linux-kernel - Re: [PATCH v1 1/1] platform/x86: ideapad-laptop: Make the scope

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Message-ID: <Ztg2ZjfuEe5PuvF8@smile.fi.intel.com>
Date: Wed, 4 Sep 2024 13:28:54 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Gergo Koteles <soyer@....hu>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Hans de Goede <hdegoede@...hat.com>,
	platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ike Panhc <ike.pan@...onical.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v1 1/1] platform/x86: ideapad-laptop: Make the
 scope_guard() clear of its scope

On Tue, Sep 03, 2024 at 06:22:42PM -0700, Nathan Chancellor wrote:
> On Tue, Sep 03, 2024 at 06:40:16PM +0300, Andy Shevchenko wrote:
> > On Tue, Sep 03, 2024 at 05:29:02PM +0200, Gergo Koteles wrote:
> > > On Tue, 2024-09-03 at 18:14 +0300, Andy Shevchenko wrote:
> > > > On Tue, Sep 03, 2024 at 05:00:51PM +0200, Gergo Koteles wrote:
> > > > > On Thu, 2024-08-29 at 19:50 +0300, Andy Shevchenko wrote:
> > > > > > First of all, it's a bit counterintuitive to have something like
> > > > > > 
> > > > > > 	int err;
> > > > > > 	...
> > > > > > 	scoped_guard(...)
> > > > > > 		err = foo(...);
> > > > > > 	if (err)
> > > > > > 		return err;
> > > > > > 
> > > > > > Second, with a particular kernel configuration and compiler version in
> > > > > > one of such cases the objtool is not happy:
> > > > > > 
> > > > > >   ideapad-laptop.o: warning: objtool: .text.fan_mode_show: unexpected end of section
> > > > > > 
> > > > > > I'm not an expert on all this, but the theory is that compiler and
> > > > > > linker in this case can't understand that 'result' variable will be
> > > > > > always initialized as long as no error has been returned. Assigning
> > > > > > 'result' to a dummy value helps with this. Note, that fixing the
> > > > > > scoped_guard() scope (as per above) does not make issue gone.
> > > > > > 
> > > > > > That said, assign dummy value and make the scope_guard() clear of its scope.
> > > > > > For the sake of consistency do it in the entire file.
> > > > > > 
> > > > > 
> > > > > Interestingly, if I open a scope manually and use the plain guard, the
> > > > > warning disappears.
> > > > 
> > > > Yes, that's what I also have, but I avoid that approach because in that case
> > > > the printing will be done inside the lock, widening the critical section for
> > > > no benefits.
> > > > 
> > > 
> > > This is intended to be an inner block scope within the function, it
> > > does not expand the critical section.
> > 
> > I'm not sure I understand.
> > 
> > scoped_guard() has a marked scope (with {} or just a line coupled with it).
> > The guard() has a scope starting at it till the end of the function. In the
> > latter case the sysfs_emit() becomes part of the critical section.
> > 
> > > > > 	unsigned long result;
> > > > > 	int err;
> > > > > 
> > > > > 	{
> > > > > 		guard(mutex)(&priv->vpc_mutex);
> > > > > 		err = read_ec_data(priv->adev->handle, VPCCMD_R_FAN,
> > > > > &result);
> > > > > 		if (err)
> > > > > 			return err;
> > > > > 	}
> > 
> > But looking again into the code above now I got what you meant.
> > You have added a nested scope inside the function, like
> > 
> > 	do {
> > 		...
> > 	} while (0);
> > 
> > Yes, this is strange and not what we want to have either. So I prefer to hear
> > what objtool / clang people may comment on this.
> 
> So this does not appear to happen when CONFIG_KCOV is disabled with the
> configuration from the original report. I have spent some time looking
> at the disassembly but I am a little out of my element there. If I
> remember correctly, the "unexpected end of section" warning from objtool
> can appear when optimizations play fast and loose with the presence of
> potential undefined behavior (or cannot prove that there is no undefined
> behavior through inlining or analysis). In this case, I wonder if KCOV
> prevents LLVM from realizing that the for loop that scoped_guard()
> results in will run at least once, meaning that err and result would be
> potentially used uninitialized? That could explain why this change
> resolves the warning, as it ensures that no undefined behavior could
> happen regardless of whether or not the loop runs?
> 
> Josh and Peter may have more insight.

Thanks for looking into this. Josh already keeps an eye on this.

-- 
With Best Regards,
Andy Shevchenko