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

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Message-ID: <6b58a3e1-f2ea-cc4c-03b2-06334b559373@gmail.com>
Date:   Sat, 6 Nov 2021 23:18:27 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Ajay Garg <ajaygargnsit@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>, jirislaby@...nel.org,
        kernel@...il.dk, David Laight <David.Laight@...lab.com>
Cc:     "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 11/6/21 22:46, Ajay Garg wrote:
> Actually, on further thoughts, even David's solution will require an
> extra check, if -E2BIG is returned.
> 
> So, I guess the solution suggested by me looks the best
> (https://lore.kernel.org/linux-serial/868025b485b94480ad17d0ec971b3ee9@AcuMS.aculab.com/T/#m1c4aaa4347b02fd4c11ce611ff5029fcb71c37a1)
> :
> 
> 1.
> == Do not use the return value from strlcpy. ==
> 
>                  len = strlcpy(kbs, func_table[kb_func] ? : "", len);
> =>
>                  strlcpy(kbs, func_table[kb_func] ? : "", len);
> 
> 
> 2.
> == Calculate the actual length of kbs, add 1, and then copy those many
> bytes to user-buffer ==
> 
> ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
>                          -EFAULT : 0;
> =>
> ret = copy_to_user(user_kdgkb->kb_string, kbs, strlen(kbs) + 1) ?
>                          -EFAULT : 0;
> 

But isn't strlen(kbs) is guaranteed to be equal to strlcpy() return 
value in this case? As I said in previous emails,
strlen(func_table[kb_func]) < sizeof(user_kdgkb->kb_string) by design of 
this function.

Do we need extra strlen() call here? Let's see what more experienced 
people think about it :)



With regards,
Pavel Skripkin