linux-kernel - Re: [PATCH] tty: vt: keyboard: do not copy an extra-byte in copy_to

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Message-ID: <CAHP4M8XtFiAa1kF5A_rPbcui3DP8L6iyfP8GbwgLLzo0Bo+TNQ@mail.gmail.com>
Date:   Sun, 7 Nov 2021 02:14:36 +0530
From:   Ajay Garg <ajaygargnsit@...il.com>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>, jirislaby@...nel.org,
        kernel@...il.dk, David Laight <David.Laight@...lab.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tty: vt: keyboard: do not copy an extra-byte in copy_to_user

> >
> > That's the whole point of the discussion :)
> >
> > The method "vt_do_kdgkb_ioctl" does not manage "func_table[kb_func]".
> > Thus, the method does not know whether or not
> > strlen(func_table[kb_func]) < sizeof(user_kdgkb->kb_string).
> >
>
> It manages. The code under `case KDSKBSENT:` sets func_table[] entries
> via vt_kdskbsent().
>
> kbs = strndup_user(..., sizeof(user_kdgkb->kb_string));
>
> is used to allocate buffer for the func_table[] entry. That's my main
> point :)

func_table is set in vt_kdskbent, which itself is external.

More importantly, vt_kdskbent is handled in case KDSKBSENT:, while the
strlcpy issue we are dealing with is in case KDGKBSENT:
In case KDGKBSENT, following are managed :

                ssize_t len = sizeof(user_kdgkb->kb_string);
                kbs = kmalloc(len, GFP_KERNEL);

while func_table[kb_func] is external entity here, so no assumption
ought to be made for it, just my 2 cents though :)

Anyhow, really, it is the maintainers' choice now :), since there
isn't a burning (compilation/runtime) issue.

>
>
>
>
> With regards,
> Pavel Skripkin