linux-kernel - Re: [PATCH v2] vc_screen: modify vcs_size() handling in vcs

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Date:   Tue, 28 Feb 2023 02:18:16 +0000
From:   Thomas Weißschuh <linux@...ssschuh.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     George Kennedy <george.kennedy@...cle.com>, jirislaby@...nel.org,
        gregkh@...uxfoundation.org, sfr@...b.auug.org.au,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org, regressions@...ts.linux.dev
Subject: Re: [PATCH v2] vc_screen: modify vcs_size() handling in vcs_read()

On Mon, Feb 27, 2023 at 05:58:12PM -0800, Linus Torvalds wrote:
> On Mon, Feb 27, 2023 at 5:46 PM <linux@...ssschuh.net> wrote:
> >
> > This still seems to be broken for me.
> 
> Looks that way.
> 
> > I still need the patch from
> >
> > https://lore.kernel.org/lkml/20230220064612.1783-1-linux@weissschuh.net/
> 
> .. and that has the same problem with "what if the error happens
> during an iteration that wasn't the first, and we already succeeded
> partially".

Indeed.

> The "goto unlock_out" is bogus, since it jumps over all the "update
> pos and check if we read something".
> 
> It was the correct thing to do *above* the loop, but not inside the loop.
> 
> IOW, I think the proper patch is to also turn the "goto unlock_out"
> into a "break". Mind testing something like this (whitespace-damaged,
> but you get the idea):

Makes sense and seems to work correctly.

Tested-By: Thomas Weißschuh <linux@...ssschuh.net>

(Or feel free to use my patch from above and fixup the goto/break line)

> 
>     --- a/drivers/tty/vt/vc_screen.c
>     +++ b/drivers/tty/vt/vc_screen.c
>     @@ -403,10 +403,11 @@ vcs_read(struct file *file, char __user
> *buf, size_t count, loff_t *ppos)
>                 unsigned int this_round, skip = 0;
>                 int size;
> 
>     -           ret = -ENXIO;
>                 vc = vcs_vc(inode, &viewed);
>     -           if (!vc)
>     -                   goto unlock_out;
>     +           if (!vc) {
>     +                   ret = -ENXIO;
>     +                   break;
>     +           }
> 
>                 /* Check whether we are above size each round,
>                  * as copy_to_user at the end of this loop
> 
> which hopefully really fixes this (at least I don't see any other
> "goto unlock_out" cases).
> 
>               Linus