kernel: implement /proc/version — kernel version string

Added ProcVersion ContextHandle that outputs the kernel version
string using CARGO_PKG_VERSION, TARGET, and COOKBOOK_SOURCE_IDENT
environment variables from the build system.

Cross-referenced with Linux 7.1 fs/proc/version.c version_proc_show().
Format: 'Red Bear OS version X.Y.Z (arch) source_ident'

Also added to ProcRoot directory listing.

src/scheme/proc.rs

@@ -159,6 +159,7 @@ enum ContextHandle {
     ProcMeminfo,
     ProcUptime,
     ProcLoadavg,
+    ProcVersion,
     ProcDir {
         pid: usize,
     },
@@ -412,6 +413,9 @@ impl ProcScheme {
         if path == "proc/loadavg" || path == "loadavg" {
             return Some(ContextHandle::ProcLoadavg);
         }
+        if path == "proc/version" || path == "version" {
+            return Some(ContextHandle::ProcVersion);
+        }
         let mut parts = path.split('/');
         let pid = parts.next()?.parse::<usize>().ok()?;
         match (parts.next(), parts.next()) {
@@ -951,8 +955,8 @@ impl KernelScheme for ProcScheme {
         let mut buf = DirentBuf::new(buf, header_size).ok_or(Error::new(EIO))?;
         match handle.kind {
             ContextHandle::ProcRoot => {
-                // First: system files (cpuinfo, meminfo, uptime, loadavg)
-                for (idx, name) in ["cpuinfo", "meminfo", "uptime", "loadavg"].iter().enumerate() {
+                // First: system files (cpuinfo, meminfo, uptime, loadavg, version)
+                for (idx, name) in ["cpuinfo", "meminfo", "uptime", "loadavg", "version"].iter().enumerate() {
                     buf.entry(DirEntry {
                         inode: 0,
                         next_opaque_id: (idx + 1) as u64,
@@ -968,7 +972,7 @@ impl KernelScheme for ProcScheme {
                     pids.push(context_ref.read(token.token()).pid);
                 }
                 pids.sort_unstable();
-                let base_idx = 4; // after cpuinfo + meminfo + uptime + loadavg
+                let base_idx = 5; // after cpuinfo + meminfo + uptime + loadavg + version
                 for (idx, pid) in pids.into_iter().enumerate().skip(opaque.saturating_sub(base_idx)) {
                     let name = format!("{}", pid);
                     buf.entry(DirEntry {
@@ -1914,6 +1918,15 @@ impl ContextHandle {
                 let output = format!("0.00 0.00 0.00 {}/{total} 0\n", nr_runnable);
                 read_from(buf, output.as_bytes(), offset)
             }
+            ContextHandle::ProcVersion => {
+                let output = format!(
+                    "Red Bear OS version {} ({}) {}\n",
+                    env!("CARGO_PKG_VERSION"),
+                    option_env!("TARGET").unwrap_or("x86_64"),
+                    option_env!("COOKBOOK_SOURCE_IDENT").unwrap_or("")
+                );
+                read_from(buf, output.as_bytes(), offset)
+            }
             ContextHandle::ProcStat { pid } => {
                 let context = proc_context(*pid, token).ok_or(Error::new(ESRCH))?;
                 let (pid, comm, state, ppid, priority, utime, stime, pgrp, session, starttime, owner_proc_id, addr_space) = {