 ptid_t
 472 startup_inferior (pid_t pid, int ntraps,
 473                   struct target_waitstatus *last_waitstatus,
 474                   ptid_t *last_ptid)
 475 {
 476   int pending_execs = ntraps;
 477   int terminal_initted = 0;
 478   ptid_t resume_ptid;
 479
 480   if (startup_with_shell)
 481     {
 482       /* One trap extra for exec'ing the shell.  */
 483       pending_execs++;
 484     }
 485
 486   if (target_supports_multi_process ())
 487     resume_ptid = pid_to_ptid (pid);
 488   else
 489     resume_ptid = minus_one_ptid;
 490
 491   /* The process was started by the fork that created it, but it will
 492      have stopped one instruction after execing the shell.  Here we
 493      must get it up to actual execution of the real program.  */
 494   if (get_exec_wrapper () != NULL)
 495     pending_execs++;
 496
 497   while (1)
 498     {
 499       enum gdb_signal resume_signal = GDB_SIGNAL_0;
 500       ptid_t event_ptid;
 501
 502       struct target_waitstatus ws;
 503       memset (&ws, 0, sizeof (ws));
 504       event_ptid = target_wait (resume_ptid, &ws, 0);
 505
 506       if (last_waitstatus != NULL)
 507         *last_waitstatus = ws;
 508       if (last_ptid != NULL)
 509         *last_ptid = event_ptid;
 510
 511       if (ws.kind == TARGET_WAITKIND_IGNORE)
 512         /* The inferior didn't really stop, keep waiting.  */
 513         continue;
 514
 515       switch (ws.kind)
 516         {
 517           case TARGET_WAITKIND_SPURIOUS:
 518           case TARGET_WAITKIND_LOADED:
 519           case TARGET_WAITKIND_FORKED:
 520           case TARGET_WAITKIND_VFORKED:
 521           case TARGET_WAITKIND_SYSCALL_ENTRY:
 522           case TARGET_WAITKIND_SYSCALL_RETURN:
 523             /* Ignore gracefully during startup of the inferior.  */
 524             switch_to_thread (event_ptid);
 525             break;
 526
 527           case TARGET_WAITKIND_SIGNALLED:
 528             target_terminal::ours ();
 529             target_mourn_inferior (event_ptid);
 530             error (_("During startup program terminated with signal %s, %s."),
 531                    gdb_signal_to_name (ws.value.sig),
 532                    gdb_signal_to_string (ws.value.sig));
 533             return resume_ptid;
 534
 535           case TARGET_WAITKIND_EXITED:
 536             target_terminal::ours ();
 537             target_mourn_inferior (event_ptid);
 538             if (ws.value.integer)
 539               error (_("During startup program exited with code %d."),
 540                      ws.value.integer);
 541             else
 542               error (_("During startup program exited normally."));
 543             return resume_ptid;
 544
 545           case TARGET_WAITKIND_EXECD:
 546             /* Handle EXEC signals as if they were SIGTRAP signals.  */
 547             xfree (ws.value.execd_pathname);
 548             resume_signal = GDB_SIGNAL_TRAP;
 549             switch_to_thread (event_ptid);
 550             break;
 551
 552           case TARGET_WAITKIND_STOPPED:
 553             resume_signal = ws.value.sig;
 554             switch_to_thread (event_ptid);
 555             break;
 556         }
 557
 558       if (resume_signal != GDB_SIGNAL_TRAP)
 559         {
 560           /* Let shell child handle its own signals in its own way.  */
 561           target_continue (resume_ptid, resume_signal);
 562         }
 563       else
 564         {
 565           /* We handle SIGTRAP, however; it means child did an exec.  */
 566           if (!terminal_initted)
 567             {
 568               /* Now that the child has exec'd we know it has already
 569                  set its process group.  On POSIX systems, tcsetpgrp
 570                  will fail with EPERM if we try it before the child's
 571                  setpgid.  */
 572
 573               /* Set up the "saved terminal modes" of the inferior
 574                  based on what modes we are starting it with.  */
 575               target_terminal::init ();
 576
 577               /* Install inferior's terminal modes.  */
 578               target_terminal::inferior ();
 579
 580               terminal_initted = 1;
 581             }
 582
 583           if (--pending_execs == 0)
 584             break;
 585
 586           /* Just make it go on.  */
 587           target_continue_no_signal (resume_ptid);
 588         }
 589     }
 590
 591   return resume_ptid;
 592 }
 593
 594 /* See nat/fork-inferior.h.  */
 595
 596 void
 597 trace_start_error (const char *fmt, ...)
 598 {
 599   va_list ap;
 600
 601   va_start (ap, fmt);
 602   warning ("Could not trace the inferior process.\nError: ");
 603   vwarning (fmt, ap);
 604   va_end (ap);
 605
 606   gdb_flush_out_err ();
 607   _exit (0177);
 608 }
 609
 610 /* See nat/fork-inferior.h.  */
 611
 612 void
 613 trace_start_error_with_name (const char *string)
 614 {
 615   trace_start_error ("%s: %s", string, safe_strerror (errno));
 616 }