proc: Add pos and mnt_id fields to /proc/[pid]/fdinfo/[fd]

Currently gVisor only outputs the flags field in fdinfo, which causes the Python library psutil to fail with IndexError when parsing fdinfo, as they expect pos to be the first line.

Add the typical base fields pos and mnt_id.
- pos: via Seek(ctx, 0, SEEK_CUR), defaults to 0 for non-seekable fds
- mnt_id: from file.Mount().ID

FUTURE_COPYBARA_INTEGRATE_REVIEW=#12819 from tanyifeng:proc-fdinfo-pos-mntid d238786
PiperOrigin-RevId: 893667695

Author: tanyifeng

pkg/sentry/fsimpl/proc/task_fds.go

@@ -349,11 +349,16 @@ func (d *fdInfoData) Generate(ctx context.Context, buf *bytes.Buffer) error {
 		return linuxerr.ENOENT
 	}
 	defer d.fs.SafeDecRefFD(ctx, file)
-	// TODO(b/121266871): Include pos, locks, and other data. For now we only
-	// have flags.
+	// Currently we output the typical base fields: pos, flags, mnt_id.
+	// TODO(b/121266871): Add ino, lock, and type-specific fields.
 	// See https://www.kernel.org/doc/Documentation/filesystems/proc.txt
+	var pos int64
+	if p, err := file.Seek(ctx, 0, linux.SEEK_CUR); err == nil {
+		pos = p
+	}
 	flags := uint(file.StatusFlags()) | descriptorFlags.ToLinuxFileFlags()
-	fmt.Fprintf(buf, "flags:\t0%o\n", flags)
+	mntID := file.Mount().ID
+	fmt.Fprintf(buf, "pos:\t%d\nflags:\t0%o\nmnt_id:\t%d\n", pos, flags, mntID)
 	return nil
 }
 

pkg/sentry/fsimpl/proc/tasks_test.go

@@ -19,6 +19,7 @@ import (
 	"math"
 	"path"
 	"strconv"
+	"strings"
 	"testing"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
@@ -510,3 +511,88 @@ func TestTree(t *testing.T) {
 	iterateDir(ctx, t, s, fd)
 	fd.DecRef(ctx)
 }
+
+func TestFdInfoContent(t *testing.T) {
+	s := setup(t)
+	defer s.Destroy()
+
+	k := kernel.KernelFromContext(s.Ctx)
+	tc := k.NewThreadGroup(k.RootPIDNamespace(), kernel.NewSignalHandlers(), linux.SIGCHLD, k.GlobalInit().Limits())
+	task, err := testutil.CreateTask(s.Ctx, "name", tc, s.MntNs, s.Root, s.Root)
+	if err != nil {
+		t.Fatalf("CreateTask(): %v", err)
+	}
+
+	// Create a test file and add it to the task's FD table.
+	pop := &vfs.PathOperation{
+		Root:  s.Root,
+		Start: s.Root,
+		Path:  fspath.Parse("test-file"),
+	}
+	opts := &vfs.OpenOptions{
+		Flags: linux.O_RDWR | linux.O_CREAT,
+		Mode:  0644,
+	}
+	file, err := s.VFS.OpenAt(s.Ctx, s.Creds, pop, opts)
+	if err != nil {
+		t.Fatalf("failed to create test file: %v", err)
+	}
+	defer file.DecRef(s.Ctx)
+
+	fdno, err := task.FDTable().NewFD(task.AsyncContext(), 0, file, kernel.FDFlags{})
+	if err != nil {
+		t.Fatalf("NewFD(): %v", err)
+	}
+
+	// Read /proc/1/fdinfo/<fd>.
+	ctx := task.AsyncContext()
+	fdInfoPath := fmt.Sprintf("/proc/1/fdinfo/%d", fdno)
+	fdInfoFD, err := s.VFS.OpenAt(
+		ctx,
+		auth.CredentialsFromContext(s.Ctx),
+		&vfs.PathOperation{Root: s.Root, Start: s.Root, Path: fspath.Parse(fdInfoPath)},
+		&vfs.OpenOptions{},
+	)
+	if err != nil {
+		t.Fatalf("OpenAt(%q) failed: %v", fdInfoPath, err)
+	}
+	defer fdInfoFD.DecRef(ctx)
+
+	buf := make([]byte, 256)
+	n, err := fdInfoFD.Read(ctx, usermem.BytesIOSequence(buf), vfs.ReadOptions{})
+	if err != nil {
+		t.Fatalf("Read(%q) failed: %v", fdInfoPath, err)
+	}
+	content := string(buf[:n])
+
+	// Verify that pos, flags, and mnt_id fields are present.
+	if !strings.HasPrefix(content, "pos:\t") {
+		t.Errorf("fdinfo content should start with 'pos:', got: %q", content)
+	}
+	if !strings.Contains(content, "flags:\t") {
+		t.Errorf("fdinfo content should contain 'flags:', got: %q", content)
+	}
+	if !strings.Contains(content, "mnt_id:\t") {
+		t.Errorf("fdinfo content should contain 'mnt_id:', got: %q", content)
+	}
+
+	// Verify the order: pos, flags, mnt_id (matching Linux's seq_show).
+	lines := strings.Split(strings.TrimSpace(content), "\n")
+	if len(lines) < 3 {
+		t.Fatalf("expected at least 3 lines in fdinfo, got %d: %q", len(lines), content)
+	}
+	if !strings.HasPrefix(lines[0], "pos:\t") {
+		t.Errorf("first line should be 'pos:', got: %q", lines[0])
+	}
+	if !strings.HasPrefix(lines[1], "flags:\t") {
+		t.Errorf("second line should be 'flags:', got: %q", lines[1])
+	}
+	if !strings.HasPrefix(lines[2], "mnt_id:\t") {
+		t.Errorf("third line should be 'mnt_id:', got: %q", lines[2])
+	}
+
+	// Verify pos is 0 for a freshly opened file.
+	if !strings.HasPrefix(lines[0], "pos:\t0") {
+		t.Errorf("pos should be 0 for freshly opened file, got: %q", lines[0])
+	}
+}