Relocation problem with MIPS kernel modules

[David Daney]

Ralf Baechle wrote:
> On Thu, Jul 30, 2009 at 11:49:23AM -0700, David VomLehn wrote:
> 
>> To: GCC Help Mailing List <gcc-help@gcc.gnu.org>,
>> 	Linux MIPS Mailing List <linux-mips@linux-mips.org>
>> Subject: Relocation problem with MIPS kernel modules
>> Content-Type: text/plain; charset=us-ascii
>>
>> I have a MIPS loadable kernel module that, when I try to insmod it, causes the
>> kernel to emit the message:
>>
>> 	module xyz: dangerous relocation
>>
>> This message appears in three different places in arch/mips/kernel/module.c,
>> but this one is coming from apply_r_mips_lo16_rel(). The module code at
> 
> I'll change the messages to indicate the relocation type.
> 
>> the location at which the error message is generated appears to be pretty
>> bland:
>> 	lw v0,28564(s1)
>> with the expected relocation type of R_MIPS_LO16. The relocation before it
>> is R_MIPS_HI16, as expected, but for a different symbol. Before *that*
>> is another R_MIPS_HI16 relocation entry for yet a third symbol.
>>
>> According to the MIPS ABI, for what it's worth, "Each relocation type of
>> R_MIPS_HI16 must have an associated R_MIPS_LO16 entry immediately following
>> it in the list of relocations." So, what's actually getting generated by
>> gcc and linker differs from the closest thing we have to an ABI of record for
>> MIPS processors.
> 
> The GNU tools as an extension over the MIPS ABI allows an arbitrary number of
> R_MIPS_HI16 relocations to be followed by a R_MIPS_LO16 symbol.  All
> relocations of this sequence must use the same symbol, of course.  This is
> a very old extension; I think it predates the Linux/MIPS port.
> 

Actually I think it is the opposite:

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE
00000000 R_MIPS_HI16       .bss+0x00000004
00000008 R_MIPS_LO16       .bss+0x00000004
00000014 R_MIPS_LO16       .bss+0x00000004

We load the hi16 value into a register and then use multiple lo16 
offsets for the follow loads and stores to the same location.  On a 
read-modify-write we only want to load the base address one time.

David Daney