linux-kernel - Re: [Stable-review] [24/28] USB: xhci

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Date:	Wed, 20 Apr 2011 07:39:25 +0200
From:	Willy Tarreau <w@....eu>
To:	Ben Hutchings <ben@...adent.org.uk>
Cc:	Dmitry Torokhov <dtor@...are.com>,
	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	stable@...nel.org, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, stable-review@...nel.org,
	alan@...rguk.ukuu.org.uk
Subject: Re: [Stable-review] [24/28] USB: xhci - fix unsafe macro definitions

On Wed, Apr 20, 2011 at 03:02:04AM +0100, Ben Hutchings wrote:
> On Tue, 2011-04-19 at 13:31 -0700, Greg KH wrote:
> > 2.6.32-longterm review patch.  If anyone has any objections, please let us know.
> > 
> > ------------------
> > 
> > From: Dmitry Torokhov <dtor@...are.com>
> > 
> > commit 5a6c2f3ff039154872ce597952f8b8900ea0d732 upstream.
> > 
> > Macro arguments used in expressions need to be enclosed in parenthesis
> > to avoid unpleasant surprises.
> 
> Do you know of any specific uses of these macros where the missing
> parentheses caused 'unpleasant surprises'?

In my opinion, this type of fix should be backported even if the current
code does not appear to be at risk, otherwise a later fix in the kernel
could cause a serious regression when backported to -stable. For instance,
if we later have to backport this patch (cut'n'pasted) :

--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -1033,7 +1033,7 @@ static inline unsigned int xhci_get_endpoint_interval(struct usb_device *udev,
        default:
                BUG();
        }
-       return EP_INTERVAL(interval);
+       return EP_INTERVAL(interval + 1);
 }

How can you guess that while works in mainline, it breaks -stable ?

As a user, I'd rather have known valid code in -stable and -longterm at the
risk of an occasional *temporary* regression than longterm unexplainable
regressions due to diverging code causing unexpected issues with backported
fixes.

Regards,
Willy

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