On Tue, 23 Nov 2010 01:20:48 +0100
Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> Le lundi 22 novembre 2010 __ 15:50 -0800, Andrew Morton a __crit :
>
> > Well. We certainly assume in many places that
> >
> > struct foo {
> > int a;
> > int b;
> > } f = {
> > .a = 1,
> > };
> >
> > will initialise b to zero. But I doubt if much code at all assumes
> > that this initialisation patterm will reliably zero out *holes* in the
> > struct.
> >
>
> We did such assertions in the past, we were wrong.
>
> Check commit 1c40be12f7d8ca1d387510d39787b12e512a7ce8 for an example
> (net sched: fix some kernel memory leaks)
>
> I guess we must make a full audit of all C99 initializers or structures
> copied to userspace, giving a name to hidden holes, to force gcc to init
> them to 0.
>
> # cat try.c
> struct s {
> char c;
> long l;
> };
>
> void bar(void *v)
> {
> unsigned long *p = v;
>
> printf("%lx %lx\n", p[0], p[1]);
> }
>
> int main()
> {
> struct s s1 = {
> .c = 1,
> .l = 2,
> };
>
> bar(&s1);
> return 0;
> }
>
> # gcc -O2 -o try try.c
> # ./try
> 8049401 2
OK, thanks. That rather settles it then. memset() it is.
> Strangely, if we remove ".l = 2," line, gcc emits code to clear al the
> fields
Maybe a glitch, maybe a small optimisation? That's the sort of thing
which will change over gcc versions too..
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