Remove map from fd_map, change to fd_table.

This renames FDMap to FDTable and drops the kernel.FD type, which had an entire
package to itself and didn't serve much use (it was freely cast between types,
and served as more of an annoyance than providing any protection.)

Based on BenchmarkFDLookupAndDecRef-12, we can expect 5-10 ns per lookup
operation, and 10-15 ns per concurrent lookup operation of savings.

This also fixes two tangential usage issues with the FDMap. Namely, non-atomic
use of NewFDFrom and associated calls to Remove (that are both racy and fail to
drop the reference on the underlying file.)

PiperOrigin-RevId: 256285890

5 files changed, 21 insertions, 28 deletions

diff --git a/pkg/sentry/fs/README.md b/pkg/sentry/fs/README.md
index c4e8faa3c..db4a1b730 100644
--- a/pkg/sentry/fs/README.md
+++ b/pkg/sentry/fs/README.md
@@ -69,7 +69,7 @@ Specifically this state is:
 
 -   An `fs.MountManager` containing mount points.
 
--   A `kernel.FDMap` containing pointers to open files.
+-   A `kernel.FDTable` containing pointers to open files.
 
 Anything else managed by the VFS that can be easily loaded into memory from a
 filesystem is synced back to those filesystems and is not saved. Examples are
diff --git a/pkg/sentry/fs/g3doc/inotify.md b/pkg/sentry/fs/g3doc/inotify.md
index e1630553b..71a577d9d 100644
--- a/pkg/sentry/fs/g3doc/inotify.md
+++ b/pkg/sentry/fs/g3doc/inotify.md
@@ -9,7 +9,7 @@ For the most part, the sentry implementation of inotify mirrors the Linux
 architecture. Inotify instances (i.e. the fd returned by inotify_init(2)) are
 backed by a pseudo-filesystem. Events are generated from various places in the
 sentry, including the [syscall layer][syscall_dir], the [vfs layer][dirent] and
-the [process fd table][fd_map]. Watches are stored in inodes and generated
+the [process fd table][fd_table]. Watches are stored in inodes and generated
 events are queued to the inotify instance owning the watches for delivery to the
 user.
 
@@ -114,7 +114,7 @@ ordering.
 
 [dirent]: https://github.com/google/gvisor/blob/master/+/master/pkg/sentry/fs/dirent.go
 [event]: https://github.com/google/gvisor/blob/master/+/master/pkg/sentry/fs/inotify_event.go
-[fd_map]: https://github.com/google/gvisor/blob/master/+/master/pkg/sentry/kernel/fd_map.go
+[fd_table]: https://github.com/google/gvisor/blob/master/+/master/pkg/sentry/kernel/fd_table.go
 [inode]: https://github.com/google/gvisor/blob/master/+/master/pkg/sentry/fs/inode.go
 [inode_watches]: https://github.com/google/gvisor/blob/master/+/master/pkg/sentry/fs/inode_inotify.go
 [inotify]: https://github.com/google/gvisor/blob/master/+/master/pkg/sentry/fs/inotify.go
diff --git a/pkg/sentry/fs/proc/BUILD b/pkg/sentry/fs/proc/BUILD
index da41a10ab..70ed854a8 100644
--- a/pkg/sentry/fs/proc/BUILD
+++ b/pkg/sentry/fs/proc/BUILD
@@ -42,7 +42,6 @@ go_library(
         "//pkg/sentry/inet",
         "//pkg/sentry/kernel",
         "//pkg/sentry/kernel/auth",
-        "//pkg/sentry/kernel/kdefs",
         "//pkg/sentry/kernel/time",
         "//pkg/sentry/limits",
         "//pkg/sentry/mm",
diff --git a/pkg/sentry/fs/proc/fds.go b/pkg/sentry/fs/proc/fds.go
index ea7aded9a..bee421d76 100644
--- a/pkg/sentry/fs/proc/fds.go
+++ b/pkg/sentry/fs/proc/fds.go
@@ -25,7 +25,6 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/fs/proc/device"
 	"gvisor.dev/gvisor/pkg/sentry/fs/ramfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
-	"gvisor.dev/gvisor/pkg/sentry/kernel/kdefs"
 	"gvisor.dev/gvisor/pkg/syserror"
 )
 
@@ -42,8 +41,8 @@ func walkDescriptors(t *kernel.Task, p string, toInode func(*fs.File, kernel.FDF
 	var file *fs.File
 	var fdFlags kernel.FDFlags
 	t.WithMuLocked(func(t *kernel.Task) {
-		if fdm := t.FDMap(); fdm != nil {
-			file, fdFlags = fdm.GetDescriptor(kdefs.FD(n))
+		if fdTable := t.FDTable(); fdTable != nil {
+			file, fdFlags = fdTable.Get(int32(n))
 		}
 	})
 	if file == nil {
@@ -56,36 +55,31 @@ func walkDescriptors(t *kernel.Task, p string, toInode func(*fs.File, kernel.FDF
 // toDentAttr callback for each to get a DentAttr, which it then emits. This is
 // a helper for implementing fs.InodeOperations.Readdir.
 func readDescriptors(t *kernel.Task, c *fs.DirCtx, offset int64, toDentAttr func(int) fs.DentAttr) (int64, error) {
-	var fds kernel.FDs
+	var fds []int32
 	t.WithMuLocked(func(t *kernel.Task) {
-		if fdm := t.FDMap(); fdm != nil {
-			fds = fdm.GetFDs()
+		if fdTable := t.FDTable(); fdTable != nil {
+			fds = fdTable.GetFDs()
 		}
 	})
 
-	fdInts := make([]int, 0, len(fds))
-	for _, fd := range fds {
-		fdInts = append(fdInts, int(fd))
-	}
-
-	// Find the fd to start at.
-	idx := sort.SearchInts(fdInts, int(offset))
-	if idx == len(fdInts) {
+	// Find the appropriate starting point.
+	idx := sort.Search(len(fds), func(i int) bool { return fds[i] >= int32(offset) })
+	if idx == len(fds) {
 		return offset, nil
 	}
-	fdInts = fdInts[idx:]
+	fds = fds[idx:]
 
-	var fd int
-	for _, fd = range fdInts {
+	// Serialize all FDs.
+	for _, fd := range fds {
 		name := strconv.FormatUint(uint64(fd), 10)
-		if err := c.DirEmit(name, toDentAttr(fd)); err != nil {
+		if err := c.DirEmit(name, toDentAttr(int(fd))); err != nil {
 			// Returned offset is the next fd to serialize.
 			return int64(fd), err
 		}
 	}
 	// We serialized them all.  Next offset should be higher than last
 	// serialized fd.
-	return int64(fd + 1), nil
+	return int64(fds[len(fds)-1] + 1), nil
 }
 
 // fd implements fs.InodeOperations for a file in /proc/TID/fd/.
@@ -154,9 +148,9 @@ func (f *fd) Close() error {
 type fdDir struct {
 	ramfs.Dir
 
-	// We hold a reference on the task's fdmap but only keep an indirect
-	// task pointer to avoid Dirent loading circularity caused by fdmap's
-	// potential back pointers into the dirent tree.
+	// We hold a reference on the task's FDTable but only keep an indirect
+	// task pointer to avoid Dirent loading circularity caused by the
+	// table's back pointers into the dirent tree.
 	t *kernel.Task
 }
 
diff --git a/pkg/sentry/fs/proc/task.go b/pkg/sentry/fs/proc/task.go
index b2e36aeee..ef0ca3301 100644
--- a/pkg/sentry/fs/proc/task.go
+++ b/pkg/sentry/fs/proc/task.go
@@ -580,8 +580,8 @@ func (s *statusData) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle) (
 	var fds int
 	var vss, rss, data uint64
 	s.t.WithMuLocked(func(t *kernel.Task) {
-		if fdm := t.FDMap(); fdm != nil {
-			fds = fdm.Size()
+		if fdTable := t.FDTable(); fdTable != nil {
+			fds = fdTable.Size()
 		}
 		if mm := t.MemoryManager(); mm != nil {
 			vss = mm.VirtualMemorySize()