Re: [PATCH] vc_screen: break from vcs_read() while loop if vcs_vc() returns NULL

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Mon, Feb 6, 2023 at 9:34 AM George Kennedy <george.kennedy@xxxxxxxxxx> wrote:
>
>
> -               ret = -ENXIO;
>                 vc = vcs_vc(inode, &viewed);
> -               if (!vc)
> +               if (!vc) {
> +                       if (read)
> +                               break;
> +                       ret = -ENXIO;
>                         goto unlock_out;
> +               }

That works, but the whole "if (read)" thing is already done after the
loop, so instead of essentially duplicating that logic, I really think
the patch should be just a plain

                vc = vcs_vc(inode, &viewed);
                if (!vc)
-                       goto unlock_out;
+                       break;

and nothing else.

And yes, the pre-existing vcs_size() error handling has that same ugly pattern.

It might be worth cleaning up too, although right now that

                size = vcs_size(vc, attr, uni_mode);
                if (size < 0) {
                        if (read)
                                break;

pattern means that if we 'break' there, 'read' is non-zero, so 'ret'
doesn't matter. Which is also ugly, but works.

I *think* it could all be rewritten to just use 'break' everywhere in
the loop, and make 'ret' handling be saner.

Something like the attached patch, but while I tried to think about
it, I didn't spend a lot of effort on it, and I certainly didn't test
it. So I'm sending this out as a "Hmm. This _looks_ better to me, but
whatever" patch.

               Linus

 drivers/tty/vt/vc_screen.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index f566eb1839dc..c599b452969f 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -406,19 +406,17 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		ret = -ENXIO;
 		vc = vcs_vc(inode, &viewed);
 		if (!vc)
-			goto unlock_out;
+			break;
 
 		/* Check whether we are above size each round,
 		 * as copy_to_user at the end of this loop
 		 * could sleep.
 		 */
-		size = vcs_size(vc, attr, uni_mode);
-		if (size < 0) {
-			if (read)
-				break;
-			ret = size;
-			goto unlock_out;
-		}
+		ret = vcs_size(vc, attr, uni_mode);
+		if (ret < 0)
+			break;
+		size = ret;
+		ret = 0;
 		if (pos >= size)
 			break;
 		if (count > size - pos)