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Date:   Tue, 18 Jun 2019 15:59:11 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "# 3.4.x" <stable@vger.kernel.org>
Subject: Re: [PATCH] ubsan: mark ubsan_type_mismatch_common inline
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On Tue, Jun 18, 2019 at 04:27:45PM +0300, Andrey Ryabinin wrote:
> 
> 
> On 6/18/19 3:56 PM, Arnd Bergmann wrote:
> > On Mon, Jun 17, 2019 at 4:02 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >>
> >> On Mon, Jun 17, 2019 at 02:31:09PM +0200, Arnd Bergmann wrote:
> >>> objtool points out a condition that it does not like:
> >>>
> >>> lib/ubsan.o: warning: objtool: __ubsan_handle_type_mismatch()+0x4a: call to stackleak_track_stack() with UACCESS enabled
> >>> lib/ubsan.o: warning: objtool: __ubsan_handle_type_mismatch_v1()+0x4a: call to stackleak_track_stack() with UACCESS enabled
> >>>
> >>> I guess this is related to the call ubsan_type_mismatch_common()
> >>> not being inline before it calls user_access_restore(), though
> >>> I don't fully understand why that is a problem.
> >>
> >> The rules are that when AC is set, one is not allowed to CALL schedule,
> >> because scheduling does not save/restore AC.  Preemption, through the
> >> exceptions is fine, because the exceptions do save/restore AC.
> >>
> >> And while most functions do not appear to call into schedule, function
> >> trace ensures that every single call does in fact call into schedule.
> >> Therefore any CALL (with AC set) is invalid.
> > 
> > I see that stackleak_track_stack is already marked 'notrace',
> > since we must ensure we don't recurse when calling into it from
> > any of the function trace logic.
> > 
> > Does that mean we could just mark it as another safe call?
> > 
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -486,6 +486,7 @@ static const char *uaccess_safe_builtin[] = {
> >         "__ubsan_handle_type_mismatch",
> >         "__ubsan_handle_type_mismatch_v1",
> >         /* misc */
> > +       "stackleak_track_stack",
> >         "csum_partial_copy_generic",
> >         "__memcpy_mcsafe",
> >         "ftrace_likely_update", /* CONFIG_TRACE_BRANCH_PROFILING */

Indeed, we could do this.

> > 
> >> Maybe we should disable stackleak when building ubsan instead? We
> >> already disable stack-protector when building ubsan.
> > 
> > I couldn't find out how that is done.
> > 
> 
> I guess this:
> ccflags-y += $(DISABLE_STACKLEAK_PLUGIN)

Or more specifically this, I guess:

CFLAGS_ubsan.o := $(call cc-option, -fno-conserve-stack -fno-stack-protector) $(DISABLE_STACKLEAK_PLUGIN)

we'd not want to exclude all of lib/ from stackleak I figure.

Of these two options, I think I prefer the latter, because a smaller
whitelist is a better whitelist and since we already disable
stack protector, it is only consistent to also disable stack leak.