Build failure (with insane config)

Build failure (with insane config)

[Roland Dreier]

For x86-64, arch/x86/lib/Makefile has:

        lib-y += csum-partial_64.o csum-copy_64.o csum-wrappers_64.o

so in particular ip_compute_csum() (from csum-partial_64.c) gets put in
libs, so even though it is exported, it gets thrown away at link time if
there are no non-modular users.

However, if someone has an insane config with CONFIG_NET=n and
CONFIG_VIDEO_BT848=m, then ip_compute_csum() gets discarded but the
modular code in drivers/media/video/bt8xx/bttv-cards.c calls it and...

        ERROR: "ip_compute_csum" [drivers/media/video/bt8xx/bttv.ko] undefined!

I guess one fix is just to put at least csum-partial_64.c in obj-y
instead; it does seem broken to have files with EXPORT_SYMBOL() be
lib-y, since it is really an invitation for this problem.  Other ideas
would be for bttv to depend on or select CONFIG_NET.

[It's a long story why I noticed this problem, but maybe we want to fix
it to save people doing randconfig test builds or something like that.
Or maybe we don't really care, since the class of real systems with
video capture cards but no networking is probably quite small]

 - R.

Re: Build failure (with insane config)

[Ingo Molnar]

* Roland Dreier <rdreier@cisco.com> wrote:

> For x86-64, arch/x86/lib/Makefile has:
> 
>         lib-y += csum-partial_64.o csum-copy_64.o csum-wrappers_64.o
> 
> so in particular ip_compute_csum() (from csum-partial_64.c) gets put in
> libs, so even though it is exported, it gets thrown away at link time if
> there are no non-modular users.
> 
> However, if someone has an insane config with CONFIG_NET=n and
> CONFIG_VIDEO_BT848=m, then ip_compute_csum() gets discarded but the
> modular code in drivers/media/video/bt8xx/bttv-cards.c calls it and...
> 
>         ERROR: "ip_compute_csum" [drivers/media/video/bt8xx/bttv.ko] undefined!
> 
> I guess one fix is just to put at least csum-partial_64.c in obj-y 
> instead; it does seem broken to have files with EXPORT_SYMBOL() be 
> lib-y, since it is really an invitation for this problem.  Other ideas 
> would be for bttv to depend on or select CONFIG_NET.

well, the TCP/IP checksumming functions are in lib portions on every 
architecture i checked, and the exports are handled by the networking 
code. And the symbol used is named ip_compute_csum(). So i think the 
correct solution is to make CONFIG_VIDEO_BT848 depend on CONFIG_NET ...

especially on embedded there's an advantage in linking out unused core 
kernel functions. But even if we did this change on x86, other 
architectures might (correctly) complain about x86 going it alone. x86 
gets the most use by far, so if we start introducing build assymetries 
like this it will just mess up the build on other architectures.

	Ingo

Re: Build failure (with insane config)

[Adrian Bunk]

On Sun, Apr 27, 2008 at 08:34:12PM -0700, Roland Dreier wrote:
> For x86-64, arch/x86/lib/Makefile has:
> 
>         lib-y += csum-partial_64.o csum-copy_64.o csum-wrappers_64.o
> 
> so in particular ip_compute_csum() (from csum-partial_64.c) gets put in
> libs, so even though it is exported, it gets thrown away at link time if
> there are no non-modular users.
> 
> However, if someone has an insane config with CONFIG_NET=n and
> CONFIG_VIDEO_BT848=m, then ip_compute_csum() gets discarded but the
> modular code in drivers/media/video/bt8xx/bttv-cards.c calls it and...
> 
>         ERROR: "ip_compute_csum" [drivers/media/video/bt8xx/bttv.ko] undefined!
> 
> I guess one fix is just to put at least csum-partial_64.c in obj-y
> instead; it does seem broken to have files with EXPORT_SYMBOL() be
> lib-y, since it is really an invitation for this problem.  Other ideas
> would be for bttv to depend on or select CONFIG_NET.
>...

This isn't the first time csum-partial_64.c is involved in such a kind 
of breakage (and we constantly have this kind of bugs with various 
exports from lib-y), and EXPORT_SYMBOL's in lib-y are simply wrong and 
always bugs waiting to happen.

Axing lib-y is somewhere low priority on my TODO list.

>  - R.

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed