Implement basic lsns utility (#554)

---------

Signed-off-by: Malhar Vora <mlvora.2010@gmail.com>

Author: vbmade2000

Cargo.lock

@@ -38,6 +38,7 @@ feat_common_core = [
   "lsipc",
   "lslocks",
   "lsmem",
+  "lsns",  
   "mcookie",
   "mesg",
   "mountpoint",
@@ -110,6 +111,7 @@ lscpu = { optional = true, version = "0.0.1", package = "uu_lscpu", path = "src/
 lsipc = { optional = true, version = "0.0.1", package = "uu_lsipc", path = "src/uu/lsipc" }
 lslocks = { optional = true, version = "0.0.1", package = "uu_lslocks", path = "src/uu/lslocks" }
 lsmem = { optional = true, version = "0.0.1", package = "uu_lsmem", path = "src/uu/lsmem" }
+lsns = { optional = true, version = "0.0.1", package = "uu_lsns", path = "src/uu/lsns" }
 mcookie = { optional = true, version = "0.0.1", package = "uu_mcookie", path = "src/uu/mcookie" }
 mesg = { optional = true, version = "0.0.1", package = "uu_mesg", path = "src/uu/mesg" }
 mountpoint = { optional = true, version = "0.0.1", package = "uu_mountpoint", path = "src/uu/mountpoint" }

Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "uu_lsns"
+version = "0.0.1"
+edition = "2024"
+
+[lib]
+path = "src/lsns.rs"
+
+[[bin]]
+name = "lsns"
+path = "src/main.rs"
+
+[dependencies]
+uucore = { workspace = true, features = ["entries"] }
+clap = { workspace = true }
+smartcols-sys = { workspace = true }

src/uu/lsns/Cargo.toml

@@ -0,0 +1,7 @@
+# lsns
+
+```
+lsns [OPTION]...
+```
+
+List the namespaces in the system.

src/uu/lsns/lsns.md

@@ -0,0 +1,105 @@
+// This file is part of the uutils util-linux package.
+//
+// For the full copyright and license information, please view the LICENSE
+// file that was distributed with this source code.
+
+use std::ffi::c_int;
+use std::fmt;
+
+use uucore::error::UError;
+
+#[derive(Debug)]
+pub enum LsnsError {
+    /// Generic I/O error with context message
+    IOError(String, std::io::Error),
+    /// CString conversion error (null byte in string)
+    NulError(String, std::ffi::NulError),
+    /// Invalid namespace type index
+    InvalidNamespaceType(usize),
+    /// Unsupported platform
+    #[cfg(not(target_os = "linux"))]
+    UnsupportedPlatform,
+    /// Invalid namespace inode format
+    InvalidNamespaceInodeFormat(String),
+    /// Invalid process stat format
+    InvalidProcessStatFormat(String),
+    /// Failed to get UID from directory entry
+    FailedToGetUid(String),
+    /// Failed to get PID from directory entry
+    FailedToGetPid(String),
+    /// Failed to read process information
+    FailedToReadProcess(String),
+}
+
+impl LsnsError {
+    /// Create an I/O error with a context message
+    pub(crate) fn io0(message: impl Into<String>, error: impl Into<std::io::Error>) -> Self {
+        Self::IOError(message.into(), error.into())
+    }
+
+    /// Helper to convert negative errno to Result
+    pub(crate) fn io_from_neg_errno(
+        message: impl Into<String>,
+        result: c_int,
+    ) -> Result<usize, LsnsError> {
+        if let Ok(result) = usize::try_from(result) {
+            Ok(result)
+        } else {
+            let err = std::io::Error::from_raw_os_error(-result);
+            Err(Self::IOError(message.into(), err))
+        }
+    }
+}
+
+impl fmt::Display for LsnsError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::IOError(message, err) => write!(f, "{message}: {err}"),
+            Self::NulError(message, err) => write!(f, "{message}: {err}"),
+            Self::InvalidNamespaceType(idx) => write!(f, "Invalid namespace type index: {}", idx),
+            #[cfg(not(target_os = "linux"))]
+            Self::UnsupportedPlatform => write!(f, "lsns is only supported on Linux"),
+            Self::InvalidNamespaceInodeFormat(s) => {
+                write!(f, "Invalid namespace inode format: {}", s)
+            }
+            Self::InvalidProcessStatFormat(s) => {
+                write!(f, "Invalid process stat format: {}", s)
+            }
+            Self::FailedToGetUid(s) => {
+                write!(f, "Failed to get UID from directory entry: {}", s)
+            }
+            Self::FailedToGetPid(s) => {
+                write!(f, "Failed to get PID from directory entry: {}", s)
+            }
+            Self::FailedToReadProcess(s) => {
+                write!(f, "Failed to read process information: {}", s)
+            }
+        }
+    }
+}
+
+impl UError for LsnsError {
+    fn code(&self) -> i32 {
+        1
+    }
+
+    fn usage(&self) -> bool {
+        false
+    }
+}
+
+impl std::error::Error for LsnsError {}
+
+// Implement From trait for automatic conversion from std::io::Error
+impl From<std::io::Error> for LsnsError {
+    fn from(err: std::io::Error) -> Self {
+        Self::IOError(String::new(), err)
+    }
+}
+
+// Implement From trait for automatic conversion from std::ffi::NulError
+impl From<std::ffi::NulError> for LsnsError {
+    fn from(err: std::ffi::NulError) -> Self {
+        Self::NulError(String::new(), err)
+    }
+}