Manual Pages  — EXECVE

An interpreter file begins with a line of the form:

amp;#! interpreter [ 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 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 path).

The argument 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 execve(). If any of the standard descriptors (0, 1, and/or 2) are closed at the time 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 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	seegetpid(2)
parent process ID	seegetppid(2)
process group ID	seegetpgrp(2)
access groups	seegetgroups(2)
working directory	seechdir(2)
root directory	seechroot(2)
control terminal	seetermios(4)
resource usages	seegetrusage(2)
interval timers	seegetitimer(2)
resource limits	seegetrlimit(2)
file mode mask	seeumask(2)
signal mask	seesigaction(2),sigprocmask(2)

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

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

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

The fexecve() ignores the file offset of fd. Since execute permission is checked by fexecve(), the file descriptor 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 fd to fexecve() is to use the O_EXEC flag when opening fd. Note that the file to be executed can not be open for writing.

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