kernel: implement /proc/filesystems — supported filesystem types

Added ProcFilesystems ContextHandle that lists supported filesystems
in Linux /proc/filesystems format. Lists sysfs, proc, devtmpfs,
and redoxfs as supported types.

Cross-referenced with Linux 7.1 fs/filesystems.c filesystems_proc_show().
'nodev' prefix indicates filesystems not requiring a block device.

Also added to ProcRoot directory listing.

src/scheme/proc.rs

@@ -160,6 +160,7 @@ enum ContextHandle {
     ProcUptime,
     ProcLoadavg,
     ProcVersion,
+    ProcFilesystems,
     ProcDir {
         pid: usize,
     },
@@ -416,6 +417,9 @@ impl ProcScheme {
         if path == "proc/version" || path == "version" {
             return Some(ContextHandle::ProcVersion);
         }
+        if path == "proc/filesystems" || path == "filesystems" {
+            return Some(ContextHandle::ProcFilesystems);
+        }
         let mut parts = path.split('/');
         let pid = parts.next()?.parse::<usize>().ok()?;
         match (parts.next(), parts.next()) {
@@ -955,8 +959,8 @@ 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, version)
-                for (idx, name) in ["cpuinfo", "meminfo", "uptime", "loadavg", "version"].iter().enumerate() {
+                // First: system files (cpuinfo, meminfo, uptime, loadavg, version, filesystems)
+                for (idx, name) in ["cpuinfo", "meminfo", "uptime", "loadavg", "version", "filesystems"].iter().enumerate() {
                     buf.entry(DirEntry {
                         inode: 0,
                         next_opaque_id: (idx + 1) as u64,
@@ -972,7 +976,7 @@ impl KernelScheme for ProcScheme {
                     pids.push(context_ref.read(token.token()).pid);
                 }
                 pids.sort_unstable();
-                let base_idx = 5; // after cpuinfo + meminfo + uptime + loadavg + version
+                let base_idx = 6; // after 6 system files
                 for (idx, pid) in pids.into_iter().enumerate().skip(opaque.saturating_sub(base_idx)) {
                     let name = format!("{}", pid);
                     buf.entry(DirEntry {
@@ -1927,6 +1931,13 @@ impl ContextHandle {
                 );
                 read_from(buf, output.as_bytes(), offset)
             }
+            ContextHandle::ProcFilesystems => {
+                // Linux /proc/filesystems lists registered filesystem types.
+                // Cross-referenced with Linux 7.1 fs/filesystems.c filesystems_proc_show().
+                // Red Bear supports redoxfs as the native filesystem.
+                let output = "nodev\tsysfs\nnodev\tproc\nnodev\tdevtmpfs\n\tredoxfs\n";
+                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) = {