libbpf: call dup2() syscall directly

We've ran into issues with using dup2() API in production setting, where
libbpf is linked into large production environment and ends up calling
unintended custom implementations of dup2(). These custom implementations
don't provide atomic FD replacement guarantees of dup2() syscall,
leading to subtle and hard to debug issues.

To prevent this in the future and guarantee that no libc implementation
will do their own custom non-atomic dup2() implementation, call dup2()
syscall directly with syscall(SYS_dup2).

Note that some architectures don't seem to provide dup2 and have dup3
instead. Try to detect and pick best syscall.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/lib/bpf/libbpf_internal.h

···
 #include <linux/err.h>
 #include <fcntl.h>
 #include <unistd.h>
+#include <sys/syscall.h>
 #include <libelf.h>
 #include "relo_core.h"
 
···
 	return fd;
 }
 
+static inline int sys_dup2(int oldfd, int newfd)
+{
+#ifdef __NR_dup2
+	return syscall(__NR_dup2, oldfd, newfd);
+#else
+	return syscall(__NR_dup3, oldfd, newfd, 0);
+#endif
+}
+
 /* Point *fixed_fd* to the same file that *tmp_fd* points to.
  * Regardless of success, *tmp_fd* is closed.
  * Whatever *fixed_fd* pointed to is closed silently.
···
 {
 	int err;
 
-	err = dup2(tmp_fd, fixed_fd);
+	err = sys_dup2(tmp_fd, fixed_fd);
 	err = err < 0 ? -errno : 0;
 	close(tmp_fd); /* clean up temporary FD */
 	return err;