linux-kernel - Re: GCC fails to spot uninitialized variable

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Date:   Fri, 15 Jul 2022 15:03:37 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Petko Manolov <petko.manolov@...sulko.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: GCC fails to spot uninitialized variable

On Fri, Jul 15, 2022 at 11:09:17PM +0300, Petko Manolov wrote:
> 	Guys,
> 
> Today i was bitten by a stupid bug that i introduced myself while writing some
> v4l2 code.  Looking at it a bit more carefully i was surprised that GCC didn't
> catch this one, as it was something that should definitely emit a warning.
> 
> When included into the driver, this particular code:
> 
> int blah(int a, int *b)
> {
> 	int ret;
> 
> 	switch (a) {
> 	case 0:
> 		ret = a;
> 		break;
> 	case 1:
> 		ret = *b;
> 		break;
> 	case 2:
> 		*b = a;
> 		break;
> 	default:
> 		ret = 0;
> 	}
> 
> 	return ret;
> }
> 
> somehow managed to defeat GCC checks.  Compiling it as a standalone .c file
> with:
> 
> 	gcc -Wall -O2 -c t.c
> 
> gives me nice:
> 
> t.c:19:16: warning: 'ret' may be used uninitialized in this function [-Wmaybe-uninitialized]
>    19 |         return ret;
>       |                ^~~
> 
> Any idea what might have gone wrong?

See commit 78a5255ffb6a ("Stop the ad-hoc games with
-Wno-maybe-initialized") in 5.7, which disabled that warning for a
default kernel build. You have to either include 'W=2' (which will
introduce other warnings which might be noisy) or
'KCFLAGS=-Wmaybe-uninitialized' (which will just add that warning) in
your make command to see those warnings.

As an aside, your mailer adds a "Mail-Followup-To:" header that was set
to LKML, meaning that you would not have seen this reply unless you were
subscribed to LKML. Might be something worth looking into.

Cheers,
Nathan