man2/execve.2

   1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
   2 .\" and Copyright (c) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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  25 .\"
  26 .\" Modified by Michael Haardt <michael@moria.de>
  27 .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
  28 .\" Modified 1994-08-21 by Michael Chastain <mec@shell.portal.com>:
  29 .\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
  30 .\" Modified 1999-11-12 by Urs Thuermann <urs@isnogud.escape.de>
  31 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
  32 .\" 2006-09-04 Michael Kerrisk <mtk.manpages@gmail.com>
  33 .\"     Added list of process attributes that are not preserved on exec().
  34 .\" 2007-09-14 Ollie Wild <aaw@google.com>, mtk
  35 .\"     Add text describing limits on command-line arguments + environment
  36 .\"
  37 .TH EXECVE 2 2016-10-08 "Linux" "Linux Programmer's Manual"
  38 .SH NAME
  39 execve \- execute program
  40 .SH SYNOPSIS
  41 .B #include <unistd.h>
  42 .sp
  43 .BI "int execve(const char *" filename ", char *const " argv "[], "
  44 .br
  45 .BI "           char *const " envp []);
  46 .SH DESCRIPTION
  47 .BR execve ()
  48 executes the program pointed to by \fIfilename\fP.
  49 \fIfilename\fP must be either a binary executable, or a script
  50 starting with a line of the form:
  51
  52 .in +4n
  53 .nf
  54 \fB#!\fP \fIinterpreter \fP[optional-arg]
  55 .fi
  56 .in
  57
  58 For details of the latter case, see "Interpreter scripts" below.
  59
  60 \fIargv\fP is an array of argument strings passed to the new program.
  61 By convention, the first of these strings (i.e.,
  62 .IR argv[0] )
  63 should contain the filename associated with the file being executed.
  64 \fIenvp\fP is an array of strings, conventionally of the form
  65 \fBkey=value\fP, which are passed as environment to the new program.
  66 The \fIargv\fP and \fIenvp\fP arrays must each include a null pointer
  67 at the end of the array.
  68
  69 The argument vector and environment can be accessed by the
  70 called program's main function, when it is defined as:
  71
  72 .in +4n
  73 .nf
  74 int main(int argc, char *argv[], char *envp[])
  75 .fi
  76 .in
  77
  78 Note, however, that the use of a third argument to the main function
  79 is not specified in POSIX.1;
  80 according to POSIX.1,
  81 the environment should be accessed via the external variable
  82 .BR environ (7).
  83
  84 .BR execve ()
  85 does not return on success, and the text, initialized data,
  86 uninitialized data (bss), and stack of the calling process are overwritten
  87 according to the contents of the newly loaded program.
  88
  89 If the current program is being ptraced, a \fBSIGTRAP\fP signal is sent to it
  90 after a successful
  91 .BR execve ().
  92
  93 If the set-user-ID bit is set on the program file pointed to by
  94 \fIfilename\fP,
  95 then the effective user ID of the calling process is changed
  96 to that of the owner of the program file.
  97 Similarly, when the set-group-ID
  98 bit of the program file is set the effective group ID of the calling
  99 process is set to the group of the program file.
 100 The aforementioned transformations of the effective IDs are
 101 .I not
 102 performed if any of the following is true:
 103 .IP * 3
 104 the underlying filesystem is mounted
 105 .I nosuid
 106 (the
 107 .B MS_NOSUID
 108 flag for
 109 .BR mount (2));
 110 or
 111 .IP *
 112 the calling process is being ptraced.
 113 .PP
 114 The effective user ID of the process is copied to the saved set-user-ID;
 115 similarly, the effective group ID is copied to the saved set-group-ID.
 116 This copying takes place after any effective ID changes that occur
 117 because of the set-user-ID and set-group-ID mode bits.
 118
 119 The process's read UID and real GID, as well its supplementary group IDs,
 120 are unchanged by a call to
 121 .BR execve (2).
 122
 123 If the executable is an a.out dynamically linked
 124 binary executable containing
 125 shared-library stubs, the Linux dynamic linker
 126 .BR ld.so (8)
 127 is called at the start of execution to bring
 128 needed shared objects into memory
 129 and link the executable with them.
 130
 131 If the executable is a dynamically linked ELF executable, the
 132 interpreter named in the PT_INTERP segment is used to load the needed
 133 shared objects.
 134 This interpreter is typically
 135 .I /lib/ld-linux.so.2
 136 for binaries linked with glibc (see
 137 .BR ld-linux.so (8)).
 138
 139 All process attributes are preserved during an
 140 .BR execve (),
 141 except the following:
 142 .IP * 3
 143 The dispositions of any signals that are being caught are
 144 reset to the default
 145 .RB ( signal (7)).
 146 .IP *
 147 Any alternate signal stack is not preserved
 148 .RB ( sigaltstack (2)).
 149 .IP *
 150 Memory mappings are not preserved
 151 .RB ( mmap (2)).
 152 .IP *
 153 Attached System\ V shared memory segments are detached
 154 .RB ( shmat (2)).
 155 .IP *
 156 POSIX shared memory regions are unmapped
 157 .RB ( shm_open (3)).
 158 .IP *
 159 Open POSIX message queue descriptors are closed
 160 .RB ( mq_overview (7)).
 161 .IP *
 162 Any open POSIX named semaphores are closed
 163 .RB ( sem_overview (7)).
 164 .IP *
 165 POSIX timers are not preserved
 166 .RB ( timer_create (2)).
 167 .IP *
 168 Any open directory streams are closed
 169 .RB ( opendir (3)).
 170 .IP *
 171 Memory locks are not preserved
 172 .RB ( mlock (2),
 173 .BR mlockall (2)).
 174 .IP *
 175 Exit handlers are not preserved
 176 .RB ( atexit (3),
 177 .BR on_exit (3)).
 178 .IP *
 179 The floating-point environment is reset to the default (see
 180 .BR fenv (3)).
 181 .PP
 182 The process attributes in the preceding list are all specified
 183 in POSIX.1.
 184 The following Linux-specific process attributes are also
 185 not preserved during an
 186 .BR execve ():
 187 .IP * 3
 188 The
 189 .BR prctl (2)
 190 .B PR_SET_DUMPABLE
 191 flag is set,
 192 unless a set-user-ID or set-group ID program is being executed,
 193 in which case it is cleared.
 194 .IP *
 195 The
 196 .BR prctl (2)
 197 .B PR_SET_KEEPCAPS
 198 flag is cleared.
 199 .IP *
 200 (Since Linux 2.4.36 / 2.6.23)
 201 If a set-user-ID or set-group-ID program is being executed,
 202 then the parent death signal set by
 203 .BR prctl (2)
 204 .B PR_SET_PDEATHSIG
 205 flag is cleared.
 206 .IP *
 207 The process name, as set by
 208 .BR prctl (2)
 209 .B PR_SET_NAME
 210 (and displayed by
 211 .IR "ps\ \-o comm" ),
 212 is reset to the name of the new executable file.
 213 .IP *
 214 The
 215 .B SECBIT_KEEP_CAPS
 216 .I securebits
 217 flag is cleared.
 218 See
 219 .BR capabilities (7).
 220 .IP *
 221 The termination signal is reset to
 222 .B SIGCHLD
 223 (see
 224 .BR clone (2)).
 225 .IP *
 226 The file descriptor table is unshared, undoing the effect of the
 227 .B CLONE_FILES
 228 flag of
 229 .BR clone (2).
 230 .PP
 231 Note the following further points:
 232 .IP * 3
 233 All threads other than the calling thread are destroyed during an
 234 .BR execve ().
 235 Mutexes, condition variables, and other pthreads objects are not preserved.
 236 .IP *
 237 The equivalent of \fIsetlocale(LC_ALL, "C")\fP
 238 is executed at program start-up.
 239 .IP *
 240 POSIX.1 specifies that the dispositions of any signals that
 241 are ignored or set to the default are left unchanged.
 242 POSIX.1 specifies one exception: if
 243 .B SIGCHLD
 244 is being ignored,
 245 then an implementation may leave the disposition unchanged or
 246 reset it to the default; Linux does the former.
 247 .IP *
 248 Any outstanding asynchronous I/O operations are canceled
 249 .RB ( aio_read (3),
 250 .BR aio_write (3)).
 251 .IP *
 252 For the handling of capabilities during
 253 .BR execve (),
 254 see
 255 .BR capabilities (7).
 256 .IP *
 257 By default, file descriptors remain open across an
 258 .BR execve ().
 259 File descriptors that are marked close-on-exec are closed;
 260 see the description of
 261 .B FD_CLOEXEC
 262 in
 263 .BR fcntl (2).
 264 (If a file descriptor is closed, this will cause the release
 265 of all record locks obtained on the underlying file by this process.
 266 See
 267 .BR fcntl (2)
 268 for details.)
 269 POSIX.1 says that if file descriptors 0, 1, and 2 would
 270 otherwise be closed after a successful
 271 .BR execve (),
 272 and the process would gain privilege because the set-user-ID or
 273 set-group_ID mode bit was set on the executed file,
 274 then the system may open an unspecified file for each of these
 275 file descriptors.
 276 As a general principle, no portable program, whether privileged or not,
 277 can assume that these three file descriptors will remain
 278 closed across an
 279 .BR execve ().
 280 .\" On Linux it appears that these file descriptors are
 281 .\" always open after an execve(), and it looks like
 282 .\" Solaris 8 and FreeBSD 6.1 are the same. -- mtk, 30 Apr 2007
 283 .SS Interpreter scripts
 284 An interpreter script is a text file that has execute
 285 permission enabled and whose first line is of the form:
 286
 287 .in +4n
 288 .nf
 289 \fB#!\fP \fIinterpreter \fP[optional-arg]
 290 .fi
 291 .in
 292
 293 The
 294 .I interpreter
 295 must be a valid pathname for an executable file.
 296 If the
 297 .I filename
 298 argument of
 299 .BR execve ()
 300 specifies an interpreter script, then
 301 .I interpreter
 302 will be invoked with the following arguments:
 303
 304 .in +4n
 305 .nf
 306 \fIinterpreter\fP [optional-arg] \fIfilename\fP arg...
 307 .fi
 308 .in
 309
 310 where
 311 .I arg...
 312 is the series of words pointed to by the
 313 .I argv
 314 argument of
 315 .BR execve (),
 316 starting at
 317 .IR argv [1].
 318
 319 For portable use,
 320 .I optional-arg
 321 should either be absent, or be specified as a single word (i.e., it
 322 should not contain white space); see NOTES below.
 323
 324 Since Linux 2.6.28,
 325 .\" commit bf2a9a39639b8b51377905397a5005f444e9a892
 326 the kernel permits the interpreter of a script to itself be a script.
 327 This permission is recursive, up to a limit of four recursions,
 328 so that the interpreter may be a script which is interpreted by a script,
 329 and so on.
 330 .SS Limits on size of arguments and environment
 331 Most UNIX implementations impose some limit on the total size
 332 of the command-line argument
 333 .RI ( argv )
 334 and environment
 335 .RI ( envp )
 336 strings that may be passed to a new program.
 337 POSIX.1 allows an implementation to advertise this limit using the
 338 .B ARG_MAX
 339 constant (either defined in
 340 .I <limits.h>
 341 or available at run time using the call
 342 .IR "sysconf(_SC_ARG_MAX)" ).
 343
 344 On Linux prior to kernel 2.6.23, the memory used to store the
 345 environment and argument strings was limited to 32 pages
 346 (defined by the kernel constant
 347 .BR MAX_ARG_PAGES ).
 348 On architectures with a 4-kB page size,
 349 this yields a maximum size of 128 kB.
 350
 351 On kernel 2.6.23 and later, most architectures support a size limit
 352 derived from the soft
 353 .B RLIMIT_STACK
 354 resource limit (see
 355 .BR getrlimit (2))
 356 that is in force at the time of the
 357 .BR execve ()
 358 call.
 359 (Architectures with no memory management unit are excepted:
 360 they maintain the limit that was in effect before kernel 2.6.23.)
 361 This change allows programs to have a much larger
 362 argument and/or environment list.
 363 .\" For some background on the changes to ARG_MAX in kernels 2.6.23 and
 364 .\" 2.6.25, see:
 365 .\"     http://sourceware.org/bugzilla/show_bug.cgi?id=5786
 366 .\"     http://bugzilla.kernel.org/show_bug.cgi?id=10095
 367 .\"     http://thread.gmane.org/gmane.linux.kernel/646709/focus=648101,
 368 .\"     checked into 2.6.25 as commit a64e715fc74b1a7dcc5944f848acc38b2c4d4ee2.
 369 For these architectures, the total size is limited to 1/4 of the allowed
 370 stack size.
 371 (Imposing the 1/4-limit
 372 ensures that the new program always has some stack space.)
 373 .\" Ollie: That doesn't include the lists of pointers, though,
 374 .\" so the actual usage is a bit higher (1 pointer per argument).
 375 Since Linux 2.6.25,
 376 the kernel places a floor of 32 pages on this size limit,
 377 so that, even when
 378 .BR RLIMIT_STACK
 379 is set very low,
 380 applications are guaranteed to have at least as much argument and
 381 environment space as was provided by Linux 2.6.23 and earlier.
 382 (This guarantee was not provided in Linux 2.6.23 and 2.6.24.)
 383 Additionally, the limit per string is 32 pages (the kernel constant
 384 .BR MAX_ARG_STRLEN ),
 385 and the maximum number of strings is 0x7FFFFFFF.
 386 .SH RETURN VALUE
 387 On success,
 388 .BR execve ()
 389 does not return, on error \-1 is returned, and
 390 .I errno
 391 is set appropriately.
 392 .SH ERRORS
 393 .TP
 394 .B E2BIG
 395 The total number of bytes in the environment
 396 .RI ( envp )
 397 and argument list
 398 .RI ( argv )
 399 is too large.
 400 .TP
 401 .B EACCES
 402 Search permission is denied on a component of the path prefix of
 403 .I filename
 404 or the name of a script interpreter.
 405 (See also
 406 .BR path_resolution (7).)
 407 .TP
 408 .B EACCES
 409 The file or a script interpreter is not a regular file.
 410 .TP
 411 .B EACCES
 412 Execute permission is denied for the file or a script or ELF interpreter.
 413 .TP
 414 .B EACCES
 415 The filesystem is mounted
 416 .IR noexec .
 417 .TP
 418 .BR EAGAIN " (since Linux 3.1)"
 419 .\" commit 72fa59970f8698023045ab0713d66f3f4f96945c
 420 Having changed its real UID using one of the
 421 .BR set*uid ()
 422 calls, the caller was\(emand is now still\(emabove its
 423 .BR RLIMIT_NPROC
 424 resource limit (see
 425 .BR setrlimit (2)).
 426 For a more detailed explanation of this error, see NOTES.
 427 .TP
 428 .B EFAULT
 429 .I filename
 430 or one of the pointers in the vectors
 431 .I argv
 432 or
 433 .I envp
 434 points outside your accessible address space.
 435 .TP
 436 .B EINVAL
 437 An ELF executable had more than one PT_INTERP segment (i.e., tried to
 438 name more than one interpreter).
 439 .TP
 440 .B EIO
 441 An I/O error occurred.
 442 .TP
 443 .B EISDIR
 444 An ELF interpreter was a directory.
 445 .TP
 446 .B ELIBBAD
 447 An ELF interpreter was not in a recognized format.
 448 .TP
 449 .B ELOOP
 450 Too many symbolic links were encountered in resolving
 451 .I filename
 452 or the name of a script or ELF interpreter.
 453 .TP
 454 .B ELOOP
 455 The maximum recursion limit was reached during recursive script
 456 interpretation (see "Interpreter scripts", above).
 457 Before Linux 3.8,
 458 .\" commit d740269867021faf4ce38a449353d2b986c34a67
 459 the error produced for this case was
 460 .BR ENOEXEC .
 461 .TP
 462 .B EMFILE
 463 The per-process limit on the number of open file descriptors has been reached.
 464 .TP
 465 .B ENAMETOOLONG
 466 .I filename
 467 is too long.
 468 .TP
 469 .B ENFILE
 470 The system-wide limit on the total number of open files has been reached.
 471 .TP
 472 .B ENOENT
 473 The file
 474 .I filename
 475 or a script or ELF interpreter does not exist, or a shared library
 476 .\" FIXME but see http://sourceware.org/bugzilla/show_bug.cgi?id=12241
 477 needed for the file or interpreter cannot be found.
 478 .TP
 479 .B ENOEXEC
 480 An executable is not in a recognized format, is for the wrong
 481 architecture, or has some other format error that means it cannot be
 482 executed.
 483 .TP
 484 .B ENOMEM
 485 Insufficient kernel memory was available.
 486 .TP
 487 .B ENOTDIR
 488 A component of the path prefix of
 489 .I filename
 490 or a script or ELF interpreter is not a directory.
 491 .TP
 492 .B EPERM
 493 The filesystem is mounted
 494 .IR nosuid ,
 495 the user is not the superuser,
 496 and the file has the set-user-ID or set-group-ID bit set.
 497 .TP
 498 .B EPERM
 499 The process is being traced, the user is not the superuser and the
 500 file has the set-user-ID or set-group-ID bit set.
 501 .TP
 502 .B EPERM
 503 A "capability-dumb" applications would not obtain the full set of
 504 permitted capabilities granted by the executable file.
 505 See
 506 .BR capabilities (7).
 507 .TP
 508 .B ETXTBSY
 509 The specified executable was open for writing by one or more processes.
 510 .SH CONFORMING TO
 511 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 512 POSIX does not document the #! behavior, but it exists
 513 (with some variations) on other UNIX systems.
 514 .\" SVr4 documents additional error
 515 .\" conditions EAGAIN, EINTR, ELIBACC, ENOLINK, EMULTIHOP; POSIX does not
 516 .\" document ETXTBSY, EPERM, EFAULT, ELOOP, EIO, ENFILE, EMFILE, EINVAL,
 517 .\" EISDIR or ELIBBAD error conditions.
 518 .SH NOTES
 519 Set-user-ID and set-group-ID processes can not be
 520 .BR ptrace (2)d.
 521
 522 The result of mounting a filesystem
 523 .I nosuid
 524 varies across Linux kernel versions:
 525 some will refuse execution of set-user-ID and set-group-ID
 526 executables when this would
 527 give the user powers she did not have already (and return
 528 .BR EPERM ),
 529 some will just ignore the set-user-ID and set-group-ID bits and
 530 .BR exec ()
 531 successfully.
 532
 533 On Linux,
 534 .I argv
 535 and
 536 .I envp
 537 can be specified as NULL.
 538 In both cases, this has the same effect as specifying the argument
 539 as a pointer to a list containing a single null pointer.
 540 .B "Do not take advantage of this nonstandard and nonportable misfeature!"
 541 On many other UNIX systems, specifying
 542 .I argv
 543 as NULL will result in an error
 544 .RB ( EFAULT ).
 545 .I Some
 546 other UNIX systems treat the
 547 .I envp==NULL
 548 case the same as Linux.
 549 .\" e.g., EFAULT on Solaris 8 and FreeBSD 6.1; but
 550 .\" HP-UX 11 is like Linux -- mtk, Apr 2007
 551 .\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
 552 .\" Bug rejected (because fix would constitute an ABI change).
 553 .\"
 554
 555 POSIX.1 says that values returned by
 556 .BR sysconf (3)
 557 should be invariant over the lifetime of a process.
 558 However, since Linux 2.6.23, if the
 559 .BR RLIMIT_STACK
 560 resource limit changes, then the value reported by
 561 .B _SC_ARG_MAX
 562 will also change,
 563 to reflect the fact that the limit on space for holding
 564 command-line arguments and environment variables has changed.
 565
 566 In most cases where
 567 .BR execve ()
 568 fails, control returns to the original executable image,
 569 and the caller of
 570 .BR execve ()
 571 can then handle the error.
 572 However, in (rare) cases (typically caused by resource exhaustion),
 573 failure may occur past the point of no return:
 574 the original executable image has been torn down,
 575 but the new image could not be completely built.
 576 In such cases, the kernel kills the process with a
 577 .BR SIGKILL
 578 signal.
 579 .\"
 580 .SS Interpreter scripts
 581 A maximum line length of 127 characters is allowed for the first line in
 582 an interpreter scripts.
 583
 584 The semantics of the
 585 .I optional-arg
 586 argument of an interpreter script vary across implementations.
 587 On Linux, the entire string following the
 588 .I interpreter
 589 name is passed as a single argument to the interpreter,
 590 and this string can include white space.
 591 However, behavior differs on some other systems.
 592 Some systems
 593 .\" e.g., Solaris 8
 594 use the first white space to terminate
 595 .IR optional-arg .
 596 On some systems,
 597 .\" e.g., FreeBSD before 6.0, but not FreeBSD 6.0 onward
 598 an interpreter script can have multiple arguments,
 599 and white spaces in
 600 .I optional-arg
 601 are used to delimit the arguments.
 602
 603 Linux ignores the set-user-ID and set-group-ID bits on scripts.
 604 .\"
 605 .\" .SH BUGS
 606 .\" Some Linux versions have failed to check permissions on ELF
 607 .\" interpreters.  This is a security hole, because it allows users to
 608 .\" open any file, such as a rewinding tape device, for reading.  Some
 609 .\" Linux versions have also had other security holes in
 610 .\" .BR execve ()
 611 .\" that could be exploited for denial of service by a suitably crafted
 612 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 613 .SS execve() and EAGAIN
 614 A more detailed explanation of the
 615 .BR EAGAIN
 616 error that can occur (since Linux 3.1) when calling
 617 .BR execve ()
 618 is as follows.
 619
 620 The
 621 .BR EAGAIN
 622 error can occur when a
 623 .I preceding
 624 call to
 625 .BR setuid (2),
 626 .BR setreuid (2),
 627 or
 628 .BR setresuid (2)
 629 caused the real user ID of the process to change,
 630 and that change caused the process to exceed its
 631 .BR RLIMIT_NPROC
 632 resource limit (i.e., the number of processes belonging
 633 to the new real UID exceeds the resource limit).
 634 From Linux 2.6.0 to 3.0, this caused the
 635 .BR set*uid ()
 636 call to fail.
 637 (Prior to 2.6,
 638 .\" commit 909cc4ae86f3380152a18e2a3c44523893ee11c4
 639 the resource limit was not imposed on processes that
 640 changed their user IDs.)
 641
 642 Since Linux 3.1, the scenario just described no longer causes the
 643 .BR set*uid ()
 644 call to fail,
 645 because it too often led to security holes where buggy applications
 646 didn't check the return status and assumed
 647 that\(emif the caller had root privileges\(emthe call would always succeed.
 648 Instead, the
 649 .BR set*uid ()
 650 calls now successfully change the real UID,
 651 but the kernel sets an internal flag, named
 652 .BR PF_NPROC_EXCEEDED ,
 653 to note that the
 654 .BR RLIMIT_NPROC
 655 resource limit has been exceeded.
 656 If the
 657 .BR PF_NPROC_EXCEEDED
 658 flag is set and the resource limit is still
 659 exceeded at the time of a subsequent
 660 .BR execve ()
 661 call, that call fails with the error
 662 .BR EAGAIN .
 663 This kernel logic ensures that the
 664 .BR RLIMIT_NPROC
 665 resource limit is still enforced for the
 666 common privileged daemon workflow\(emnamely,
 667 .BR fork (2)
 668 +
 669 .BR set*uid ()
 670 +
 671 .BR execve ().
 672
 673 If the resource limit was not still exceeded at the time of the
 674 .BR execve ()
 675 call
 676 (because other processes belonging to this real UID terminated between the
 677 .BR set*uid ()
 678 call and the
 679 .BR execve ()
 680 call), then the
 681 .BR execve ()
 682 call succeeds and the kernel clears the
 683 .BR PF_NPROC_EXCEEDED
 684 process flag.
 685 The flag is also cleared if a subsequent call to
 686 .BR fork (2)
 687 by this process succeeds.
 688 .SS Historical
 689 With UNIX\ V6, the argument list of an
 690 .BR exec ()
 691 call was ended by 0,
 692 while the argument list of
 693 .I main
 694 was ended by \-1.
 695 Thus, this argument list was not directly usable in a further
 696 .BR exec ()
 697 call.
 698 Since UNIX\ V7, both are NULL.
 699 .\"
 700 .\" .SH BUGS
 701 .\" Some Linux versions have failed to check permissions on ELF
 702 .\" interpreters.  This is a security hole, because it allows users to
 703 .\" open any file, such as a rewinding tape device, for reading.  Some
 704 .\" Linux versions have also had other security holes in
 705 .\" .BR execve ()
 706 .\" that could be exploited for denial of service by a suitably crafted
 707 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 708 .SH EXAMPLE
 709 The following program is designed to be execed by the second program below.
 710 It just echoes its command-line arguments, one per line.
 711
 712 .in +4n
 713 .nf
 714 /* myecho.c */
 715
 716 #include <stdio.h>
 717 #include <stdlib.h>
 718
 719 int
 720 main(int argc, char *argv[])
 721 {
 722     int j;
 723
 724     for (j = 0; j < argc; j++)
 725         printf("argv[%d]: %s\\n", j, argv[j]);
 726
 727     exit(EXIT_SUCCESS);
 728 }
 729 .fi
 730 .in
 731
 732 This program can be used to exec the program named in its command-line
 733 argument:
 734 .in +4n
 735 .nf
 736
 737 /* execve.c */
 738
 739 #include <stdio.h>
 740 #include <stdlib.h>
 741 #include <unistd.h>
 742
 743 int
 744 main(int argc, char *argv[])
 745 {
 746     char *newargv[] = { NULL, "hello", "world", NULL };
 747     char *newenviron[] = { NULL };
 748
 749     if (argc != 2) {
 750         fprintf(stderr, "Usage: %s <file\-to\-exec>\\n", argv[0]);
 751         exit(EXIT_FAILURE);
 752     }
 753
 754     newargv[0] = argv[1];
 755
 756     execve(argv[1], newargv, newenviron);
 757     perror("execve");   /* execve() returns only on error */
 758     exit(EXIT_FAILURE);
 759 }
 760 .fi
 761 .in
 762
 763 We can use the second program to exec the first as follows:
 764
 765 .in +4n
 766 .nf
 767 .RB "$" " cc myecho.c \-o myecho"
 768 .RB "$" " cc execve.c \-o execve"
 769 .RB "$" " ./execve ./myecho"
 770 argv[0]: ./myecho
 771 argv[1]: hello
 772 argv[2]: world
 773 .fi
 774 .in
 775
 776 We can also use these programs to demonstrate the use of a script
 777 interpreter.
 778 To do this we create a script whose "interpreter" is our
 779 .I myecho
 780 program:
 781
 782 .in +4n
 783 .nf
 784 .RB "$" " cat > script"
 785 .B #!./myecho script-arg
 786 .B ^D
 787 .RB "$" " chmod +x script"
 788 .fi
 789 .in
 790
 791 We can then use our program to exec the script:
 792
 793 .in +4n
 794 .nf
 795 .RB "$" " ./execve ./script"
 796 argv[0]: ./myecho
 797 argv[1]: script-arg
 798 argv[2]: ./script
 799 argv[3]: hello
 800 argv[4]: world
 801 .fi
 802 .in
 803 .SH SEE ALSO
 804 .BR chmod (2),
 805 .BR execveat (2),
 806 .BR fork (2),
 807 .BR ptrace (2),
 808 .BR execl (3),
 809 .BR fexecve (3),
 810 .BR getopt (3),
 811 .BR system (3),
 812 .BR credentials (7),
 813 .BR environ (7),
 814 .BR path_resolution (7),
 815 .BR ld.so (8)