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

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Message-ID: <08c9e717-4367-5316-87cd-90b5ceb13ed9@kernel.org>
Date:   Wed, 10 Nov 2021 06:22:21 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Pavel Skripkin <paskripkin@...il.com>,
        Ajay Garg <ajaygargnsit@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Greg KH <gregkh@...uxfoundation.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

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@...nel.org>
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