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.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/fcntl/fcntl.c b/src/fcntl/fcntl.c
index fa5ad32f..fb7806a3 100644
--- a/src/fcntl/fcntl.c
+++ b/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);
 }