Manpage of EXECVE

EXECVE

Section: Linux Programmer's Manual (2)
Updated: 2017-03-13
Index
 

NAME

execve - execute program  

SYNOPSIS

#include <unistd.h>

int execve(const char *filename, char *const argv[],
char *const envp[]); 

DESCRIPTION

execve() executes the program pointed to by filename. filename must be either a binary executable, or a script starting with a line of the form:

#! interpreter [optional-arg]

For details of the latter case, see "Interpreter scripts" below.

argv is an array of argument strings passed to the new program. By convention, the first of these strings (i.e., argv[0]) should contain the filename associated with the file being executed. envp is an array of strings, conventionally of the form key=value, which are passed as environment to the new program. The argv and envp arrays must each include a null pointer at the end of the array.

The argument vector and environment can be accessed by the called program's main function, when it is defined as:

int main(int argc, char *argv[], char *envp[])

Note, however, that the use of a third argument to the main function is not specified in POSIX.1; according to POSIX.1, the environment should be accessed via the external variable environ(7).

execve() does not return on success, and the text, initialized data, uninitialized data (bss), and stack of the calling process are overwritten according to the contents of the newly loaded program.

If the current program is being ptraced, a SIGTRAP signal is sent to it after a successful execve().

If the set-user-ID bit is set on the program file pointed to by filename, then the effective user ID of the calling process is changed to that of the owner of the program file. Similarly, when the set-group-ID bit of the program file is set the effective group ID of the calling process is set to the group of the program file. The aforementioned transformations of the effective IDs are notperformed (i.e., the set-user-ID and set-group-ID bits are ignored) if any of the following is true:

*
the no_new_privsattribute is set for the calling thread (see prctl(2));
*
the underlying filesystem is mounted nosuid(the MS_NOSUIDflag for mount(2)); or
*
the calling process is being ptraced.

The capabilities of the program file (see capabilities(7)) are also if any of the above are true.

The effective user ID of the process is copied to the saved set-user-ID; similarly, the effective group ID is copied to the saved set-group-ID. This copying takes place after any effective ID changes that occur because of the set-user-ID and set-group-ID mode bits.

The process's read UID and real GID, as well its supplementary group IDs, are unchanged by a call to execve().

If the executable is an a.out dynamically linked binary executable containing shared-library stubs, the Linux dynamic linker ld.so(8) is called at the start of execution to bring needed shared objects into memory and link the executable with them.

If the executable is a dynamically linked ELF executable, the interpreter named in the PT_INTERP segment is used to load the needed shared objects. This interpreter is typically /lib/ld-linux.so.2for binaries linked with glibc (see ld-linux.so(8)).

All process attributes are preserved during an execve(), except the following:

*
The dispositions of any signals that are being caught are reset to the default (signal(7)).
*
Any alternate signal stack is not preserved (sigaltstack(2)).
*
Memory mappings are not preserved (mmap(2)).
*
Attached System V shared memory segments are detached (shmat(2)).
*
POSIX shared memory regions are unmapped (shm_open(3)).
*
Open POSIX message queue descriptors are closed (mq_overview(7)).
*
Any open POSIX named semaphores are closed (sem_overview(7)).
*
POSIX timers are not preserved (timer_create(2)).
*
Any open directory streams are closed (opendir(3)).
*
Memory locks are not preserved (mlock(2), mlockall(2)).
*
Exit handlers are not preserved (atexit(3), on_exit(3)).
*
The floating-point environment is reset to the default (see fenv(3)).

The process attributes in the preceding list are all specified in POSIX.1. The following Linux-specific process attributes are also not preserved during an execve():

*
The prctl(2) PR_SET_DUMPABLEflag is set, unless a set-user-ID or set-group ID program is being executed, in which case it is cleared.
*
The prctl(2) PR_SET_KEEPCAPSflag is cleared.
*
(Since Linux 2.4.36 / 2.6.23) If a set-user-ID or set-group-ID program is being executed, then the parent death signal set by prctl(2) PR_SET_PDEATHSIGflag is cleared.
*
The process name, as set by prctl(2) PR_SET_NAME(and displayed by ps -o comm), is reset to the name of the new executable file.
*
The SECBIT_KEEP_CAPSsecurebitsflag is cleared. See capabilities(7).
*
The termination signal is reset to SIGCHLD(see clone(2)).
*
The file descriptor table is unshared, undoing the effect of the CLONE_FILESflag of clone(2).

Note the following further points:

*
All threads other than the calling thread are destroyed during an execve(). Mutexes, condition variables, and other pthreads objects are not preserved.
*
The equivalent of setlocale(LC_ALL, "C")is executed at program start-up.
*
POSIX.1 specifies that the dispositions of any signals that are ignored or set to the default are left unchanged. POSIX.1 specifies one exception: if SIGCHLDis being ignored, then an implementation may leave the disposition unchanged or reset it to the default; Linux does the former.
*
Any outstanding asynchronous I/O operations are canceled (aio_read(3), aio_write(3)).
*
For the handling of capabilities during execve(), see capabilities(7).
*
By default, file descriptors remain open across an execve(). File descriptors that are marked close-on-exec are closed; see the description of FD_CLOEXECin fcntl(2). (If a file descriptor is closed, this will cause the release of all record locks obtained on the underlying file by this process. See fcntl(2) for details.) POSIX.1 says that if file descriptors 0, 1, and 2 would otherwise be closed after a successful execve(), and the process would gain privilege because the set-user-ID or set-group_ID mode bit was set on the executed file, then the system may open an unspecified file for each of these file descriptors. As a general principle, no portable program, whether privileged or not, can assume that these three file descriptors will remain closed across an execve().
 

Interpreter scripts

An interpreter script is a text file that has execute permission enabled and whose first line is of the form:

#! interpreter [optional-arg]

The interpretermust be a valid pathname for an executable file. If the filenameargument of execve() specifies an interpreter script, then interpreterwill be invoked with the following arguments:

interpreter [optional-arg] filename arg...

where arg...is the series of words pointed to by the argvargument of execve(), starting at argv[1].

For portable use, optional-argshould either be absent, or be specified as a single word (i.e., it should not contain white space); see NOTES below.

Since Linux 2.6.28, the kernel permits the interpreter of a script to itself be a script. This permission is recursive, up to a limit of four recursions, so that the interpreter may be a script which is interpreted by a script, and so on.  

Limits on size of arguments and environment

Most UNIX implementations impose some limit on the total size of the command-line argument (argv) and environment (envp) strings that may be passed to a new program. POSIX.1 allows an implementation to advertise this limit using the ARG_MAXconstant (either defined in <limits.h>or available at run time using the call sysconf(_SC_ARG_MAX)).

On Linux prior to kernel 2.6.23, the memory used to store the environment and argument strings was limited to 32 pages (defined by the kernel constant MAX_ARG_PAGES). On architectures with a 4-kB page size, this yields a maximum size of 128 kB.

On kernel 2.6.23 and later, most architectures support a size limit derived from the soft RLIMIT_STACKresource limit (see getrlimit(2)) that is in force at the time of the execve() call. (Architectures with no memory management unit are excepted: they maintain the limit that was in effect before kernel 2.6.23.) This change allows programs to have a much larger argument and/or environment list. For these architectures, the total size is limited to 1/4 of the allowed stack size. (Imposing the 1/4-limit ensures that the new program always has some stack space.) Since Linux 2.6.25, the kernel places a floor of 32 pages on this size limit, so that, even when RLIMIT_STACKis set very low, applications are guaranteed to have at least as much argument and environment space as was provided by Linux 2.6.23 and earlier. (This guarantee was not provided in Linux 2.6.23 and 2.6.24.) Additionally, the limit per string is 32 pages (the kernel constant MAX_ARG_STRLEN), and the maximum number of strings is 0x7FFFFFFF.  

RETURN VALUE

On success, execve() does not return, on error -1 is returned, and errnois set appropriately.  

ERRORS

E2BIG
The total number of bytes in the environment (envp) and argument list (argv) is too large.
EACCES
Search permission is denied on a component of the path prefix of filenameor the name of a script interpreter. (See also path_resolution(7).)
EACCES
The file or a script interpreter is not a regular file.
EACCES
Execute permission is denied for the file or a script or ELF interpreter.
EACCES
The filesystem is mounted noexec.
EAGAIN (since Linux 3.1)
Having changed its real UID using one of the set*uid() calls, the caller was---and is now still---above its RLIMIT_NPROCresource limit (see setrlimit(2)). For a more detailed explanation of this error, see NOTES.
EFAULT
filenameor one of the pointers in the vectors argvor envppoints outside your accessible address space.
EINVAL
An ELF executable had more than one PT_INTERP segment (i.e., tried to name more than one interpreter).
EIO
An I/O error occurred.
EISDIR
An ELF interpreter was a directory.
ELIBBAD
An ELF interpreter was not in a recognized format.
ELOOP
Too many symbolic links were encountered in resolving filenameor the name of a script or ELF interpreter.
ELOOP
The maximum recursion limit was reached during recursive script interpretation (see "Interpreter scripts", above). Before Linux 3.8, the error produced for this case was ENOEXEC.
EMFILE
The per-process limit on the number of open file descriptors has been reached.
ENAMETOOLONG
filenameis too long.
ENFILE
The system-wide limit on the total number of open files has been reached.
ENOENT
The file filenameor a script or ELF interpreter does not exist, or a shared library needed for the file or interpreter cannot be found.
ENOEXEC
An executable is not in a recognized format, is for the wrong architecture, or has some other format error that means it cannot be executed.
ENOMEM
Insufficient kernel memory was available.
ENOTDIR
A component of the path prefix of filenameor a script or ELF interpreter is not a directory.
EPERM
The filesystem is mounted nosuid, the user is not the superuser, and the file has the set-user-ID or set-group-ID bit set.
EPERM
The process is being traced, the user is not the superuser and the file has the set-user-ID or set-group-ID bit set.
EPERM
A "capability-dumb" applications would not obtain the full set of permitted capabilities granted by the executable file. See capabilities(7).
ETXTBSY
The specified executable was open for writing by one or more processes.
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. POSIX does not document the #! behavior, but it exists (with some variations) on other UNIX systems.  

NOTES

Set-user-ID and set-group-ID processes can not be ptrace(2)d.

The result of mounting a filesystem nosuidvaries across Linux kernel versions: some will refuse execution of set-user-ID and set-group-ID executables when this would give the user powers she did not have already (and return EPERM), some will just ignore the set-user-ID and set-group-ID bits and exec() successfully.

On Linux, argvand envpcan be specified as NULL. In both cases, this has the same effect as specifying the argument as a pointer to a list containing a single null pointer. Do not take advantage of this nonstandard and nonportable misfeature!On many other UNIX systems, specifying argvas NULL will result in an error (EFAULT). Someother UNIX systems treat the envp==NULLcase the same as Linux.

POSIX.1 says that values returned by sysconf(3) should be invariant over the lifetime of a process. However, since Linux 2.6.23, if the RLIMIT_STACKresource limit changes, then the value reported by _SC_ARG_MAXwill also change, to reflect the fact that the limit on space for holding command-line arguments and environment variables has changed.

In most cases where execve() fails, control returns to the original executable image, and the caller of execve() can then handle the error. However, in (rare) cases (typically caused by resource exhaustion), failure may occur past the point of no return: the original executable image has been torn down, but the new image could not be completely built. In such cases, the kernel kills the process with a SIGKILLsignal.  

Interpreter scripts

A maximum line length of 127 characters is allowed for the first line in an interpreter scripts.

The semantics of the optional-argargument of an interpreter script vary across implementations. On Linux, the entire string following the interpretername is passed as a single argument to the interpreter, and this string can include white space. However, behavior differs on some other systems. Some systems use the first white space to terminate optional-arg. On some systems, an interpreter script can have multiple arguments, and white spaces in optional-argare used to delimit the arguments.

Linux ignores the set-user-ID and set-group-ID bits on scripts.  

execve() and EAGAIN

A more detailed explanation of the EAGAINerror that can occur (since Linux 3.1) when calling execve() is as follows.

The EAGAINerror can occur when a precedingcall to setuid(2), setreuid(2), or setresuid(2) caused the real user ID of the process to change, and that change caused the process to exceed its RLIMIT_NPROCresource limit (i.e., the number of processes belonging to the new real UID exceeds the resource limit). From Linux 2.6.0 to 3.0, this caused the set*uid() call to fail. (Prior to 2.6, the resource limit was not imposed on processes that changed their user IDs.)

Since Linux 3.1, the scenario just described no longer causes the set*uid() call to fail, because it too often led to security holes where buggy applications didn't check the return status and assumed that---if the caller had root privileges---the call would always succeed. Instead, the set*uid() calls now successfully change the real UID, but the kernel sets an internal flag, named PF_NPROC_EXCEEDED, to note that the RLIMIT_NPROCresource limit has been exceeded. If the PF_NPROC_EXCEEDEDflag is set and the resource limit is still exceeded at the time of a subsequent execve() call, that call fails with the error EAGAIN. This kernel logic ensures that the RLIMIT_NPROCresource limit is still enforced for the common privileged daemon workflow---namely, fork(2) + set*uid() + execve().

If the resource limit was not still exceeded at the time of the execve() call (because other processes belonging to this real UID terminated between the set*uid() call and the execve() call), then the execve() call succeeds and the kernel clears the PF_NPROC_EXCEEDEDprocess flag. The flag is also cleared if a subsequent call to fork(2) by this process succeeds.  

Historical

With UNIX V6, the argument list of an exec() call was ended by 0, while the argument list of mainwas ended by -1. Thus, this argument list was not directly usable in a further exec() call. Since UNIX V7, both are NULL.  

EXAMPLE

The following program is designed to be execed by the second program below. It just echoes its command-line arguments, one per line.

/* myecho.c */

#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char *argv[])
{
    int j;

    for (j = 0; j < argc; j++)
        printf("argv[%d]: %s\n", j, argv[j]);

    exit(EXIT_SUCCESS);
}

This program can be used to exec the program named in its command-line argument:

/* execve.c */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    char *newargv[] = { NULL, "hello", "world", NULL };
    char *newenviron[] = { NULL };

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <file-to-exec>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    newargv[0] = argv[1];

    execve(argv[1], newargv, newenviron);
    perror("execve");   /* execve() returns only on error */
    exit(EXIT_FAILURE);
}

We can use the second program to exec the first as follows:

$ cc myecho.c -o myecho$ cc execve.c -o execve$ ./execve ./myechoargv[0]: ./myecho
argv[1]: hello
argv[2]: world

We can also use these programs to demonstrate the use of a script interpreter. To do this we create a script whose "interpreter" is our myechoprogram:

$ cat > script#!./myecho script-arg^D$ chmod +x script

We can then use our program to exec the script:

$ ./execve ./scriptargv[0]: ./myecho
argv[1]: script-arg
argv[2]: ./script
argv[3]: hello
argv[4]: world
 

SEE ALSO

chmod(2), execveat(2), fork(2), ptrace(2), execl(3), fexecve(3), getopt(3), system(3), credentials(7), environ(7), path_resolution(7), ld.so(8)


 

Index

NAME
SYNOPSIS
DESCRIPTION
Interpreter scripts
Limits on size of arguments and environment
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
Interpreter scripts
execve() and EAGAIN
Historical
EXAMPLE
SEE ALSO

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