linux-kernel - Re: [PATCH] tty: vt: keyboard: Fix uninitialized variables in vt_do_kdgkb

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Message-ID: <641cb805-b279-48af-a3a9-492a8738c841@gmail.com>
Date: Fri, 11 Apr 2025 18:48:13 +0530
From: Purva Yeshi <purvayeshi550@...il.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: jirislaby@...nel.org, tglx@...utronix.de, hdegoede@...hat.com,
 mingo@...nel.org, linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH] tty: vt: keyboard: Fix uninitialized variables in
 vt_do_kdgkb_ioctl

On 11/04/25 16:58, Greg KH wrote:
> On Fri, Apr 11, 2025 at 04:45:48PM +0530, Purva Yeshi wrote:
>> Fix Smatch-detected issue:
>>
>> drivers/tty/vt/keyboard.c:2106 vt_do_kdgkb_ioctl() error:
>> uninitialized symbol 'kbs'.
>> drivers/tty/vt/keyboard.c:2108 vt_do_kdgkb_ioctl() error:
>> uninitialized symbol 'ret'.
>>
>> Fix uninitialized variable warnings reported by Smatch in
>> vt_do_kdgkb_ioctl(). The variables kbs and ret were used in the kfree
>> and return statements without guaranteed initialization paths, leading to
>> potential undefined behavior or false positives during static analysis.
>>
>> Initialize char *kbs to NULL and int ret to -EINVAL at declaration.
>> This ensures safe use of kfree(kbs) and return ret regardless of control
>> flow. Also add a default case in the switch to preserve fallback behavior.
> 
> When you say "also" in a patch, that is a HUGE flag that this should be
> split up into a separate change.  Please do that here, don't mix changes
> that have nothing to do with each other together into one.
> 
> Also, why isn't the compilers noticing that these are uninitialized
> variables?  Are you sure the warning is correct?
> 
> thanks,
> 
> greg k-h

Hi Greg,

Thank you for the feedback.

Got it. I will remove the default case from this patch and resend it 
with only the fix for the uninitialized variables.

Yes, Smatch reports uninitialized variable warnings for kbs and ret 
because, in the function vt_do_kdgkb_ioctl(), both variables are used 
outside the switch block but are only initialized conditionally within 
certain case branches. If the cmd value passed to the function does not 
match any of the explicitly handled cases (KDGKBSENT or KDSKBSENT), then 
the switch body is skipped entirely. In such a scenario, kbs remains 
uninitialized, yet kfree(kbs) is still called, which could result in 
undefined behavior.

Similarly, ret is returned at the end of the function even though it may 
not have been assigned a value, leading to unpredictable results.

Best regards,
Purva