umh: Capture the pid in umh_pipe_setup

The pid in struct subprocess_info is only used by umh_clean_and_save_pid to
write the pid into umh_info.

Instead always capture the pid on struct umh_info in umh_pipe_setup, removing
code that is specific to user mode drivers from the common user path of
user mode helpers.

v1: https://lkml.kernel.org/r/87h7uygf9i.fsf_-_@x220.int.ebiederm.org
v2: https://lkml.kernel.org/r/875zb97iix.fsf_-_@x220.int.ebiederm.org
Link: https://lkml.kernel.org/r/20200702164140.4468-1-ebiederm@xmission.comReviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/umh.h

@@ -25,7 +25,6 @@ struct subprocess_info {
 	struct file *file;
 	int wait;
 	int retval;
-	pid_t pid;
 	int (*init)(struct subprocess_info *info, struct cred *new);
 	void (*cleanup)(struct subprocess_info *info);
 	void *data;

kernel/umh.c

@@ -102,7 +102,6 @@ static int call_usermodehelper_exec_async(void *data)
 
 	commit_creds(new);
 
-	sub_info->pid = task_pid_nr(current);
 	if (sub_info->file) {
 		retval = do_execve_file(sub_info->file,
 					sub_info->argv, sub_info->envp);
@@ -468,6 +467,7 @@ static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
 
 	umh_info->pipe_to_umh = to_umh[1];
 	umh_info->pipe_from_umh = from_umh[0];
+	umh_info->pid = task_pid_nr(current);
 	return 0;
 }
 
@@ -476,13 +476,12 @@ static void umh_clean_and_save_pid(struct subprocess_info *info)
 	struct umh_info *umh_info = info->data;
 
 	/* cleanup if umh_pipe_setup() was successful but exec failed */
-	if (info->pid && info->retval) {
+	if (info->retval) {
 		fput(umh_info->pipe_to_umh);
 		fput(umh_info->pipe_from_umh);
 	}
 
 	argv_free(info->argv);
-	umh_info->pid = info->pid;
 }
 
 /**