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Date:   Wed, 16 Sep 2020 19:32:06 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Quentin Perret <qperret@google.com>
Cc:     stern@rowland.harvard.edu, linux-usb@vger.kernel.org,
        linux-kernel@vger.kernel.org, gprocida@google.com,
        maennich@google.com, kernel-team@android.com
Subject: Re: [PATCH] ehci-hcd: Move include to keep CRC stable
Message-ID: <20200916173206.GA3070014@kroah.com>
References: <20200916171825.3228122-1-qperret@google.com>
 <20200916173127.GB3068942@kroah.com>
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On Wed, Sep 16, 2020 at 07:31:27PM +0200, Greg KH wrote:
> On Wed, Sep 16, 2020 at 06:18:25PM +0100, Quentin Perret wrote:
> > The CRC calculation done by genksyms is triggered when the parser hits
> > EXPORT_SYMBOL*() macros. At this point, genksyms recursively expands the
> > types of the function parameters, and uses that as the input for the CRC
> > calculation. In the case of forward-declared structs, the type expands
> > to 'UNKNOWN'. Following this, it appears that the result of the
> > expansion of each type is cached somewhere, and seems to be re-used
> > when/if the same type is seen again for another exported symbol in the
> > same C file.
> > 
> > Unfortunately, this can cause CRC 'stability' issues when a struct
> > definition becomes visible in the middle of a C file. For example, let's
> > assume code with the following pattern:
> > 
> >     struct foo;
> > 
> >     int bar(struct foo *arg)
> >     {
> > 	/* Do work ... */
> >     }
> >     EXPORT_SYMBOL_GPL(bar);
> > 
> >     /* This contains struct foo's definition */
> >     #include "foo.h"
> > 
> >     int baz(struct foo *arg)
> >     {
> > 	/* Do more work ... */
> >     }
> >     EXPORT_SYMBOL_GPL(baz);
> > 
> > Here, baz's CRC will be computed using the expansion of struct foo that
> > was cached after bar's CRC calculation ('UNKOWN' here). But if
> > EXPORT_SYMBOL_GPL(bar) is removed from the file (because of e.g. symbol
> > trimming using CONFIG_TRIM_UNUSED_KSYMS), struct foo will be expanded
> > late, during baz's CRC calculation, which now has visibility over the
> > full struct definition, hence resulting in a different CRC for baz.
> > 
> > The proper fix for this certainly is in genksyms, but that will take me
> > some time to get right. In the meantime, we have seen one occurrence of
> > this in the ehci-hcd code which hits this problem because of the way it
> > includes C files halfway through the code together with an unlucky mix
> > of symbol trimming.
> > 
> > In order to workaround this, move the include done in ehci-hub.c early
> > in ehci-hcd.c, hence making sure the struct definitions are visible to
> > the entire file. This improves CRC stability of the ehci-hcd exports
> > even when symbol trimming is enabled.
> > 
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> >  drivers/usb/host/ehci-hcd.c | 1 +
> >  drivers/usb/host/ehci-hub.c | 1 -
> >  2 files changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/usb/host/ehci-hcd.c b/drivers/usb/host/ehci-hcd.c
> > index 6257be4110ca..3575b7201881 100644
> > --- a/drivers/usb/host/ehci-hcd.c
> > +++ b/drivers/usb/host/ehci-hcd.c
> > @@ -22,6 +22,7 @@
> >  #include <linux/interrupt.h>
> >  #include <linux/usb.h>
> >  #include <linux/usb/hcd.h>
> > +#include <linux/usb/otg.h>
> >  #include <linux/moduleparam.h>
> >  #include <linux/dma-mapping.h>
> >  #include <linux/debugfs.h>
> > diff --git a/drivers/usb/host/ehci-hub.c b/drivers/usb/host/ehci-hub.c
> > index ce0eaf7d7c12..087402aec5cb 100644
> > --- a/drivers/usb/host/ehci-hub.c
> > +++ b/drivers/usb/host/ehci-hub.c
> > @@ -14,7 +14,6 @@
> >   */
> >  
> >  /*-------------------------------------------------------------------------*/
> > -#include <linux/usb/otg.h>
> >  
> >  #define	PORT_WAKE_BITS	(PORT_WKOC_E|PORT_WKDISC_E|PORT_WKCONN_E)
> 
> Thanks for root-causing this issue.  genksyms is a fragile beast, good
> luck trying to resolve that!
> 
> I'll go queue this up and mark it for stable kernels, thanks.

I'll queue it up after I get Alan's review first :)

thanks,

greg k-h