syscall, internal/poll: fall back to accept on linux-arm

Our minimum Linux version is 2.6.32, and the accept4 system call was
introduced in 2.6.28, so we use accept4 everywhere. Unfortunately,
it turns out that the accept4 system call was only added to
linux-arm in 2.6.36, so for linux-arm only we need to try the accept4
system call and then fall back to accept if it doesn't work.

The code we use on linux-arm is the code we used in Go 1.17.
On non-arm platforms we continue using the simpler code introduced
in Go 1.18.

Adding accept4 to the ARM Linux kernel was:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=21d93e2e29722d7832f61cc56d73fb953ee6578e

Fixes #57333

Change-Id: I6680cb54dd4d3514a6887dda8906e6708c64459d
Reviewed-on: https://go-review.googlesource.com/c/go/+/457995
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/internal/poll/sock_cloexec.go

@@ -5,7 +5,7 @@
 // This file implements accept for platforms that provide a fast path for
 // setting SetNonblock and CloseOnExec.
 
-//go:build dragonfly || freebsd || linux || netbsd || openbsd || solaris
+//go:build dragonfly || freebsd || (linux && !arm) || netbsd || openbsd || solaris
 
 package poll
 

src/internal/poll/sock_cloexec_accept.go

@@ -0,0 +1,51 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements accept for platforms that provide a fast path for
+// setting SetNonblock and CloseOnExec, but don't necessarily have accept4.
+// This is the code we used for accept in Go 1.17 and earlier.
+// On Linux the accept4 system call was introduced in 2.6.28 kernel,
+// and our minimum requirement is 2.6.32, so we simplified the function.
+// Unfortunately, on ARM accept4 wasn't added until 2.6.36, so for ARM
+// only we continue using the older code.
+
+//go:build linux && arm
+
+package poll
+
+import "syscall"
+
+// Wrapper around the accept system call that marks the returned file
+// descriptor as nonblocking and close-on-exec.
+func accept(s int) (int, syscall.Sockaddr, string, error) {
+	ns, sa, err := Accept4Func(s, syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC)
+	switch err {
+	case nil:
+		return ns, sa, "", nil
+	default: // errors other than the ones listed
+		return -1, sa, "accept4", err
+	case syscall.ENOSYS: // syscall missing
+	case syscall.EINVAL: // some Linux use this instead of ENOSYS
+	case syscall.EACCES: // some Linux use this instead of ENOSYS
+	case syscall.EFAULT: // some Linux use this instead of ENOSYS
+	}
+
+	// See ../syscall/exec_unix.go for description of ForkLock.
+	// It is probably okay to hold the lock across syscall.Accept
+	// because we have put fd.sysfd into non-blocking mode.
+	// However, a call to the File method will put it back into
+	// blocking mode. We can't take that risk, so no use of ForkLock here.
+	ns, sa, err = AcceptFunc(s)
+	if err == nil {
+		syscall.CloseOnExec(ns)
+	}
+	if err != nil {
+		return -1, nil, "accept", err
+	}
+	if err = syscall.SetNonblock(ns, true); err != nil {
+		CloseFunc(ns)
+		return -1, nil, "setnonblock", err
+	}
+	return ns, sa, "", nil
+}

src/syscall/syscall_linux.go

@@ -643,21 +643,6 @@ func anyToSockaddr(rsa *RawSockaddrAny) (Sockaddr, error) {
 	return nil, EAFNOSUPPORT
 }
 
-func Accept(fd int) (nfd int, sa Sockaddr, err error) {
-	var rsa RawSockaddrAny
-	var len _Socklen = SizeofSockaddrAny
-	nfd, err = accept4(fd, &rsa, &len, 0)
-	if err != nil {
-		return
-	}
-	sa, err = anyToSockaddr(&rsa)
-	if err != nil {
-		Close(nfd)
-		nfd = 0
-	}
-	return
-}
-
 func Accept4(fd int, flags int) (nfd int, sa Sockaddr, err error) {
 	var rsa RawSockaddrAny
 	var len _Socklen = SizeofSockaddrAny

src/syscall/syscall_linux_accept.go

@@ -0,0 +1,34 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// We require Linux kernel version 2.6.32. The accept4 system call was
+// added in version 2.6.28, so in general we can use accept4.
+// Unfortunately, for ARM only, accept4 was added in version 2.6.36.
+// Handle that case here, by using a copy of the Accept function that
+// we used in Go 1.17.
+
+//go:build linux && arm
+
+package syscall
+
+//sys	accept(s int, rsa *RawSockaddrAny, addrlen *_Socklen) (fd int, err error)
+
+func Accept(fd int) (nfd int, sa Sockaddr, err error) {
+	var rsa RawSockaddrAny
+	var len _Socklen = SizeofSockaddrAny
+	// Try accept4 first for Android and newer kernels.
+	nfd, err = accept4(fd, &rsa, &len, 0)
+	if err == ENOSYS {
+		nfd, err = accept(fd, &rsa, &len)
+	}
+	if err != nil {
+		return
+	}
+	sa, err = anyToSockaddr(&rsa)
+	if err != nil {
+		Close(nfd)
+		nfd = 0
+	}
+	return
+}

src/syscall/syscall_linux_accept4.go

@@ -0,0 +1,25 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file provides the Accept function used on all systems
+// other than arm. See syscall_linux_accept.go for why.
+
+//go:build linux && !arm
+
+package syscall
+
+func Accept(fd int) (nfd int, sa Sockaddr, err error) {
+	var rsa RawSockaddrAny
+	var len _Socklen = SizeofSockaddrAny
+	nfd, err = accept4(fd, &rsa, &len, 0)
+	if err != nil {
+		return
+	}
+	sa, err = anyToSockaddr(&rsa)
+	if err != nil {
+		Close(nfd)
+		nfd = 0
+	}
+	return
+}