pkg/osutil: use memfd_create on Linux

Mmapping plain files as shared memory leads to extra burden on the
system being fuzzed, because the OS will periodically sync that data to
the disk.

On Linux, use memfd_create to obtain a file handle for the shared memory
region. Experiments have demonstrated than it leads to 5-10% increase in
the fuzzing performance.

3 files changed, 84 insertions, 26 deletions

diff --git a/pkg/osutil/osutil_unix.go b/pkg/osutil/osutil_unix.go
index 0ab5dfd05..33cb09e40 100644
--- a/pkg/osutil/osutil_unix.go
+++ b/pkg/osutil/osutil_unix.go
@@ -96,33 +96,28 @@ func LongPipe() (io.ReadCloser, io.WriteCloser, error) {
 	return r, w, err
 }
 
+// ProcessExitStatus returns process exit status.
+// This is here only because of fuchsia that does not implement WaitStatus.
+func ProcessExitStatus(ps *os.ProcessState) int {
+	return ps.Sys().(syscall.WaitStatus).ExitStatus()
+}
+
 // CreateMemMappedFile creates a temp file with the requested size and maps it into memory.
 func CreateMemMappedFile(size int) (f *os.File, mem []byte, err error) {
-	f, err = ioutil.TempFile("./", "syzkaller-shm")
+	f, err = CreateSharedMemFile(size)
 	if err != nil {
-		err = fmt.Errorf("failed to create temp file: %v", err)
 		return
 	}
 	if err = f.Truncate(int64(size)); err != nil {
-		err = fmt.Errorf("failed to truncate shm file: %v", err)
-		f.Close()
-		os.Remove(f.Name())
-		return
-	}
-	f.Close()
-	fname := f.Name()
-	f, err = os.OpenFile(f.Name(), os.O_RDWR, DefaultFilePerm)
-	if err != nil {
-		err = fmt.Errorf("failed to open shm file: %v", err)
-		os.Remove(fname)
+		err = fmt.Errorf("failed to truncate shared mem file: %v", err)
+		CloseSharedMemFile(f)
 		return
 	}
+
 	mem, err = syscall.Mmap(int(f.Fd()), 0, size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
 	if err != nil {
 		err = fmt.Errorf("failed to mmap shm file: %v", err)
-		f.Close()
-		os.Remove(f.Name())
-		return
+		CloseSharedMemFile(f)
 	}
 	return
 }
@@ -130,22 +125,13 @@ func CreateMemMappedFile(size int) (f *os.File, mem []byte, err error) {
 // CloseMemMappedFile destroys memory mapping created by CreateMemMappedFile.
 func CloseMemMappedFile(f *os.File, mem []byte) error {
 	err1 := syscall.Munmap(mem)
-	err2 := f.Close()
-	err3 := os.Remove(f.Name())
+	err2 := CloseSharedMemFile(f)
 	switch {
 	case err1 != nil:
 		return err1
 	case err2 != nil:
 		return err2
-	case err3 != nil:
-		return err3
 	default:
 		return nil
 	}
 }
-
-// ProcessExitStatus returns process exit status.
-// This is here only because of fuchsia that does not implement WaitStatus.
-func ProcessExitStatus(ps *os.ProcessState) int {
-	return ps.Sys().(syscall.WaitStatus).ExitStatus()
-}
diff --git a/pkg/osutil/sharedmem_file.go b/pkg/osutil/sharedmem_file.go
new file mode 100644
index 000000000..1e969cdba
--- /dev/null
+++ b/pkg/osutil/sharedmem_file.go
@@ -0,0 +1,42 @@
+// Copyright 2021 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+//go:build freebsd || netbsd || openbsd || darwin
+// +build freebsd netbsd openbsd darwin
+
+package osutil
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+)
+
+func CreateSharedMemFile(size int) (f *os.File, err error) {
+	f, err = ioutil.TempFile("./", "syzkaller-shm")
+	if err != nil {
+		err = fmt.Errorf("failed to create temp file: %v", err)
+		return
+	}
+	f.Close()
+	fname := f.Name()
+	f, err = os.OpenFile(f.Name(), os.O_RDWR, DefaultFilePerm)
+	if err != nil {
+		err = fmt.Errorf("failed to open shm file: %v", err)
+		os.Remove(fname)
+	}
+	return
+}
+
+func CloseSharedMemFile(f *os.File) error {
+	err1 := f.Close()
+	err2 := os.Remove(f.Name())
+	switch {
+	case err1 != nil:
+		return err1
+	case err2 != nil:
+		return err2
+	default:
+		return nil
+	}
+}
diff --git a/pkg/osutil/sharedmem_memfd.go b/pkg/osutil/sharedmem_memfd.go
new file mode 100644
index 000000000..bdcea486f
--- /dev/null
+++ b/pkg/osutil/sharedmem_memfd.go
@@ -0,0 +1,30 @@
+// Copyright 2021 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+//go:build linux
+// +build linux
+
+package osutil
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// In the case of Linux, we can just use the memfd_create syscall.
+func CreateSharedMemFile(size int) (f *os.File, err error) {
+	// The name is actually irrelevant and can even be the same for all such files.
+	fd, err := unix.MemfdCreate("syz-shared-mem", 0)
+	if err != nil {
+		err = fmt.Errorf("failed to do memfd_create: %v", err)
+		return
+	}
+	f = os.NewFile(uintptr(fd), fmt.Sprintf("/proc/self/fd/%d", fd))
+	return
+}
+
+func CloseSharedMemFile(f *os.File) error {
+	return f.Close()
+}