proper error handling for fcntl F_GETOWN on modern kernels

on old kernels, there's no way to detect errors; we must assume
negative syscall return values are pgrp ids. but if the F_GETOWN_EX
fcntl works, we can get a reliable answer.

arch/arm/bits/fcntl.h

@@ -31,3 +31,6 @@
 #define F_GETLK 12
 #define F_SETLK 13
 #define F_SETLKW 14
+
+#define F_SETOWN_EX 15
+#define F_GETOWN_EX 16

arch/i386/bits/fcntl.h

@@ -31,3 +31,6 @@
 #define F_GETLK 12
 #define F_SETLK 13
 #define F_SETLKW 14
+
+#define F_SETOWN_EX 15
+#define F_GETOWN_EX 16

arch/x86_64/bits/fcntl.h

@@ -31,3 +31,6 @@
 #define F_GETLK 5
 #define F_SETLK 6
 #define F_SETLKW 7
+
+#define F_SETOWN_EX 15
+#define F_GETOWN_EX 16

include/fcntl.h

@@ -99,6 +99,17 @@ int posix_fallocate(int, off_t, off_t);
 int lockf(int, int, off_t);
 #endif
 
+#if defined(_GNU_SOURCE)
+#define F_OWNER_TID 0
+#define F_OWNER_PID 1
+#define F_OWNER_PGRP 2
+#define F_OWNER_GID 2
+struct f_owner_ex {
+	int type;
+	pid_t pid;
+};
+#endif
+
 #if defined(_LARGEFILE64_SOURCE) || defined(_GNU_SOURCE)
 #define open64 open
 #define openat64 openat

src/fcntl/fcntl.c

@@ -1,6 +1,8 @@
+#define _GNU_SOURCE
 #include <fcntl.h>
 #include <unistd.h>
 #include <stdarg.h>
+#include <errno.h>
 #include "syscall.h"
 #include "libc.h"
 
@@ -13,6 +15,12 @@ int fcntl(int fd, int cmd, ...)
 	va_end(ap);
 	if (cmd == F_SETFL) arg |= O_LARGEFILE;
 	if (cmd == F_SETLKW) return syscall_cp(SYS_fcntl, fd, cmd, arg);
-	if (cmd == F_GETOWN) return __syscall(SYS_fcntl, fd, cmd, arg);
+	if (cmd == F_GETOWN) {
+		struct f_owner_ex ex;
+		int ret = __syscall(SYS_fcntl, fd, F_GETOWN_EX, &ex);
+		if (ret == -EINVAL) return __syscall(SYS_fcntl, fd, cmd, arg);
+		if (ret) return __syscall_ret(ret);
+		return ex.type == F_OWNER_PGRP ? -ex.pid : ex.pid;
+	}
 	return syscall(SYS_fcntl, fd, cmd, arg);
 }