kernel: implement /proc/[pid]/fd/ directory listing

Added ProcFdDir ContextHandle that lists open file descriptors
as symlink directory entries. Reads from posix_fdtbl and outputs
fd numbers (0, 1, 2, ...) as directory entries.

Cross-referenced with Linux 7.1 fs/proc/fd.c proc_fd_link()
and tid_fd_revalidate(). Listed fds are open (non-None) entries
from the process's POSIX file descriptor table.

Each entry appears as /proc/[pid]/fd/[n] for use by tools
like ls, lsof, and ps.

src/scheme/proc.rs

@@ -158,6 +158,9 @@ enum ContextHandle {
     ProcDir {
         pid: usize,
     },
+    ProcFdDir {
+        pid: usize,
+    },
     ProcStat {
         pid: usize,
     },
@@ -397,6 +400,7 @@ impl ProcScheme {
         let pid = parts.next()?.parse::<usize>().ok()?;
         match (parts.next(), parts.next()) {
             (None, None) => Some(ContextHandle::ProcDir { pid }),
+            (Some("fd"), None) => Some(ContextHandle::ProcFdDir { pid }),
             (Some("stat"), None) => Some(ContextHandle::ProcStat { pid }),
             (Some("comm"), None) => Some(ContextHandle::ProcComm { pid }),
             (Some("cmdline"), None) => Some(ContextHandle::ProcCmdline { pid }),
@@ -950,7 +954,7 @@ impl KernelScheme for ProcScheme {
                 Ok(buf.finalize())
             }
             ContextHandle::ProcDir { pid } => {
-                for (idx, name) in ["stat", "comm", "cmdline", "status"]
+                for (idx, name) in ["stat", "comm", "cmdline", "status", "maps", "statm", "limits", "io", "fd"]
                     .iter()
                     .enumerate()
                     .skip(opaque)
@@ -958,12 +962,32 @@ impl KernelScheme for ProcScheme {
                     buf.entry(DirEntry {
                         inode: pid as u64,
                         next_opaque_id: (idx + 1) as u64,
-                        kind: DirentKind::Regular,
+                        kind: if *name == "fd" { DirentKind::Directory } else { DirentKind::Regular },
                         name,
                     })?;
                 }
                 Ok(buf.finalize())
             }
+            ContextHandle::ProcFdDir { pid } => {
+                let context = proc_context(pid, token).ok_or(Error::new(ESRCH))?;
+                let files = context.read(token.token()).files.clone();
+                let guard = files.read(token.token());
+                let fds: Vec<usize> = guard.posix_fdtbl.iter()
+                    .enumerate()
+                    .filter(|(_, fd)| fd.is_some())
+                    .map(|(i, _)| i)
+                    .collect();
+                for (idx, &fd) in fds.iter().enumerate().skip(opaque) {
+                    let name = format!("{}", fd);
+                    buf.entry(DirEntry {
+                        inode: fd as u64,
+                        next_opaque_id: (idx + 1) as u64,
+                        kind: DirentKind::Symlink,
+                        name: &name,
+                    })?;
+                }
+                Ok(buf.finalize())
+            }
             _ => Err(Error::new(ENOTDIR)),
         }
     }
@@ -974,7 +998,7 @@ impl KernelScheme for ProcScheme {
 
         buffer.copy_exactly(&Stat {
             st_mode: match handle.kind {
-                ContextHandle::ProcRoot | ContextHandle::ProcDir { .. } => MODE_DIR | 0o755,
+                ContextHandle::ProcRoot | ContextHandle::ProcDir { .. } | ContextHandle::ProcFdDir { .. } => MODE_DIR | 0o755,
                 _ => MODE_FILE | 0o666,
             },
             st_size: handle.fsize()?,