Problem creating PIC executable

[Stefano Bonifazi]

Hi thank you very much for your answer!

Did you have a look at the logs of the readelf -r I posted in the other 
answer (to you) of mine?
crtbegin.o contained a lot of R_PPC_ADDR16_HA / R_PPC_ADDR16_LO pairs, I 
dunno if I can state crtbegin was not compiled -fPIC..
I did not compile gcc myself.. so maybe it is a Debian6 PPC bug.. what 
shall I do now? Where to post this problem rather than here?

For my work I tried to solve this way: I created a PowerPC cross 
toolchain, where I forced some -fPIC inside gcc sources where it makes 
crt stuff..
My new crtbegin.o with readelf -r gives this:

> Relocation section '.rela.text' at offset 0x818 contains 2 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name + Addend
> 00000000  0000051a R_PPC_REL32       00000000   .got2 + 7ff0
> 000000d4  0000051a R_PPC_REL32       00000000   .got2 + 7ff0
>
> Relocation section '.rela.got2' at offset 0x830 contains 6 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name + Addend
> 00000000  00000401 R_PPC_ADDR32      00000000   .bss + 0
> 00000004  00000401 R_PPC_ADDR32      00000000   .bss + 4
> 00000008  00001801 R_PPC_ADDR32      00000000   __DTOR_END__ + 0
> 0000000c  00000801 R_PPC_ADDR32      00000000   .dtors + 0
> 00000010  00000a01 R_PPC_ADDR32      00000000   .jcr + 0
> 00000014  00001901 R_PPC_ADDR32      00000000   _Jv_RegisterClasses + 0
>
> Relocation section '.rela.fini' at offset 0x878 contains 1 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name + Addend
> 00000000  00000217 R_PPC_LOCAL24PC   00000000   .text + 4
>
> Relocation section '.rela.init' at offset 0x884 contains 1 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name + Addend
> 00000000  00000217 R_PPC_LOCAL24PC   00000000   .text + d8
There are no more R_PPC_ADDR16_HA/R_PPC_ADDR16_LO pairs, can I say it is 
PIC?

Anyway also the simple PIC "hello world" produced with this crosstool 
chain (  Linux kernel version (2.6.35.10 (longterm))
binutils version (2.20.1) gcc version (4.4.3) glibc version (2.9) ) 
mantains the relocation problem:

> powerpc-unknown-linux-gnu-readelf -d bin/ppc-test|grep TEXT
>  0x00000016 (TEXTREL)                    0x0
>  0x0000001e (FLAGS)                      TEXTREL

I did the readelf -r to crti.o that is not part of gcc crtstuff but it 
is linked to the simple executable at link time, the output is:

> Relocation section '.rela.text' at offset 0x468 contains 3 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name + Addend
> 0000001a  00000306 R_PPC_ADDR16_HA   00000000   .rodata + 0
> 0000001e  00000304 R_PPC_ADDR16_LO   00000000   .rodata + 0
> 00000020  0000140a R_PPC_REL24       00000000   __libc_start_main + 0
>
> Relocation section '.rela.rodata' at offset 0x48c contains 4 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name + Addend
> 00000000  00000f01 R_PPC_ADDR32      00000000   _SDA_BASE_ + 0
> 00000004  00001101 R_PPC_ADDR32      00000000   main + 0
> 00000008  00001001 R_PPC_ADDR32      00000000   __libc_csu_init + 0
> 0000000c  00000d01 R_PPC_ADDR32      00000000   __libc_csu_fini + 0
>
> Relocation section '.rela.eh_frame' at offset 0x4bc contains 1 entries:
>  Offset     Info    Type            Sym.Value  Sym. Name + Addend
> 0000001c  0000021a R_PPC_REL32       00000000   .text + 0

  and there is the ugly pair R_PPC_ADDR16_HA/R_PPC_ADDR16_LO, and just 
after the R_PPC_REL24 of __libc_start_main that I get as error at runtime..
Can you tell me if this is the NON PIC code causing the mess??
It is produced inside glibc .. I even tried to force -fPIC in the 
makefile for that crt?, but crti.o comes up always the same..

Please help me.. other developers told me I can only get such help from 
gnu linkers experts!

On 02/22/2011 11:53 PM, Ian Lance Taylor wrote:
>
> You just have to know which relocations are compatible with PIC and
> which are not.
>
Where can I study that? Consider I am seeing for the first time all of 
it.. most of all on precious articles like yours :)
>> Are those not PIC all "R_PPC_ADDR16_HA/R_PPC_ADDR16_LO pairs" ?
> I could be wrong, but I think that those relocs are not PIC.
>
Is there any official paper where to check it?
>> They are almost everywhere inside those .o !! What are magic symbols?
>> How to distinguish them?
> There is typically one magic symbol named something like
> _GLOBAL_OFFSET_TABLE_.
>
Thank you.. I met it often while checking the relocs .. so this is a 
fine one..
>> I didn't compile gcc myself.. I am simply using what Debian6 PowerPC
>> installed during installation.. so if this is bugged, all those who
>> have installed Debian6 PPC have a bugged one!
>> What can I do? Is there a way to force gcc to create all PIC code when
>> installing(or compiling) it?
> On GNU/Linux it seems very odd to me that if you use -fPIC on the link
> line that you get crtbegin.o rather than crtbeginS.o.  Normally
> crtbeginS.o is the version compiled with -fPIC.
>
Anyway to fix the compiler to use the correct one? .. Is it a bug?
Maybe I have to use directly the linker forcing crtbeginS.o?


Thank you very much in advance!
Stefano B.