unistd: add fexecve()

This adds fexecve() to `nix::unistd`. It is available in libc since 0.2.29.

Ref: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html

Author: lucab

CHANGELOG.md

@@ -16,6 +16,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#647](https://github.com/nix-rust/nix/pull/647))
 - Added the `pid()` method to `WaitStatus` for extracting the PID.
   ([#722](https://github.com/nix-rust/nix/pull/722))
+- Added `nix::unistd:fexecve`.
+  ([#727](https://github.com/nix-rust/nix/pull/727))
 
 ### Changed
 - Renamed existing `ptrace` wrappers to encourage namespacing ([#692](https://github.com/nix-rust/nix/pull/692))

src/unistd.rs

@@ -593,6 +593,37 @@ pub fn execvp(filename: &CString, args: &[CString]) -> Result<Void> {
     Err(Error::Sys(Errno::last()))
 }
 
+/// Replace the current process image with a new one (see
+/// [fexecve(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html)).
+///
+/// The `fexecve` function allows for another process to be "called" which will
+/// replace the current process image.  That is, this process becomes the new
+/// command that is run. On success, this function will not return. Instead,
+/// the new program will run until it exits.
+///
+/// This function is similar to `execve`, except that the program to be executed
+/// is referenced as a file descriptor instead of a path.
+///
+/// # Errors
+///
+/// If an error occurs, this function will return with an indication of the
+/// cause of failure.  See
+/// [fexecve(2)#errors](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html#tag_16_111_05)
+/// for a list of error conditions.
+#[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
+          target_os = "netbsd", target_os = "openbsd", target_os = "linux"))]
+#[inline]
+pub fn fexecve(fd: RawFd, args: &[CString], env: &[CString]) -> Result<Void> {
+    let args_p = to_exec_array(args);
+    let env_p = to_exec_array(env);
+
+    unsafe {
+        libc::fexecve(fd, args_p.as_ptr(), env_p.as_ptr())
+    };
+
+    Err(Error::Sys(Errno::last()))
+}
+
 /// Daemonize this process by detaching from the controlling terminal (see
 /// [daemon(3)](http://man7.org/linux/man-pages/man3/daemon.3.html)).
 ///

test/test.rs

@@ -1,4 +1,6 @@
 #[macro_use]
+extern crate cfg_if;
+#[macro_use]
 extern crate nix;
 #[macro_use]
 extern crate lazy_static;

test/test_unistd.rs

@@ -105,7 +105,7 @@ mod linux_android {
 }
 
 macro_rules! execve_test_factory(
-    ($test_name:ident, $syscall:ident, $unix_sh:expr, $android_sh:expr) => (
+    ($test_name:ident, $syscall:ident, $exe: expr) => (
     #[test]
     fn $test_name() {
         #[allow(unused_variables)]
@@ -119,19 +119,13 @@ macro_rules! execve_test_factory(
         //       The tests make sure not to do that, though.
         match fork().unwrap() {
             Child => {
-                #[cfg(not(target_os = "android"))]
-                const SH_PATH: &'static [u8] = $unix_sh;
-
-                #[cfg(target_os = "android")]
-                const SH_PATH: &'static [u8] = $android_sh;
-
                 // Close stdout.
                 close(1).unwrap();
                 // Make `writer` be the stdout of the new process.
                 dup(writer).unwrap();
                 // exec!
                 $syscall(
-                    &CString::new(SH_PATH).unwrap(),
+                    $exe,
                     &[CString::new(b"".as_ref()).unwrap(),
                       CString::new(b"-c".as_ref()).unwrap(),
                       CString::new(b"echo nix!!! && echo foo=$foo && echo baz=$baz"
@@ -156,6 +150,23 @@ macro_rules! execve_test_factory(
     )
 );
 
+cfg_if!{
+    if #[cfg(target_os = "android")] {
+        execve_test_factory!(test_execve, execve, &CString::new("/system/bin/sh").unwrap());
+        execve_test_factory!(test_fexecve, fexecve, File::open("/system/bin/sh").unwrap().into_raw_fd());
+    } else if #[cfg(any(target_os = "dragonfly",
+                        target_os = "freebsd",
+                        target_os = "netbsd",
+                        target_os = "openbsd",
+                        target_os = "linux", ))] {
+        execve_test_factory!(test_execve, execve, &CString::new("/bin/sh").unwrap());
+        execve_test_factory!(test_fexecve, fexecve, File::open("/bin/sh").unwrap().into_raw_fd());
+    } else if #[cfg(any(target_os = "ios", target_os = "macos", ))] {
+        execve_test_factory!(test_execve, execve, &CString::new("/bin/sh").unwrap());
+        // No fexecve() on macos/ios.
+    }
+}
+
 #[test]
 fn test_fchdir() {
     // fchdir changes the process's cwd
@@ -231,8 +242,6 @@ fn test_lseek64() {
     close(tmpfd).unwrap();
 }
 
-execve_test_factory!(test_execve, execve, b"/bin/sh", b"/system/bin/sh");
-
 #[test]
 fn test_fpathconf_limited() {
     let f = tempfile().unwrap();