linux-kernel - Re: [PATCH 1/1] container_of: Document container

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Message-ID: <2025052138-carport-applaud-61b8@gregkh>
Date: Wed, 21 May 2025 15:27:19 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] container_of: Document container_of() is not to be
 used in new code

On Tue, May 20, 2025 at 10:09:18PM +0000, Sakari Ailus wrote:
> Hi Greg,
> 
> On Tue, May 20, 2025 at 04:30:19PM +0200, Greg Kroah-Hartman wrote:
> > On Tue, May 20, 2025 at 05:16:45PM +0300, Andy Shevchenko wrote:
> > > On Tue, May 20, 2025 at 01:34:37PM +0300, Sakari Ailus wrote:
> > > > There is a warning in the kerneldoc documentation of container_of() that
> > > > constness of its ptr argument is lost. While this is a faible suggestion
> > > > container_of_const() should be used instead, the vast majority of new code
> > > > still uses container_of():
> > > > 
> > > > $ git diff v6.13 v6.14|grep container_of\(|wc -l
> > > > 646
> > > > $ git diff v6.13 v6.14|grep container_of_const|wc -l
> > > > 9
> > > > 
> > > > Make an explicit recommendation to use container_of_const().
> > > > 
> > > > Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > > > ---
> > > > Hi Greg, Andy,
> > > > 
> > > > I guess we generally agree the additional constness check in
> > > > container_of_const() is useful, but adding the same check to
> > > > container_of() generates warnings -- there are some errors, too -- such as
> > > > this one currently:
> > > > 
> > > > In file included from /home/sailus/src/linux/include/linux/bcma/bcma.h:14,
> > > >                  from /home/sailus/src/linux/arch/x86/kernel/early-quirks.c:17:
> > > > /home/sailus/src/linux/include/linux/ssb/ssb.h: In function ‘dev_to_ssb_dev’:
> > > > /home/sailus/src/linux/include/linux/ssb/ssb.h:291:14: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
> > > >   291 |         wrap = container_of(dev, struct __ssb_dev_wrapper, dev);
> > > >       |              ^
> > > > 
> > > > As noted above, 646 new missing constness checks were introduced through
> > > > container_of() macro use during the 6.14 cycle alone. Most of these are
> > > > likely harmless, but with so many new users some are bound to be ignoring
> > > > constness.
> > > > 
> > > > Once the warnings from bad container_of() use are worked out in a way or
> > > > another, the constness check could be added to the container_of() macro
> > > > and the current container_of_const() be dropped altogether.
> > > > 
> > > > If this patch is accepted, I'll see how to add a warning on container_of()
> > > > to checkpatch.pl.
> > > 
> > > Hmm... Wouldn't be better to fix non-const cases and add the const check, etc
> > > to the container_of() instead of doing these comments?
> > 
> > Yes, fixing up the existing places where it is broken would be best, how
> > many of them are there now?
> 
> Adding constness check for container_of(), with my partial build on x86-64
> I'm getting 893 such warnings. A fair number are probably duplicates or
> repeat of the same pattern, but also the compilation didn't succeed --
> there were multiple compilation failures.

So who is going to do that work?  I just did it for drivers/usb and it
was pretty trivial, just declaring "do not use this!" feels like the
easy way out, absolving yourself from any responsibility here :)

thanks,

greg k-h