linux-kernel - Re: [PATCH] firmware: vpd: Fix section enabled flag on vpd_section

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Message-ID: <20180723183927.GH100814@dtor-ws>
Date:   Mon, 23 Jul 2018 11:39:27 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Anton Vasilyev <vasilyev@...ras.ru>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Samuel Holland <samuel@...lland.org>,
        Pan Bian <bianpan2016@....com>, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org
Subject: Re: [PATCH] firmware: vpd: Fix section enabled flag on
 vpd_section_destroy

On Mon, Jul 23, 2018 at 11:27:10AM -0700, Guenter Roeck wrote:
> On Mon, Jul 23, 2018 at 10:23:05AM -0700, Dmitry Torokhov wrote:
> > On Mon, Jul 23, 2018 at 10:13:36AM -0700, Guenter Roeck wrote:
> > > On Mon, Jul 23, 2018 at 07:48:57PM +0300, Anton Vasilyev wrote:
> > > > static struct ro_vpd and rw_vpd are initialized by vpd_sections_init()
> > > > in vpd_probe() based on header's ro and rw sizes.
> > > > In vpd_remove() vpd_section_destroy() performs deinitialization based
> > > > on enabled flag, which is set to true by vpd_sections_init().
> > > > This leads to call of vpd_section_destroy() on already destroyed section
> > > > for probe-release-probe-release sequence if first probe performs
> > > > ro_vpd initialization and second probe does not initialize it.
> > > > 
> > > 
> > > I am not sure if the situation described can be seen in the first place.
> > > The second probe would only not perform ro_vpd initialization if it fails
> > > prior to that, ie if it fails to allocate memory or if there is a
> > > consistency problem. In that case the remove function would not be called.
> > > 
> > > However, there is a problem in the code: A partially failed probe will
> > > leave the system in inconsistent state. Example: ro section initializes,
> > > rw section fails to initialize. The probe will fail, but the ro section
> > > will not be destroyed, its sysfs attributes still exist, and its memory
> > > is still mapped. It would make more sense to fix _that_ problem.
> > > Essentially, vpd_sections_init() should clean up after itself after it
> > > fails to initialize a section.
> > > 
> > > Note that I am not convinced that the "enabled" flag is needed in the first
> > > place. It is only relevant if vpd_section_destroy() is called, which only
> > > happens from the remove function. The remove function is only called if the
> > > probe function succeeded. In that case it is always set for both sections.
> > 
> > The problem will happen if coreboot memory changes between 2 probes so
> > that header.ro_size is not 0 on the first pass and is 0 on the second
> > pass. Not quite likely to ever happen in real life, but resetting a flag
> > is pretty cheap to not do it.
> > 
> 
> If that can happen between probes, meaning it is not guaranteed to be
> constant during the lifetime of the system, doesn't that mean it can
> happen anytime ?

I think we can assume that the data is stable while coreboot device is
registered, but I can imagine one can theoretically have a debug
coreboot data provider that can supply different coreboot parameters
across load/unload. I.e. we have coreboot_table-acpi.c and
coreboot_table-of.c, we might create coreboot_table-test.c to feed
arbitrary data to the subsystem.

Thanks.

-- 
Dmitry