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

[Jiri Slaby]

On 06. 11. 21, 21:48, Pavel Skripkin wrote:
> On 11/6/21 23:44, Ajay Garg wrote:
> > > >
> > > > 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.
>
> I fully agree here, it's maintainer's choice. Let's sit down and wait 
> what experienced people thing about this :)

I don't quite understand what the problem is. Provided I wrote the code, 
is there something wrong with this commit (and its explanation), in 
particular?

commit 6ca03f90527e499dd5e32d6522909e2ad390896b
Author: Jiri Slaby <jirislaby@xxxxxxxxxx>
Date:   Mon Oct 19 10:55:16 2020 +0200

    vt: keyboard, simplify vt_kdgkbsent

    Use 'strlen' of the string, add one for NUL terminator and simply do
    'copy_to_user' instead of the explicit 'for' loop. This makes the
    KDGKBSENT case more compact.

    The only thing we need to take care about is NULL 'func_table[i]'. Use
    an empty string in that case.

    The original check for overflow could never trigger as the func_buf
    strings are always shorter or equal to 'struct kbsentry's.

thanks,
--
js
suse labs