Help about calculation of addend for ELF relocations for MIPS arch

["Pan ruochen" <panruochen at gmail dot com>]

Hi All,

I am studying relocation of ELF for MIPS ABI (SYSTEM V APPLICATION
BINARY INTERFACE,
MIPS RISC Processor, Supplement, 3rd Edition). I do not understand
how to calculate `A' specified in the specification.

The specification says (in chapter 4-17) :
A      Represents the addend used to compute the value of the relocatable
       field.
...
A relocation entryr_offset value designates the offset or virtual address of the
first byte of the affected storage unit. The relocation type specifies
which bits to
change and how to calculate their values.  Because MIPS uses onlyElf32_Rel re-
location entries, the relocated field holds the addend.

Does this mean the addend is saved in the storage location where relocation
will be applied to ? For example, for a R_MIPS_26 relocation entry, the addend
is saved in the `targ26' field.

I have an experiment with a relocatable ELF file `foo.out'. And `foo.img'
is the runtime image of `foo.out' after being relocated to the base address.

$mips-elf-objdump -S foo.out
c0000000:	27bdffe0 	addiu	sp,sp,-32
c0000004:	24040003 	li	a0,3
c0000008:	00002821 	move	a1,zero
c000000c:	00003021 	move	a2,zero
c0000010:	00003821 	move	a3,zero
c0000014:	afbf0018 	sw	ra,24(sp)
c0000018:	0c000000 	jal	c0000000
...
$mips-elf-readelf -r foo.out
Relocation section '.rel.text' at offset 0x3a74 contains 254 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00000018  00009704 R_MIPS_26         000002e8   entry
00000020  00007705 R_MIPS_HI16       00000000   table
00000030  00007706 R_MIPS_LO16       00000000   table
...
$mips-elf-readelf -s foo.out
Symbol table '.symtab' contains 171 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
   ...
   151: 000002e8     0 FUNC    GLOBAL DEFAULT    1 entry
   ...
$mips-elf-ld -Ttext 0x188e1000 foo.out -o foo.img
$mips-elf-objdump -S foo.img
188e1000:	27bdffe0 	addiu	sp,sp,-32
188e1004:	24040003 	li	a0,3
188e1008:	00002821 	move	a1,zero
188e100c:	00003021 	move	a2,zero
188e1010:	00003821 	move	a3,zero
188e1014:	afbf0018 	sw	ra,24(sp)
188e1018:	0e2384ba 	jal	188e12e8
...

Let's deduce back from the final results. The relocation entry is at
the offset 0x18,
and of the type `R_MIPS_26'. The symbol is not local, so the foluma
(sign-extend(A<<2) + S) >> 2
should be applied. And S will be the value of the symbol `entry',
which is 0x2e8.

(sign-extend(A<<2) + 0x2e8) >> 2 = targ26(0e2384ba) = 0x022384ba
signed-extend(A<<2) = 0x088e1000
This value seems to be the offset to the PC-region (0x10000000).
So, here the addend seems to be (segment_base - PC_region) >> 2. But
there is not
a word in the specification explicitly says that.

So, how `A' is calculated for R_MIPS_26, R_MIPS_32 and other types.


--------------
Best Regards
PRC
Aug 12, 2008