RE: [PATCH] input/vmmouse: Fix device name copies

[David Laight <David.Laight@ACULAB.COM>]

From: Arnd Bergmann
> Sent: 03 December 2023 20:51
> 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@vmware.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.

That rather depends on whether any of the earlier char[] lengths
have been rounded up to a 'nice' value.

I'd also have thought that dangerous overflows would come from
unbounded %s formats, not fixed size strings or integers that are
always small.

There really ought to be a sane method of telling gcc not to bleat
about snprintf() potentially overflowing the target.

I've tried a few thing but none of them work.
IIRC using the result (in some ways) is enough, but neither
(void)snprintf(...); or if (snprintf(...)); is enough
(but they 'fix' 'warn unused result').

	David

> 
>      Arnd

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