September 21, 2010
execve, fexecve - execute a file
.In unistd.h int
execve const char *path char *const argv char *const envpint
fexecve int fd char *const argv char *const envp
execvesystem call transforms the calling process into a new process. The new process is constructed from an ordinary file, whose name is pointed to by path, called the new process file. The
fexecvesystem call is equivalent to
execveexcept that the file to be executed is determined by the file descriptor fd instead of a 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.out(5).
An interpreter file begins with a line of the form:
#! 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
execveis 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
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),
When a program is executed as a result of an
execvesystem 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.
fexecveignores the file offset of fd. Since execute permission is checked by
fexecve, the file descriptor fd need not have been opened with the
O_EXECflag. 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
fexecveis to use the
O_EXECflag when opening fd. Note that the file to be executed can not be open for writing.
execvesystem call overlays the current process image with a new process image the successful call has no process to return to. If
execvedoes 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.
execvesystem call will fail and return to the calling process if:
|A component of the path prefix is not a directory.|
|A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.|
|When invoking an interpreted script, the length of the first line, inclusive of the #! prefix and terminating newline, exceeds
|The new process file does not exist.|
|Too many symbolic links were encountered in translating the pathname.|
|Search permission is denied for a component of the path prefix.|
|The new process file is not an ordinary file.|
|The new process file mode denies execute permission.|
|The new process file has the appropriate access permission, but has an invalid magic number in its header.|
|The new process file is a pure procedure (shared text) file that is currently open for writing by some process.|
|The new process requires more virtual memory than is allowed by the imposed maximum (getrlimit(2)).|
|The number of bytes in the new process’ argument list is larger than the system-imposed limit. This limit is specified by the sysctl(3) MIB variable
|The new process file is not as long as indicated by the size values in its header.|
|The path, argv, or envp arguments point to an illegal address.|
||An I/O error occurred while reading from the file system.|
In addition, the
fexecvewill fail and return to the calling process if:
|The fd argument is not a valid file descriptor open for executing.|
execvesystem call conforms to -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
fexecvesystem call conforms to The Open Group Extended API Set 2 specification.
.Fx 8.0 .
execvesystem call appeared in BSD 4.2 . The
fexecvesystem call appeared in
.Fx 8.0 .
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
fexecve, kernel supplies
/dev/fd/nas 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