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 2018-04-30 "Linux" "Linux Programmer's Manual"
  38 .SH NAME
  39 execve \- execute program
  40 .SH SYNOPSIS
  41 .B #include <unistd.h>
  42 .PP
  43 .BI "int execve(const char *" pathname ", char *const " argv "[], "
  44 .br
  45 .BI "           char *const " envp []);
  46 .SH DESCRIPTION
  47 .BR execve ()
  48 executes the program referred to by \fIpathname\fP.
  49 This causes the program that is currently being run by the calling process
  50 to be replaced with a new program, with newly initialized stack, heap,
  51 and (initialized and uninitialized) data segments.
  52 .PP
  53 \fIpathname\fP must be either a binary executable, or a script
  54 starting with a line of the form:
  55 .PP
  56 .in +4n
  57 .EX
  58 \fB#!\fP \fIinterpreter \fP[optional-arg]
  59 .EE
  60 .in
  61 .PP
  62 For details of the latter case, see "Interpreter scripts" below.
  63 .PP
  64 \fIargv\fP is an array of argument strings passed to the new program.
  65 By convention, the first of these strings (i.e.,
  66 .IR argv[0] )
  67 should contain the filename associated with the file being executed.
  68 \fIenvp\fP is an array of strings, conventionally of the form
  69 \fBkey=value\fP, which are passed as environment to the new program.
  70 The \fIargv\fP and \fIenvp\fP arrays must each include a null pointer
  71 at the end of the array.
  72 .PP
  73 The argument vector and environment can be accessed by the
  74 called program's main function, when it is defined as:
  75 .PP
  76 .in +4n
  77 .EX
  78 int main(int argc, char *argv[], char *envp[])
  79 .EE
  80 .in
  81 .PP
  82 Note, however, that the use of a third argument to the main function
  83 is not specified in POSIX.1;
  84 according to POSIX.1,
  85 the environment should be accessed via the external variable
  86 .BR environ (7).
  87 .PP
  88 .BR execve ()
  89 does not return on success, and the text, initialized data,
  90 uninitialized data (bss), and stack of the calling process are overwritten
  91 according to the contents of the newly loaded program.
  92 .PP
  93 If the current program is being ptraced, a \fBSIGTRAP\fP signal is sent to it
  94 after a successful
  95 .BR execve ().
  96 .PP
  97 If the set-user-ID bit is set on the program file referred to by
  98 \fIpathname\fP,
  99 then the effective user ID of the calling process is changed
 100 to that of the owner of the program file.
 101 Similarly, when the set-group-ID
 102 bit of the program file is set the effective group ID of the calling
 103 process is set to the group of the program file.
 104 .PP
 105 The aforementioned transformations of the effective IDs are
 106 .I not
 107 performed (i.e., the set-user-ID and set-group-ID bits are ignored)
 108 if any of the following is true:
 109 .IP * 3
 110 the
 111 .I no_new_privs
 112 attribute is set for the calling thread (see
 113 .BR prctl (2));
 114 .IP *
 115 the underlying filesystem is mounted
 116 .I nosuid
 117 (the
 118 .B MS_NOSUID
 119 flag for
 120 .BR mount (2));
 121 or
 122 .IP *
 123 the calling process is being ptraced.
 124 .PP
 125 The capabilities of the program file (see
 126 .BR capabilities (7))
 127 are also ignored if any of the above are true.
 128 .PP
 129 The effective user ID of the process is copied to the saved set-user-ID;
 130 similarly, the effective group ID is copied to the saved set-group-ID.
 131 This copying takes place after any effective ID changes that occur
 132 because of the set-user-ID and set-group-ID mode bits.
 133 .PP
 134 The process's real UID and real GID, as well its supplementary group IDs,
 135 are unchanged by a call to
 136 .BR execve ().
 137 .PP
 138 If the executable is an a.out dynamically linked
 139 binary executable containing
 140 shared-library stubs, the Linux dynamic linker
 141 .BR ld.so (8)
 142 is called at the start of execution to bring
 143 needed shared objects into memory
 144 and link the executable with them.
 145 .PP
 146 If the executable is a dynamically linked ELF executable, the
 147 interpreter named in the PT_INTERP segment is used to load the needed
 148 shared objects.
 149 This interpreter is typically
 150 .I /lib/ld-linux.so.2
 151 for binaries linked with glibc (see
 152 .BR ld-linux.so (8)).
 153 .PP
 154 All process attributes are preserved during an
 155 .BR execve (),
 156 except the following:
 157 .IP * 3
 158 The dispositions of any signals that are being caught are
 159 reset to the default
 160 .RB ( signal (7)).
 161 .IP *
 162 Any alternate signal stack is not preserved
 163 .RB ( sigaltstack (2)).
 164 .IP *
 165 Memory mappings are not preserved
 166 .RB ( mmap (2)).
 167 .IP *
 168 Attached System\ V shared memory segments are detached
 169 .RB ( shmat (2)).
 170 .IP *
 171 POSIX shared memory regions are unmapped
 172 .RB ( shm_open (3)).
 173 .IP *
 174 Open POSIX message queue descriptors are closed
 175 .RB ( mq_overview (7)).
 176 .IP *
 177 Any open POSIX named semaphores are closed
 178 .RB ( sem_overview (7)).
 179 .IP *
 180 POSIX timers are not preserved
 181 .RB ( timer_create (2)).
 182 .IP *
 183 Any open directory streams are closed
 184 .RB ( opendir (3)).
 185 .IP *
 186 Memory locks are not preserved
 187 .RB ( mlock (2),
 188 .BR mlockall (2)).
 189 .IP *
 190 Exit handlers are not preserved
 191 .RB ( atexit (3),
 192 .BR on_exit (3)).
 193 .IP *
 194 The floating-point environment is reset to the default (see
 195 .BR fenv (3)).
 196 .PP
 197 The process attributes in the preceding list are all specified
 198 in POSIX.1.
 199 The following Linux-specific process attributes are also
 200 not preserved during an
 201 .BR execve ():
 202 .IP * 3
 203 The
 204 .BR prctl (2)
 205 .B PR_SET_DUMPABLE
 206 flag is set,
 207 unless a set-user-ID or set-group ID program is being executed,
 208 in which case it is cleared.
 209 .IP *
 210 The
 211 .BR prctl (2)
 212 .B PR_SET_KEEPCAPS
 213 flag is cleared.
 214 .IP *
 215 (Since Linux 2.4.36 / 2.6.23)
 216 If a set-user-ID or set-group-ID program is being executed,
 217 then the parent death signal set by
 218 .BR prctl (2)
 219 .B PR_SET_PDEATHSIG
 220 flag is cleared.
 221 .IP *
 222 The process name, as set by
 223 .BR prctl (2)
 224 .B PR_SET_NAME
 225 (and displayed by
 226 .IR "ps\ \-o comm" ),
 227 is reset to the name of the new executable file.
 228 .IP *
 229 The
 230 .B SECBIT_KEEP_CAPS
 231 .I securebits
 232 flag is cleared.
 233 See
 234 .BR capabilities (7).
 235 .IP *
 236 The termination signal is reset to
 237 .B SIGCHLD
 238 (see
 239 .BR clone (2)).
 240 .IP *
 241 The file descriptor table is unshared, undoing the effect of the
 242 .B CLONE_FILES
 243 flag of
 244 .BR clone (2).
 245 .PP
 246 Note the following further points:
 247 .IP * 3
 248 All threads other than the calling thread are destroyed during an
 249 .BR execve ().
 250 Mutexes, condition variables, and other pthreads objects are not preserved.
 251 .IP *
 252 The equivalent of \fIsetlocale(LC_ALL, "C")\fP
 253 is executed at program start-up.
 254 .IP *
 255 POSIX.1 specifies that the dispositions of any signals that
 256 are ignored or set to the default are left unchanged.
 257 POSIX.1 specifies one exception: if
 258 .B SIGCHLD
 259 is being ignored,
 260 then an implementation may leave the disposition unchanged or
 261 reset it to the default; Linux does the former.
 262 .IP *
 263 Any outstanding asynchronous I/O operations are canceled
 264 .RB ( aio_read (3),
 265 .BR aio_write (3)).
 266 .IP *
 267 For the handling of capabilities during
 268 .BR execve (),
 269 see
 270 .BR capabilities (7).
 271 .IP *
 272 By default, file descriptors remain open across an
 273 .BR execve ().
 274 File descriptors that are marked close-on-exec are closed;
 275 see the description of
 276 .B FD_CLOEXEC
 277 in
 278 .BR fcntl (2).
 279 (If a file descriptor is closed, this will cause the release
 280 of all record locks obtained on the underlying file by this process.
 281 See
 282 .BR fcntl (2)
 283 for details.)
 284 POSIX.1 says that if file descriptors 0, 1, and 2 would
 285 otherwise be closed after a successful
 286 .BR execve (),
 287 and the process would gain privilege because the set-user-ID or
 288 set-group_ID mode bit was set on the executed file,
 289 then the system may open an unspecified file for each of these
 290 file descriptors.
 291 As a general principle, no portable program, whether privileged or not,
 292 can assume that these three file descriptors will remain
 293 closed across an
 294 .BR execve ().
 295 .\" On Linux it appears that these file descriptors are
 296 .\" always open after an execve(), and it looks like
 297 .\" Solaris 8 and FreeBSD 6.1 are the same. -- mtk, 30 Apr 2007
 298 .SS Interpreter scripts
 299 An interpreter script is a text file that has execute
 300 permission enabled and whose first line is of the form:
 301 .PP
 302 .in +4n
 303 .EX
 304 \fB#!\fP \fIinterpreter \fP[optional-arg]
 305 .EE
 306 .in
 307 .PP
 308 The
 309 .I interpreter
 310 must be a valid pathname for an executable file.
 311 If the
 312 .I pathname
 313 argument of
 314 .BR execve ()
 315 specifies an interpreter script, then
 316 .I interpreter
 317 will be invoked with the following arguments:
 318 .PP
 319 .in +4n
 320 .EX
 321 \fIinterpreter\fP [optional-arg] \fIpathname\fP arg...
 322 .EE
 323 .in
 324 .PP
 325 where
 326 .I arg...
 327 is the series of words pointed to by the
 328 .I argv
 329 argument of
 330 .BR execve (),
 331 starting at
 332 .IR argv [1].
 333 .PP
 334 For portable use,
 335 .I optional-arg
 336 should either be absent, or be specified as a single word (i.e., it
 337 should not contain white space); see NOTES below.
 338 .PP
 339 Since Linux 2.6.28,
 340 .\" commit bf2a9a39639b8b51377905397a5005f444e9a892
 341 the kernel permits the interpreter of a script to itself be a script.
 342 This permission is recursive, up to a limit of four recursions,
 343 so that the interpreter may be a script which is interpreted by a script,
 344 and so on.
 345 .SS Limits on size of arguments and environment
 346 Most UNIX implementations impose some limit on the total size
 347 of the command-line argument
 348 .RI ( argv )
 349 and environment
 350 .RI ( envp )
 351 strings that may be passed to a new program.
 352 POSIX.1 allows an implementation to advertise this limit using the
 353 .B ARG_MAX
 354 constant (either defined in
 355 .I <limits.h>
 356 or available at run time using the call
 357 .IR "sysconf(_SC_ARG_MAX)" ).
 358 .PP
 359 On Linux prior to kernel 2.6.23, the memory used to store the
 360 environment and argument strings was limited to 32 pages
 361 (defined by the kernel constant
 362 .BR MAX_ARG_PAGES ).
 363 On architectures with a 4-kB page size,
 364 this yields a maximum size of 128\ kB.
 365 .PP
 366 On kernel 2.6.23 and later, most architectures support a size limit
 367 derived from the soft
 368 .B RLIMIT_STACK
 369 resource limit (see
 370 .BR getrlimit (2))
 371 that is in force at the time of the
 372 .BR execve ()
 373 call.
 374 (Architectures with no memory management unit are excepted:
 375 they maintain the limit that was in effect before kernel 2.6.23.)
 376 This change allows programs to have a much larger
 377 argument and/or environment list.
 378 .\" For some background on the changes to ARG_MAX in kernels 2.6.23 and
 379 .\" 2.6.25, see:
 380 .\"     http://sourceware.org/bugzilla/show_bug.cgi?id=5786
 381 .\"     http://bugzilla.kernel.org/show_bug.cgi?id=10095
 382 .\"     http://thread.gmane.org/gmane.linux.kernel/646709/focus=648101,
 383 .\"     checked into 2.6.25 as commit a64e715fc74b1a7dcc5944f848acc38b2c4d4ee2.
 384 For these architectures, the total size is limited to 1/4 of the allowed
 385 stack size.
 386 (Imposing the 1/4-limit
 387 ensures that the new program always has some stack space.)
 388 .\" Ollie: That doesn't include the lists of pointers, though,
 389 .\" so the actual usage is a bit higher (1 pointer per argument).
 390 Since Linux 2.6.25,
 391 the kernel places a floor of 32 pages on this size limit,
 392 so that, even when
 393 .BR RLIMIT_STACK
 394 is set very low,
 395 applications are guaranteed to have at least as much argument and
 396 environment space as was provided by Linux 2.6.23 and earlier.
 397 (This guarantee was not provided in Linux 2.6.23 and 2.6.24.)
 398 Additionally, the limit per string is 32 pages (the kernel constant
 399 .BR MAX_ARG_STRLEN ),
 400 and the maximum number of strings is 0x7FFFFFFF.
 401 .SH RETURN VALUE
 402 On success,
 403 .BR execve ()
 404 does not return, on error \-1 is returned, and
 405 .I errno
 406 is set appropriately.
 407 .SH ERRORS
 408 .TP
 409 .B E2BIG
 410 The total number of bytes in the environment
 411 .RI ( envp )
 412 and argument list
 413 .RI ( argv )
 414 is too large.
 415 .TP
 416 .B EACCES
 417 Search permission is denied on a component of the path prefix of
 418 .I pathname
 419 or the name of a script interpreter.
 420 (See also
 421 .BR path_resolution (7).)
 422 .TP
 423 .B EACCES
 424 The file or a script interpreter is not a regular file.
 425 .TP
 426 .B EACCES
 427 Execute permission is denied for the file or a script or ELF interpreter.
 428 .TP
 429 .B EACCES
 430 The filesystem is mounted
 431 .IR noexec .
 432 .TP
 433 .BR EAGAIN " (since Linux 3.1)"
 434 .\" commit 72fa59970f8698023045ab0713d66f3f4f96945c
 435 Having changed its real UID using one of the
 436 .BR set*uid ()
 437 calls, the caller was\(emand is now still\(emabove its
 438 .BR RLIMIT_NPROC
 439 resource limit (see
 440 .BR setrlimit (2)).
 441 For a more detailed explanation of this error, see NOTES.
 442 .TP
 443 .B EFAULT
 444 .I pathname
 445 or one of the pointers in the vectors
 446 .I argv
 447 or
 448 .I envp
 449 points outside your accessible address space.
 450 .TP
 451 .B EINVAL
 452 An ELF executable had more than one PT_INTERP segment (i.e., tried to
 453 name more than one interpreter).
 454 .TP
 455 .B EIO
 456 An I/O error occurred.
 457 .TP
 458 .B EISDIR
 459 An ELF interpreter was a directory.
 460 .TP
 461 .B ELIBBAD
 462 An ELF interpreter was not in a recognized format.
 463 .TP
 464 .B ELOOP
 465 Too many symbolic links were encountered in resolving
 466 .I pathname
 467 or the name of a script or ELF interpreter.
 468 .TP
 469 .B ELOOP
 470 The maximum recursion limit was reached during recursive script
 471 interpretation (see "Interpreter scripts", above).
 472 Before Linux 3.8,
 473 .\" commit d740269867021faf4ce38a449353d2b986c34a67
 474 the error produced for this case was
 475 .BR ENOEXEC .
 476 .TP
 477 .B EMFILE
 478 The per-process limit on the number of open file descriptors has been reached.
 479 .TP
 480 .B ENAMETOOLONG
 481 .I pathname
 482 is too long.
 483 .TP
 484 .B ENFILE
 485 The system-wide limit on the total number of open files has been reached.
 486 .TP
 487 .B ENOENT
 488 The file
 489 .I pathname
 490 or a script or ELF interpreter does not exist, or a shared library
 491 .\" FIXME but see http://sourceware.org/bugzilla/show_bug.cgi?id=12241
 492 needed for the file or interpreter cannot be found.
 493 .TP
 494 .B ENOEXEC
 495 An executable is not in a recognized format, is for the wrong
 496 architecture, or has some other format error that means it cannot be
 497 executed.
 498 .TP
 499 .B ENOMEM
 500 Insufficient kernel memory was available.
 501 .TP
 502 .B ENOTDIR
 503 A component of the path prefix of
 504 .I pathname
 505 or a script or ELF interpreter is not a directory.
 506 .TP
 507 .B EPERM
 508 The filesystem is mounted
 509 .IR nosuid ,
 510 the user is not the superuser,
 511 and the file has the set-user-ID or set-group-ID bit set.
 512 .TP
 513 .B EPERM
 514 The process is being traced, the user is not the superuser and the
 515 file has the set-user-ID or set-group-ID bit set.
 516 .TP
 517 .B EPERM
 518 A "capability-dumb" applications would not obtain the full set of
 519 permitted capabilities granted by the executable file.
 520 See
 521 .BR capabilities (7).
 522 .TP
 523 .B ETXTBSY
 524 The specified executable was open for writing by one or more processes.
 525 .SH CONFORMING TO
 526 POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
 527 POSIX does not document the #! behavior, but it exists
 528 (with some variations) on other UNIX systems.
 529 .\" SVr4 documents additional error
 530 .\" conditions EAGAIN, EINTR, ELIBACC, ENOLINK, EMULTIHOP; POSIX does not
 531 .\" document ETXTBSY, EPERM, EFAULT, ELOOP, EIO, ENFILE, EMFILE, EINVAL,
 532 .\" EISDIR or ELIBBAD error conditions.
 533 .SH NOTES
 534 One sometimes sees
 535 .BR execve ()
 536 (and the related functions described in
 537 .BR exec (3))
 538 described as "executing a
 539 .I new
 540 process" (or similar).
 541 This is a highly misleading description:
 542 there is no new process;
 543 many attributes of the calling process remain unchanged
 544 (in particular, its PID).
 545 All that
 546 .BR execve (2)
 547 does is arrange for an existing process (the calling process)
 548 to execute a new program.
 549 .PP
 550 Set-user-ID and set-group-ID processes can not be
 551 .BR ptrace (2)d.
 552 .PP
 553 The result of mounting a filesystem
 554 .I nosuid
 555 varies across Linux kernel versions:
 556 some will refuse execution of set-user-ID and set-group-ID
 557 executables when this would
 558 give the user powers she did not have already (and return
 559 .BR EPERM ),
 560 some will just ignore the set-user-ID and set-group-ID bits and
 561 .BR exec ()
 562 successfully.
 563 .PP
 564 On Linux,
 565 .I argv
 566 and
 567 .I envp
 568 can be specified as NULL.
 569 In both cases, this has the same effect as specifying the argument
 570 as a pointer to a list containing a single null pointer.
 571 .B "Do not take advantage of this nonstandard and nonportable misfeature!"
 572 On many other UNIX systems, specifying
 573 .I argv
 574 as NULL will result in an error
 575 .RB ( EFAULT ).
 576 .I Some
 577 other UNIX systems treat the
 578 .I envp==NULL
 579 case the same as Linux.
 580 .\" e.g., EFAULT on Solaris 8 and FreeBSD 6.1; but
 581 .\" HP-UX 11 is like Linux -- mtk, Apr 2007
 582 .\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
 583 .\" Bug rejected (because fix would constitute an ABI change).
 584 .\"
 585 .PP
 586 POSIX.1 says that values returned by
 587 .BR sysconf (3)
 588 should be invariant over the lifetime of a process.
 589 However, since Linux 2.6.23, if the
 590 .BR RLIMIT_STACK
 591 resource limit changes, then the value reported by
 592 .B _SC_ARG_MAX
 593 will also change,
 594 to reflect the fact that the limit on space for holding
 595 command-line arguments and environment variables has changed.
 596 .PP
 597 In most cases where
 598 .BR execve ()
 599 fails, control returns to the original executable image,
 600 and the caller of
 601 .BR execve ()
 602 can then handle the error.
 603 However, in (rare) cases (typically caused by resource exhaustion),
 604 failure may occur past the point of no return:
 605 the original executable image has been torn down,
 606 but the new image could not be completely built.
 607 In such cases, the kernel kills the process with a
 608 .BR SIGKILL
 609 signal.
 610 .\"
 611 .SS Interpreter scripts
 612 A maximum line length of 127 characters is allowed for the first line in
 613 an interpreter script.
 614 .PP
 615 The semantics of the
 616 .I optional-arg
 617 argument of an interpreter script vary across implementations.
 618 On Linux, the entire string following the
 619 .I interpreter
 620 name is passed as a single argument to the interpreter,
 621 and this string can include white space.
 622 However, behavior differs on some other systems.
 623 Some systems
 624 .\" e.g., Solaris 8
 625 use the first white space to terminate
 626 .IR optional-arg .
 627 On some systems,
 628 .\" e.g., FreeBSD before 6.0, but not FreeBSD 6.0 onward
 629 an interpreter script can have multiple arguments,
 630 and white spaces in
 631 .I optional-arg
 632 are used to delimit the arguments.
 633 .PP
 634 Linux ignores the set-user-ID and set-group-ID bits on scripts.
 635 .\"
 636 .\" .SH BUGS
 637 .\" Some Linux versions have failed to check permissions on ELF
 638 .\" interpreters.  This is a security hole, because it allows users to
 639 .\" open any file, such as a rewinding tape device, for reading.  Some
 640 .\" Linux versions have also had other security holes in
 641 .\" .BR execve ()
 642 .\" that could be exploited for denial of service by a suitably crafted
 643 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 644 .SS execve() and EAGAIN
 645 A more detailed explanation of the
 646 .BR EAGAIN
 647 error that can occur (since Linux 3.1) when calling
 648 .BR execve ()
 649 is as follows.
 650 .PP
 651 The
 652 .BR EAGAIN
 653 error can occur when a
 654 .I preceding
 655 call to
 656 .BR setuid (2),
 657 .BR setreuid (2),
 658 or
 659 .BR setresuid (2)
 660 caused the real user ID of the process to change,
 661 and that change caused the process to exceed its
 662 .BR RLIMIT_NPROC
 663 resource limit (i.e., the number of processes belonging
 664 to the new real UID exceeds the resource limit).
 665 From Linux 2.6.0 to 3.0, this caused the
 666 .BR set*uid ()
 667 call to fail.
 668 (Prior to 2.6,
 669 .\" commit 909cc4ae86f3380152a18e2a3c44523893ee11c4
 670 the resource limit was not imposed on processes that
 671 changed their user IDs.)
 672 .PP
 673 Since Linux 3.1, the scenario just described no longer causes the
 674 .BR set*uid ()
 675 call to fail,
 676 because it too often led to security holes where buggy applications
 677 didn't check the return status and assumed
 678 that\(emif the caller had root privileges\(emthe call would always succeed.
 679 Instead, the
 680 .BR set*uid ()
 681 calls now successfully change the real UID,
 682 but the kernel sets an internal flag, named
 683 .BR PF_NPROC_EXCEEDED ,
 684 to note that the
 685 .BR RLIMIT_NPROC
 686 resource limit has been exceeded.
 687 If the
 688 .BR PF_NPROC_EXCEEDED
 689 flag is set and the resource limit is still
 690 exceeded at the time of a subsequent
 691 .BR execve ()
 692 call, that call fails with the error
 693 .BR EAGAIN .
 694 This kernel logic ensures that the
 695 .BR RLIMIT_NPROC
 696 resource limit is still enforced for the
 697 common privileged daemon workflow\(emnamely,
 698 .BR fork (2)
 699 +
 700 .BR set*uid ()
 701 +
 702 .BR execve ().
 703 .PP
 704 If the resource limit was not still exceeded at the time of the
 705 .BR execve ()
 706 call
 707 (because other processes belonging to this real UID terminated between the
 708 .BR set*uid ()
 709 call and the
 710 .BR execve ()
 711 call), then the
 712 .BR execve ()
 713 call succeeds and the kernel clears the
 714 .BR PF_NPROC_EXCEEDED
 715 process flag.
 716 The flag is also cleared if a subsequent call to
 717 .BR fork (2)
 718 by this process succeeds.
 719 .SS Historical
 720 With UNIX\ V6, the argument list of an
 721 .BR exec ()
 722 call was ended by 0,
 723 while the argument list of
 724 .I main
 725 was ended by \-1.
 726 Thus, this argument list was not directly usable in a further
 727 .BR exec ()
 728 call.
 729 Since UNIX\ V7, both are NULL.
 730 .\"
 731 .\" .SH BUGS
 732 .\" Some Linux versions have failed to check permissions on ELF
 733 .\" interpreters.  This is a security hole, because it allows users to
 734 .\" open any file, such as a rewinding tape device, for reading.  Some
 735 .\" Linux versions have also had other security holes in
 736 .\" .BR execve ()
 737 .\" that could be exploited for denial of service by a suitably crafted
 738 .\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.
 739 .SH EXAMPLE
 740 The following program is designed to be execed by the second program below.
 741 It just echoes its command-line arguments, one per line.
 742 .PP
 743 .in +4n
 744 .EX
 745 /* myecho.c */
 746
 747 #include <stdio.h>
 748 #include <stdlib.h>
 749
 750 int
 751 main(int argc, char *argv[])
 752 {
 753     int j;
 754
 755     for (j = 0; j < argc; j++)
 756         printf("argv[%d]: %s\en", j, argv[j]);
 757
 758     exit(EXIT_SUCCESS);
 759 }
 760 .EE
 761 .in
 762 .PP
 763 This program can be used to exec the program named in its command-line
 764 argument:
 765 .PP
 766 .in +4n
 767 .EX
 768 /* execve.c */
 769
 770 #include <stdio.h>
 771 #include <stdlib.h>
 772 #include <unistd.h>
 773
 774 int
 775 main(int argc, char *argv[])
 776 {
 777     char *newargv[] = { NULL, "hello", "world", NULL };
 778     char *newenviron[] = { NULL };
 779
 780     if (argc != 2) {
 781         fprintf(stderr, "Usage: %s <file\-to\-exec>\en", argv[0]);
 782         exit(EXIT_FAILURE);
 783     }
 784
 785     newargv[0] = argv[1];
 786
 787     execve(argv[1], newargv, newenviron);
 788     perror("execve");   /* execve() returns only on error */
 789     exit(EXIT_FAILURE);
 790 }
 791 .EE
 792 .in
 793 .PP
 794 We can use the second program to exec the first as follows:
 795 .PP
 796 .in +4n
 797 .EX
 798 .RB "$" " cc myecho.c \-o myecho"
 799 .RB "$" " cc execve.c \-o execve"
 800 .RB "$" " ./execve ./myecho"
 801 argv[0]: ./myecho
 802 argv[1]: hello
 803 argv[2]: world
 804 .EE
 805 .in
 806 .PP
 807 We can also use these programs to demonstrate the use of a script
 808 interpreter.
 809 To do this we create a script whose "interpreter" is our
 810 .I myecho
 811 program:
 812 .PP
 813 .in +4n
 814 .EX
 815 .RB "$" " cat > script"
 816 .B #!./myecho script-arg
 817 .B ^D
 818 .RB "$" " chmod +x script"
 819 .EE
 820 .in
 821 .PP
 822 We can then use our program to exec the script:
 823 .PP
 824 .in +4n
 825 .EX
 826 .RB "$" " ./execve ./script"
 827 argv[0]: ./myecho
 828 argv[1]: script-arg
 829 argv[2]: ./script
 830 argv[3]: hello
 831 argv[4]: world
 832 .EE
 833 .in
 834 .SH SEE ALSO
 835 .BR chmod (2),
 836 .BR execveat (2),
 837 .BR fork (2),
 838 .BR get_robust_list (2),
 839 .BR ptrace (2),
 840 .BR execl (3),
 841 .BR fexecve (3),
 842 .BR getopt (3),
 843 .BR system (3),
 844 .BR credentials (7),
 845 .BR environ (7),
 846 .BR path_resolution (7),
 847 .BR ld.so (8)