linux-kernel - Re: [PATCH] input/vmmouse: Fix device name copies

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Message-Id: <d180f06b-64b0-4885-9794-5127c297a0f0@app.fastmail.com>
Date:   Sun, 03 Dec 2023 21:50:51 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Dmitry Torokhov" <dmitry.torokhov@...il.com>,
        "Zack Rusin" <zackr@...are.com>
Cc:     linux-kernel@...r.kernel.org,
        "VMware Graphics Reviewers" <linux-graphics-maintainer@...are.com>,
        "Robert Jarzmik" <robert.jarzmik@...e.fr>,
        "Raul Rangel" <rrangel@...omium.org>, linux-input@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] input/vmmouse: Fix device name copies

On Sun, Dec 3, 2023, at 19:41, Dmitry Torokhov wrote:
> On Mon, Nov 27, 2023 at 03:42:06PM -0500, Zack Rusin wrote:
>> From: Zack Rusin <zackr@...are.com>
>> 
>> Make sure vmmouse_data::phys can hold serio::phys (which is 32 bytes)
>> plus an extra string, extend it to 64.
>> 
>> Fixes gcc13 warnings:
>> drivers/input/mouse/vmmouse.c: In function ‘vmmouse_init’:
>> drivers/input/mouse/vmmouse.c:455:53: warning: ‘/input1’ directive output may be truncated writing 7 bytes into a region of size between 1 and 32 [-Wformat-truncation=]
>>   455 |         snprintf(priv->phys, sizeof(priv->phys), "%s/input1",
>>       |                                                     ^~~~~~~
>> drivers/input/mouse/vmmouse.c:455:9: note: ‘snprintf’ output between 8 and 39 bytes into a destination of size 32
>>   455 |         snprintf(priv->phys, sizeof(priv->phys), "%s/input1",
>>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>   456 |                  psmouse->ps2dev.serio->phys);
>>       |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> This simply wastes 32 bytes. It is perfectly fine to truncate phys
> (which does not happen in real life).
>
> -Wformat-truncation is disabled in normal builds, folks should stop
> using it with W=1 as well.

It does find real bugs, and we are fairly close to being able
to enable it by default once the remaining warnings are all
fixed.

It also doesn't waste any memory in this specific case since
vmmouse_data is currently at 168 bytes, which gets rounded
up to either 192 or 256 bytes anyway. I'd suggest using
the minimum size that is large enough though, in this case
39 bytes for the string I guess.

     Arnd