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Date:   Sat, 6 Nov 2021 23:18:27 +0300
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Subject: Re: [PATCH] tty: vt: keyboard: do not copy an extra-byte in
 copy_to_user
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To:     Ajay Garg <ajaygargnsit@gmail.com>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>, jirislaby@kernel.org,
        kernel@esmil.dk, David Laight <David.Laight@aculab.com>
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20211106092041.43745-1-ajaygargnsit@gmail.com>
 <9eafae1f-d9f0-298d-cf20-212865d0becc@gmail.com>
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From:   Pavel Skripkin <paskripkin@gmail.com>
In-Reply-To: <CAHP4M8UcZ=ttB8jbN1yOY6YH8SiQ27NhdEKi9SDH1CWG-GY6eg@mail.gmail.com>
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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