linux-kernel - Re: [PATCH] parisc: fix module loading failure of large kernel modules (take 2)

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Date:	Wed, 31 Dec 2008 00:02:41 +0100
From:	Helge Deller <deller@....de>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	linux-parisc <linux-parisc@...r.kernel.org>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	Kyle McMartin <kyle@...artin.ca>,
	Randolph Chung <randolph@...sq.org>,
	Linus <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	John David Anglin <dave@...uly1.hia.nrc.ca>
Subject: Re: [PATCH] parisc: fix module loading failure of large kernel modules
 (take 2)

Rusty Russell wrote:
> On Tuesday 30 December 2008 07:04:54 Helge Deller wrote:
>> This is the second take of the patch series.
>> Changes to previous version:
>> - new CONFIG_HAVE_MODULE_SECTION_STUBS config option
>> - put stub entries of a code section in front of the section
>>
>> ____________
>> The parisc port (esp. the 32bit kernel) currently lacks the ability to
>> load large kernel modules like xfs or ipv6. This is a long outstanding
>> bug and has already been reported a few times, e.g.:
>> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=350482,
>> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=401439,
>> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=508489
>>
>> The symptom is like this:
>> # modprobe xfs
>> FATAL: Error inserting xfs
>> (/lib/modules/2.6.26-1-parisc/kernel/fs/xfs/xfs.ko): Invalid module
>> format
>>
>> In dmesg:
>> module xfs relocation of symbol xfs_btree_read_bufs is out of range
>> (0x3ffefffe in 17 bits)
>>
>> The reason for the failure is, that the architecture only provides the
>> R_PARISC_PCREL17F (for 32bit kernels) and R_PARISC_PCREL22F (for PA2.0
>> and 64bit kernels) relocations, which sometimes can't reach the target
>> address of the stub entry if the kernel module is too large. Currently
>> parisc (like other architectures) creates one big PLT section for all
>> stubs at the beginning of the init and core sections.
>>
>> The following two patches changes the parisc module loader to put stubs
>> for the code sections in front of each section, so that the distance to
>> the stubs more easily fits into the available 17/22 bits.
> 
> So now any one section has to pass 17 bits to break?  How close are you with
> the xfs module?

I did not tested it very much, but xfs is around 1.1M on disk, and ~750K when 
loaded. I think it breaked being 1/4 through the relocations.

> But it's kind of nasty, overloading sh_entsize further.  Could we instead
> do something like add a arch_module_section_size() weak fn which you can
> overload?

Sure. I'll respin the patch.

> We'd use that in get_offset() so our layout and size calculations
> were correct, 

Yes, good.

> and use sh_size everywhere else.

BTW, although the comment states that arches can change section sizes 
in the module_frob_arch_sections() function:
/* Allow arches to frob section contents and sizes.  */
it will break horrible if you do so.
What I found was, that if you change sh_size, at least the module
references / dependency chain will break when running lsmod.

Helge
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