syscall: use Android O friendly fstatat syscall to implement Stat on linux/amd64

The Android O seccomp policy disallows the stat syscall on amd64, see
https://android.googlesource.com/platform/bionic/+/android-4.2.2_r1.2/libc/SYSCALLS.TXT

Use the fstatat syscall with AT_FDCWD and zero flags instead to achieve
the same behavior.

Fixes #24403

Change-Id: I36fc9ec9bc938cd8e9de30f66c0eb9d2e24debf6
Reviewed-on: https://go-review.googlesource.com/100878
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/syscall_linux_amd64.go

@@ -39,7 +39,6 @@ const (
 //sysnb	Setreuid(ruid int, euid int) (err error)
 //sys	Shutdown(fd int, how int) (err error)
 //sys	Splice(rfd int, roff *int64, wfd int, woff *int64, len int, flags int) (n int64, err error)
-//sys	Stat(path string, stat *Stat_t) (err error)
 //sys	Statfs(path string, buf *Statfs_t) (err error)
 //sys	SyncFileRange(fd int, off int64, n int64, flags int) (err error)
 //sys	Truncate(path string, length int64) (err error)
@@ -47,6 +46,7 @@ const (
 //sys	accept4(s int, rsa *RawSockaddrAny, addrlen *_Socklen, flags int) (fd int, err error)
 //sys	bind(s int, addr unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	connect(s int, addr unsafe.Pointer, addrlen _Socklen) (err error)
+//sys	fstatat(fd int, path string, stat *Stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sysnb	getgroups(n int, list *_Gid_t) (nn int, err error)
 //sysnb	setgroups(n int, list *_Gid_t) (err error)
 //sys	getsockopt(s int, level int, name int, val unsafe.Pointer, vallen *_Socklen) (err error)
@@ -61,6 +61,10 @@ const (
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flags int, fd int, offset int64) (xaddr uintptr, err error)
 
+func Stat(path string, stat *Stat_t) (err error) {
+	return fstatat(_AT_FDCWD, path, stat, 0)
+}
+
 //go:noescape
 func gettimeofday(tv *Timeval) (err Errno)
 

src/syscall/zsyscall_linux_amd64.go

@@ -1476,21 +1476,6 @@ func Splice(rfd int, roff *int64, wfd int, woff *int64, len int, flags int) (n i
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func Stat(path string, stat *Stat_t) (err error) {
-	var _p0 *byte
-	_p0, err = BytePtrFromString(path)
-	if err != nil {
-		return
-	}
-	_, _, e1 := Syscall(SYS_STAT, uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), 0)
-	if e1 != 0 {
-		err = errnoErr(e1)
-	}
-	return
-}
-
-// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
-
 func Statfs(path string, buf *Statfs_t) (err error) {
 	var _p0 *byte
 	_p0, err = BytePtrFromString(path)
@@ -1573,6 +1558,21 @@ func connect(s int, addr unsafe.Pointer, addrlen _Socklen) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(fd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_NEWFSTATAT, uintptr(fd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func getgroups(n int, list *_Gid_t) (nn int, err error) {
 	r0, _, e1 := RawSyscall(SYS_GETGROUPS, uintptr(n), uintptr(unsafe.Pointer(list)), 0)
 	nn = int(r0)