LIBRARY

Lb libc

SYNOPSIS

In unistd.h Ft int Fn execve const char *path char *const argv[] char *const envp[] Ft int Fn fexecve int fd char *const argv[] char *const envp[]

DESCRIPTION

The Fn execve system call transforms the calling process into a new process. The new process is constructed from an ordinary file, whose name is pointed to by Fa path , called the new process file The Fn fexecve system call is equivalent to Fn execve except that the file to be executed is determined by the file descriptor Fa fd instead of a Fa path . This file is either an executable object file, or a file of data for an interpreter. An executable object file consists of an identifying header, followed by pages of data representing the initial program (text) and initialized data pages. Additional pages may be specified by the header to be initialized with zero data; see elf(5) and a.out5.

An interpreter file begins with a line of the form:

#! interpreter Bq Em arg

When an interpreter file is execve Ap d the system actually execve Ap s the specified interpreter If the optional arg is specified, it becomes the first argument to the interpreter and the name of the originally execve Ap d file becomes the second argument; otherwise, the name of the originally execve Ap d file becomes the first argument. The original arguments are shifted over to become the subsequent arguments. The zeroth argument is set to the specified interpreter

The argument Fa argv is a pointer to a null-terminated array of character pointers to null-terminated character strings. These strings construct the argument list to be made available to the new process. At least one argument must be present in the array; by custom, the first element should be the name of the executed program (for example, the last component of Fa path ) .

The argument Fa envp is also a pointer to a null-terminated array of character pointers to null-terminated strings. A pointer to this array is normally stored in the global variable environ These strings pass information to the new process that is not directly an argument to the command (see environ(7)).

File descriptors open in the calling process image remain open in the new process image, except for those for which the close-on-exec flag is set (see close(2) and fcntl(2)). Descriptors that remain open are unaffected by Fn execve . If any of the standard descriptors (0, 1, and/or 2) are closed at the time Fn execve is called, and the process will gain privilege as a result of set-id semantics, those descriptors will be re-opened automatically. No programs, whether privileged or not, should assume that these descriptors will remain closed across a call to Fn execve .

Signals set to be ignored in the calling process are set to be ignored in the new process. Signals which are set to be caught in the calling process image are set to default action in the new process image. Blocked signals remain blocked regardless of changes to the signal action. The signal stack is reset to be undefined (see sigaction(2) for more information).

If the set-user-ID mode bit of the new process image file is set (see chmod(2)), the effective user ID of the new process image is set to the owner ID of the new process image file. If the set-group-ID mode bit of the new process image file is set, the effective group ID of the new process image is set to the group ID of the new process image file. (The effective group ID is the first element of the group list.) The real user ID, real group ID and other group IDs of the new process image remain the same as the calling process image. After any set-user-ID and set-group-ID processing, the effective user ID is recorded as the saved set-user-ID, and the effective group ID is recorded as the saved set-group-ID. These values may be used in changing the effective IDs later (see setuid(2)).

The set-ID bits are not honored if the respective file system has the nosuid option enabled or if the new process file is an interpreter file. Syscall tracing is disabled if effective IDs are changed.

The new process also inherits the following attributes from the calling process:

process ID Ta see getpid(2)

parent process ID Ta see getppid(2)

process group ID Ta see getpgrp(2)

access groups Ta see getgroups(2)

working directory Ta see chdir(2)

root directory Ta see chroot(2)

control terminal Ta see termios(4)

resource usages Ta see getrusage(2)

interval timers Ta see getitimer(2)

resource limits Ta see getrlimit(2)

file mode mask Ta see umask(2)

signal mask Ta see sigaction(2)

sigprocmask(2)

When a program is executed as a result of an Fn execve system call, it is entered as follows:

main(argc, argv, envp)
int argc;
char **argv, **envp;

where Fa argc is the number of elements in Fa argv (the ``arg count'') and Fa argv points to the array of character pointers to the arguments themselves.

The Fn fexecve ignores the file offset of Fa fd . Since execute permission is checked by Fn fexecve , the file descriptor Fa fd need not have been opened with the O_EXEC flag. However, if the file to be executed denies read permission for the process preparing to do the exec, the only way to provide the Fa fd to Fn fexecve is to use the O_EXEC flag when opening Fa fd . Note that the file to be executed can not be open for writing.

RETURN VALUES

As the Fn execve system call overlays the current process image with a new process image the successful call has no process to return to. If Fn execve does return to the calling process an error has occurred; the return value will be -1 and the global variable errno is set to indicate the error.

STANDARDS

The Fn execve system call conforms to St -p1003.1-2001 , with the exception of reopening descriptors 0, 1, and/or 2 in certain circumstances. A future update of the Standard is expected to require this behavior, and it may become the default for non-privileged processes as well. The support for executing interpreted programs is an extension. The Fn fexecve system call conforms to The Open Group Extended API Set 2 specification.

HISTORY

The Fn execve system call appeared in BSD 4.2 The Fn fexecve system call appeared in Fx 8.0 .

CAVEATS

If a program is setuid to a non-super-user, but is executed when the real uid is ``root'', then the program has some of the powers of a super-user as well.

When executing an interpreted program through Fn fexecve , kernel supplies /dev/fd/n as a second argument to the interpreter, where n is the file descriptor passed in the Fa fd argument to Fn fexecve . For this construction to work correctly, the fdescfs(5) filesystem shall be mounted on /dev/fd