fs/proc: re-factor proc_pid_cmdline_read() a bit

This is a pure refactoring of the function, preparing for some further
cleanups.  The thing was pretty illegible, and the core functionality
still is, but now the core loop is a bit more isolated from the thing
that goes on around it.

This was "inspired" by the confluence of kworker workqueue name cleanups
by Tejun, currently scheduled for 4.18, and commit 7f7ccc2c ("proc:
do not access cmdline nor environ from file-backed areas").
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -205,11 +205,9 @@ static int proc_root_link(struct dentry *dentry, struct path *path)
 	return result;
 }
 
-static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
-				     size_t _count, loff_t *pos)
+static ssize_t get_mm_cmdline(struct mm_struct *mm, char __user *buf,
+			      size_t _count, loff_t *pos)
 {
-	struct task_struct *tsk;
-	struct mm_struct *mm;
 	char *page;
 	unsigned long count = _count;
 	unsigned long arg_start, arg_end, env_start, env_end;
@@ -218,26 +216,13 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 	char c;
 	ssize_t rv;
 
-	BUG_ON(*pos < 0);
-
-	tsk = get_proc_task(file_inode(file));
-	if (!tsk)
-		return -ESRCH;
-	mm = get_task_mm(tsk);
-	put_task_struct(tsk);
-	if (!mm)
-		return 0;
 	/* Check if process spawned far enough to have cmdline. */
-	if (!mm->env_end) {
-		rv = 0;
-		goto out_mmput;
-	}
+	if (!mm->env_end)
+		return 0;
 
 	page = (char *)__get_free_page(GFP_KERNEL);
-	if (!page) {
-		rv = -ENOMEM;
-		goto out_mmput;
-	}
+	if (!page)
+		return -ENOMEM;
 
 	down_read(&mm->mmap_sem);
 	arg_start = mm->arg_start;
@@ -365,13 +350,42 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 
 out_free_page:
 	free_page((unsigned long)page);
-out_mmput:
-	mmput(mm);
-	if (rv > 0)
-		*pos += rv;
 	return rv;
 }
 
+static ssize_t get_task_cmdline(struct task_struct *tsk, char __user *buf,
+				size_t count, loff_t *pos)
+{
+	struct mm_struct *mm;
+	ssize_t ret;
+
+	mm = get_task_mm(tsk);
+	if (!mm)
+		return 0;
+
+	ret = get_mm_cmdline(mm, buf, count, pos);
+	mmput(mm);
+	return ret;
+}
+
+static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
+				     size_t count, loff_t *pos)
+{
+	struct task_struct *tsk;
+	ssize_t ret;
+
+	BUG_ON(*pos < 0);
+
+	tsk = get_proc_task(file_inode(file));
+	if (!tsk)
+		return -ESRCH;
+	ret = get_task_cmdline(tsk, buf, count, pos);
+	put_task_struct(tsk);
+	if (ret > 0)
+		*pos += ret;
+	return ret;
+}
+
 static const struct file_operations proc_pid_cmdline_ops = {
 	.read	= proc_pid_cmdline_read,
 	.llseek	= generic_file_llseek,