shim: trivial documentation enhancements

Trivial documentation enhancements:
1) comments for unsafe code
2) list supported platforms

Signed-off-by: Liu Jiang <gerry@linux.alibaba.com>

crates/shim/README.md

@@ -127,3 +127,9 @@ $ cat log
 [INFO] reaper thread exited
 [INFO] reaper thread stopped
 ```
+
+## Supported Platforms
+Currently following OSs and hardware architectures are supported, and more efforts are needed to enable and validate other OSs and architectures.
+- Linux
+- Mac OS
+- x86_64

crates/shim/src/lib.rs

@@ -284,7 +284,7 @@ pub const SOCKET_ROOT: &str = "/run/containerd";
 #[cfg(target_os = "macos")]
 pub const SOCKET_ROOT: &str = "/var/run/containerd";
 
-/// Make socket path from namespace and id.
+/// Make socket path from containerd socket path, namespace and id.
 pub fn socket_address(socket_path: &str, namespace: &str, id: &str) -> String {
     let path = PathBuf::from(socket_path)
         .join(namespace)

crates/shim/src/reap.rs

@@ -14,21 +14,23 @@
    limitations under the License.
 */
 
-use std::io;
+use std::io::Result;
 
 #[cfg(target_os = "linux")]
-pub fn set_subreaper() -> Result<(), io::Error> {
+pub fn set_subreaper() -> Result<()> {
     use libc::PR_SET_CHILD_SUBREAPER;
+    use std::io::Error;
 
+    // Safe because we trust the kernel and have checked the result.
     let code = unsafe { libc::prctl(PR_SET_CHILD_SUBREAPER, 0, 0, 0) };
     if code != 0 {
-        Err(io::Error::from_raw_os_error(code))
+        Err(Error::from_raw_os_error(code))
     } else {
         Ok(())
     }
 }
 
 #[cfg(not(target_os = "linux"))]
-pub fn set_subreaper() -> Result<(), io::Error> {
+pub fn set_subreaper() -> Result<()> {
     Ok(())
 }