linux-kernel - Re: genksyms: separating public headers from private header files

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Message-Id: <53CF8E500200007800024FD1@mail.emea.novell.com>
Date:	Wed, 23 Jul 2014 09:28:32 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Don Zickus" <dzickus@...hat.com>
Cc:	<mmarek@...e.cz>, "LKML" <linux-kernel@...r.kernel.org>
Subject: Re: genksyms: separating public headers from private header
 files

>>> On 16.07.14 at 17:19, <dzickus@...hat.com> wrote:
> Hi Jan, Michal,
> 
> I am not sure who maintains genksyms officially, so I am sending this
> question to the two of you as folks who seemed to have contributed to the
> tool. :-)
> 
> I noticed with genksyms that a symbol is opaquely defined in a
> public header file (on purpose) and then fully defined in a private
> header.  This is normal practice.  Further, symbol checksumming is done on
> EXPORT_SYMBOLs in a private c file that includes the private header
> files.
> 
> As a result, even though a struct symbol is intentionally opaquely defined
> in a public header file consumed by a third party module, the symbol
> checksumming still includes the full definition (because the private c
> file with the actual export symbol has the full definition).  This has
> made it difficult to modify the private header file struct because it
> breaks the symbol checksumming.
> 
> For example, let's consider
> 
> block/blk-core.c:EXPORT_SYMBOL(blk_put_queue);
> 
> blk_put_queue will eventually depend on struct blkcq_gq.
> 
> Now publicly blkcg_gq is defined opaquely in 
> 
> include/linux/blkdev.h
> 
> and privately in
> 
> block/block-cgroup.h
> 
> Now when we checksum blk_put_queue both include/linux/blkdev.h and
> block/block-cgroup.h are included in block/blk-core.c, so blkcg_gq is
> fully defined for checksumming.
> 
> Later if we modify blkcq_gq in block/block-cgroup.h the checksum changes,
> even though it can debated that block-cgroup.h is a private header file
> and it should not impact kabi for third party modules.
> 
> Have either of you run into this?  Or is the argument that private files
> should not impact the checksum not as strong as I might think?  Or is it a
> technical problem of how to separate the public includes from the private
> includes in the preprocessed file?

Yes, I think we've run into this (if not elsewhere then by seeing [and
having to wave] false positive kABI changes). Besides being a
technical problem of separating one kind of header from the other, I'm
also unsure whether uniformly ignoring definitions in private headers
would always be correct. Hence I think a possible solution to this ought
to involve manual annotation of structures not to participate in CRC
calculations.

Jan

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