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?
cheers,
Petko
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
On 22-07-15 15:03:37, Nathan Chancellor wrote:
> 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.
I see. I guess i'll end up enabling W=2 only for this particular driver and
only while in development.
> 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.
That would be "set followup_to=no" in mutt speak. Thanks for looking into this.
I am subscribed to all list i'm replying to but, if i understand this properly,
with the old setup non-subscribers may not get my messages.
cheers,
Petko