[U-Boot-Users] Changing u-boot relocation scheme

[J. William Campbell <jwilliamcampbell@comcast.net>]

Haavard Skinnemoen wrote:
> On Fri, 25 Jul 2008 19:28:48 +0200
> kenneth johansson <kenneth@southpole.se> wrote:
>
>   
>> I was afraid that what was needed was more or less a complete linker but
>> it looks like if one generate the dynamic reloc table a much simpler
>> linker(relocation function) is needed. Still probably a lot more complex
>> than the GOT and fixup code that is just a loop over a table. We may
>> need a case also ;-) 
>>     
>
>   
First of all, you are correct that "fully linked" executables contain no 
relocation information unless the user does something like 
--emit-relocs. I was wrong about that.
> Hmm...looks like linking with -pie (or --pic-executable) does
> something vaguely resembling the trick. But I don't know what vintage
> of ld you need for this to be available and actually work...
>
> hskinnemoen at siona:~/work/u-boot/master$ avr32-linux-readelf -d u-boot
>
> Dynamic section at offset 0x1b8f0 contains 12 entries:
>   Tag        Type                         Name/Value
>  0x00000004 (HASH)                       0x182ac
>  0x00000005 (STRTAB)                     0x18260
>  0x00000006 (SYMTAB)                     0x18150
>  0x0000000a (STRSZ)                      74 (bytes)
>  0x0000000b (SYMENT)                     16 (bytes)
>  0x00000003 (PLTGOT)                     0x1ae70
>  0x70000001 (Processor Specific: 70000001) 0xe6c
>  0x00000015 (DEBUG)                      0x0
>  0x00000007 (RELA)                       0x1833c
>  0x00000008 (RELASZ)                     6996 (bytes)
>  0x00000009 (RELAENT)                    12 (bytes)
>  0x00000000 (NULL)                       0x0
>
> The dynamic relocation section became surprisingly large. I wonder if
> we actually manage to manually relocate all those buggers...I checked a
> couple of them and in both cases it looked like something that needed
> to be relocated, but I couldn't find any code that actually did it.
>
> One example from common/cmd_itest.c:
>
> op_tbl_t op_table [] = {
>         { "-lt", LT },
>         { "<"  , LT },
>         { "-gt", GT },
>         { ">"  , GT },
>         { "-eq", EQ },
>         { "==" , EQ },
>         { "-ne", NE },
>         { "!=" , NE },
>         { "<>" , NE },
>         { "-ge", GE },
>         { ">=" , GE },
>         { "-le", LE },
>         { "<=" , LE },
> };
>
> Does this really work without any relocation? Or is it being relocated
> and I'm just too blind to see it?
>   
Is your u-boot built with -fPIC on all the C compiles? If so, op_table 
references may work without any actual relocation if the distance from 
the code segment to the segment containing op_table is not changed when 
u-boot is "relocated" to the new address. If you look at the assembly 
code for op_table being used, it may be done via an offset from the 
start of the code + a register pointing to the start of the code. The 
offset will not change when the code is moved, and the register will 
contain the correct start of code address, so it works. If it is not 
-fPIC, and it is not being executed at the address for which it was 
linked, I have no idea how it would work without a "loader" relocating 
all the references. There are "only" 583 relocation entries specified!

I think you are correct that the --pic-executable switch is the 
"generic" way to get the relocation addresses. I expect it is much 
better supported than the PPC specific compiler switches, because ld 
needs to have the relocation data on all platforms. The only additional 
requirement is for ld to actually output this data. It seems pretty 
likely that this is implemented on all platforms, because at least a 
subset of it is required for shared libraries and dynamically linked 
executables.

Best Regards,
Bill Campbell
> The symbol tables can probably be made smaller with some careful
> ldscript tweaking.
>
> Haavard
>
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