linux-user: Use qemu_set_cloexec() to mark pidfd as FD_CLOEXEC

In the linux-user do_fork() function we try to set the FD_CLOEXEC
flag on a pidfd like this:

    fcntl(pid_fd, F_SETFD, fcntl(pid_fd, F_GETFL) | FD_CLOEXEC);

This has two problems:
 (1) it doesn't check errors, which Coverity complains about
 (2) we use F_GETFL when we mean F_GETFD

Deal with both of these problems by using qemu_set_cloexec() instead.
That function will assert() if the fcntls fail, which is fine (we are
inside fork_start()/fork_end() so we know nothing can mess around
with our file descriptors here, and we just got this one from
pidfd_open()).

(As we are touching the if() statement here, we correct the
indentation.)

Coverity: CID 1508111
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Message-ID: <[email protected]>
(cherry picked from commit d6390204c61e148488f034d1f79be35cd3318d93)
Signed-off-by: Michael Tokarev <[email protected]>

Author: pm215

linux-user/syscall.c

@@ -6746,10 +6746,9 @@ static int do_fork(CPUArchState *env, unsigned int flags, abi_ulong newsp,
                 int pid_child = ret;
                 pid_fd = pidfd_open(pid_child, 0);
                 if (pid_fd >= 0) {
-                        fcntl(pid_fd, F_SETFD, fcntl(pid_fd, F_GETFL)
-                                               | FD_CLOEXEC);
+                    qemu_set_cloexec(pid_fd);
                 } else {
-                        pid_fd = 0;
+                    pid_fd = 0;
                 }
 #endif
                 put_user_u32(pid_fd, parent_tidptr);