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Date: Tue, 6 Mar 2018 22:47:54 +0100
From: Arnd Bergmann <arnd@...db.de>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>, Daniel Micay <danielmicay@...il.com>,
Ingo Molnar <mingo@...nel.org>, Kees Cook <keescook@...omium.org>,
Dave Hansen <dave.hansen@...ux.intel.com>, Alexander Popov <alex.popov@...ux.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
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LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()
On Tue, Mar 6, 2018 at 10:36 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 6 Mar 2018 13:01:20 -0800
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>> Then the optimization passes will just remove the initialization in
>> 99.9% of all cases. Only very occasionally - when gcc cannot see it
>> being overwritten - would it remain. And those are exactly the cases
>> where you *want* it to remain.
>
> You mean have gcc fill in the variables that it thinks is used
> uninitialized with zeros? As long as it still warns about it, because
> that usually catches some real bugs where zeroing the variable doesn't
> actually fix the bug.
>
> I also tried the example Arnd posted with:
>
>
> int g(int c)
> {
> int i;
> if (c) /* gcc optimizes out the condition as nothing else sets i */
> i = 1;
> return i;
> }
>
> And he's right. -O2 doesn't warn :-( I think that it should.
See this bug that has been open since 2004 and the 31 bugs
that have been marked as duplicates since then:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501
I don't really understand it myself, but I do understand that
the gcc developers think this is a hard problem to solve.
Arnd
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