Q: why didn't GCC warn about this uninitialized variable? (was: Re: [PATCH] perf tests: initialize sa.sa_flags)

[Ingo Molnar <mingo@kernel.org>]

* Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> Em Wed, Mar 02, 2016 at 02:21:27PM +0100, Peter Zijlstra escreveu:
> > On Wed, Mar 02, 2016 at 10:03:50AM -0300, Arnaldo Carvalho de Melo wrote:
> > > > Would not something like:
> > > > 
> > > > 	sa = (struct sigaction){
> > > > 		.sa_sigaction = segfault_handler,
> > > > 	};
> > > > 	sigfillset(&sa.sa_mask);
> > > > 
> > > > Be better?
> > > 
> > > I thought about that, but isn't that set in stone? This would be a 4
> > > liner, while his is a one' :-)
> > 
> > Dunno, you're right that its rather unlikely struct sigaction is going
> > to grow another member, but I like the above pattern better in general,
> > makes it harder to end up with uninitalized bits.
> > 
> > When performance matters the above pattern isn't ideal, but that should
> > not be a concern here.
> 
> Right, I also always use :
> 
> 
> 	struct foo bar = {
> 		.baz = 1,
> 		.name = "whatever",
> 	};
> 
> Even more compact than using that cast. But didn't bother changing in
> this case.

So the source of the bug was:

        struct sigaction sa;

	...

        sigfillset(&sa.sa_mask);
        sa.sa_sigaction = segfault_handler;
        sigaction(SIGSEGV, &sa, NULL);

... which uninitialized sa.sa_flags field GCC merrily accepted as proper C code, 
despite us turning on essentially _all_ GCC warnings for the perf build that exist 
under the sun:

 gcc -Wbad-function-cast -Wdeclaration-after-statement -Wformat-security -Wformat-y2k \
    -Winit-self -Wmissing-declarations -Wmissing-prototypes -Wnested-externs \
    -Wno-system-headers -Wold-style-definition -Wpacked -Wredundant-decls \
    -Wshadow -Wstrict-aliasing=3 -Wstrict-prototypes -Wswitch-default -Wswitch-enum \
    -Wundef -Wwrite-strings -Wformat \
    -Werror -O6 -fno-omit-frame-pointer -ggdb3 -funwind-tables -Wall -Wextra -std=gnu99 -fstack-protector-all -D_FORTIFY_SOURCE=2

This is a _trivial_ uninitialized variable bug, yet GCC never warned about it. 
Why?

People build perf with a wide range of GCC versions, from old ones to trunk. I 
cannot believe it that none of those GCC versions warned about this trivial 
looking bug!

And yes, I know that unitialized structures on the stack are valid C code, yet 
it's one of the most fragile aspects of C and it was the source of countless 
security holes in the past...

Thanks,

	Ingo