Encode stat to bytes manually, instead of calling CopyObjectOut.

CopyObjectOut grows its destination byte slice incrementally, causing
many small slice allocations on the heap. This leads to increased GC and
noticeably slower stat calls.

PiperOrigin-RevId: 233140904
Change-Id: Ieb90295dd8dd45b3e56506fef9d7f86c92e97d97

2 files changed, 50 insertions, 15 deletions

diff --git a/pkg/abi/linux/file.go b/pkg/abi/linux/file.go
index ae33f4a4d..e5a51a9fd 100644
--- a/pkg/abi/linux/file.go
+++ b/pkg/abi/linux/file.go
@@ -19,6 +19,7 @@ import (
 	"strings"
 
 	"gvisor.googlesource.com/gvisor/pkg/abi"
+	"gvisor.googlesource.com/gvisor/pkg/binary"
 )
 
 // Constants for open(2).
@@ -177,6 +178,9 @@ type Stat struct {
 	X_unused [3]int64
 }
 
+// SizeOfStat is the size of a Stat struct.
+var SizeOfStat = binary.Size(Stat{})
+
 // FileMode represents a mode_t.
 type FileMode uint
 
diff --git a/pkg/sentry/syscalls/linux/sys_stat.go b/pkg/sentry/syscalls/linux/sys_stat.go
index 9c433c45d..95f161aac 100644
--- a/pkg/sentry/syscalls/linux/sys_stat.go
+++ b/pkg/sentry/syscalls/linux/sys_stat.go
@@ -16,6 +16,7 @@ package linux
 
 import (
 	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+	"gvisor.googlesource.com/gvisor/pkg/binary"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/fs"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel"
@@ -124,21 +125,51 @@ func stat(t *kernel.Task, d *fs.Dirent, dirPath bool, statAddr usermem.Addr) err
 		mode |= linux.ModeSocket
 	}
 
-	_, err = t.CopyOut(statAddr, linux.Stat{
-		Dev:     uint64(d.Inode.StableAttr.DeviceID),
-		Rdev:    uint64(linux.MakeDeviceID(d.Inode.StableAttr.DeviceFileMajor, d.Inode.StableAttr.DeviceFileMinor)),
-		Ino:     uint64(d.Inode.StableAttr.InodeID),
-		Nlink:   uattr.Links,
-		Mode:    mode | uint32(uattr.Perms.LinuxMode()),
-		UID:     uint32(uattr.Owner.UID.In(t.UserNamespace()).OrOverflow()),
-		GID:     uint32(uattr.Owner.GID.In(t.UserNamespace()).OrOverflow()),
-		Size:    uattr.Size,
-		Blksize: d.Inode.StableAttr.BlockSize,
-		Blocks:  uattr.Usage / 512,
-		ATime:   uattr.AccessTime.Timespec(),
-		MTime:   uattr.ModificationTime.Timespec(),
-		CTime:   uattr.StatusChangeTime.Timespec(),
-	})
+	// We encode the stat struct to bytes manually, as stat() is a very
+	// common syscall for many applications, and t.CopyObjectOut has
+	// noticeable performance impact due to its many slice allocations and
+	// use of reflection.
+	b := make([]byte, 0, linux.SizeOfStat)
+
+	// Dev (uint64)
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(d.Inode.StableAttr.DeviceID))
+	// Ino (uint64)
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(d.Inode.StableAttr.InodeID))
+	// Nlink (uint64)
+	b = binary.AppendUint64(b, usermem.ByteOrder, uattr.Links)
+	// Mode (uint32)
+	b = binary.AppendUint32(b, usermem.ByteOrder, mode|uint32(uattr.Perms.LinuxMode()))
+	// UID (uint32)
+	b = binary.AppendUint32(b, usermem.ByteOrder, uint32(uattr.Owner.UID.In(t.UserNamespace()).OrOverflow()))
+	// GID (uint32)
+	b = binary.AppendUint32(b, usermem.ByteOrder, uint32(uattr.Owner.GID.In(t.UserNamespace()).OrOverflow()))
+	// Padding (uint32)
+	b = binary.AppendUint32(b, usermem.ByteOrder, 0)
+	// Rdev (uint64)
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(linux.MakeDeviceID(d.Inode.StableAttr.DeviceFileMajor, d.Inode.StableAttr.DeviceFileMinor)))
+	// Size (uint64)
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(uattr.Size))
+	// Blksize (uint64)
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(d.Inode.StableAttr.BlockSize))
+	// Blocks (uint64)
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(uattr.Usage/512))
+
+	// ATime
+	atime := uattr.AccessTime.Timespec()
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(atime.Sec))
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(atime.Nsec))
+
+	// MTime
+	mtime := uattr.ModificationTime.Timespec()
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(mtime.Sec))
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(mtime.Nsec))
+
+	// CTime
+	ctime := uattr.StatusChangeTime.Timespec()
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(ctime.Sec))
+	b = binary.AppendUint64(b, usermem.ByteOrder, uint64(ctime.Nsec))
+
+	_, err = t.CopyOutBytes(statAddr, b)
 	return err
 }