llvm dev - Aug 2012 - [LLVMdev] llvm-objdump does not give information about all relocations

Hi,

We are trying to use LLVM API to programmatically obtain a list of
relocations in an ELF file. The way we are doing this is exactly as
llvm-objdump does it: we are iterating through sections and in each
section we are iterating over relocations (see PrintRelocations()
function at
https://llvm.org/svn/llvm-project/llvm/trunk/tools/llvm-objdump/llvm-objdump.cpp).
However, it does not give us relocation entries in .rela.dyn section.

Consider, for instance, the output of "readelf -r /bin/true":

$ readelf -r /bin/true

Relocation section '.rela.dyn' at offset 0xa80 contains 5 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000604c38  001800000006 R_X86_64_GLOB_DAT 0000000000000000 __gmon_start__ + 0
000000604de0  002800000005 R_X86_64_COPY     0000000000604de0 __progname + 0
000000604df0  002700000005 R_X86_64_COPY     0000000000604df0 stdout + 0
000000604e00  002a00000005 R_X86_64_COPY     0000000000604e00
__progname_full + 0
000000604e08  002c00000005 R_X86_64_COPY     0000000000604e08 stderr + 0

Relocation section '.rela.plt' at offset 0xaf8 contains 37 entries:
  Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000604c60  000100000007 R_X86_64_JUMP_SLO 0000000000000000 free + 0
000000604c68  000200000007 R_X86_64_JUMP_SLO 0000000000000000 abort + 0
000000604c70  000300000007 R_X86_64_JUMP_SLO 0000000000000000
__errno_location + 0
000000604c78  000400000007 R_X86_64_JUMP_SLO 0000000000000000 _exit + 0
000000604c80  000500000007 R_X86_64_JUMP_SLO 0000000000000000 __fpending + 0
...

(The list for .rela.plt has been truncated for brevity.)

If we now run "llvm-objdump-3.1 -r /bin/true", we will get a list of
entries in .rela.plt section:

$ llvm-objdump-3.1 -r /bin/true | grep -c JUMP_SLOT
37

So the question is: why llvm-objdump does not list relocation entries
in .rela.dyn section and is it possible to get at them somehow?

Aleksandrs

Received no answer, so reported a bug: see
http://llvm.org/bugs/show_bug.cgi?id=13650 .

On Fri, Aug 3, 2012 at 1:10 PM, Aleksandrs Saveljevs
<aleksandrs.saveljevs at gmail.com> wrote:
> Hi,
>
> We are trying to use LLVM API to programmatically obtain a list of
> relocations in an ELF file. The way we are doing this is exactly as
> llvm-objdump does it: we are iterating through sections and in each
> section we are iterating over relocations (see PrintRelocations()
> function at
https://llvm.org/svn/llvm-project/llvm/trunk/tools/llvm-objdump/llvm-objdump.cpp).
> However, it does not give us relocation entries in .rela.dyn section.
>
> Consider, for instance, the output of "readelf -r /bin/true":
>
> $ readelf -r /bin/true
>
> Relocation section '.rela.dyn' at offset 0xa80 contains 5 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name +
Addend
> 000000604c38  001800000006 R_X86_64_GLOB_DAT 0000000000000000
__gmon_start__ + 0
> 000000604de0  002800000005 R_X86_64_COPY     0000000000604de0 __progname +
0
> 000000604df0  002700000005 R_X86_64_COPY     0000000000604df0 stdout + 0
> 000000604e00  002a00000005 R_X86_64_COPY     0000000000604e00
> __progname_full + 0
> 000000604e08  002c00000005 R_X86_64_COPY     0000000000604e08 stderr + 0
>
> Relocation section '.rela.plt' at offset 0xaf8 contains 37 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name +
Addend
> 000000604c60  000100000007 R_X86_64_JUMP_SLO 0000000000000000 free + 0
> 000000604c68  000200000007 R_X86_64_JUMP_SLO 0000000000000000 abort + 0
> 000000604c70  000300000007 R_X86_64_JUMP_SLO 0000000000000000
> __errno_location + 0
> 000000604c78  000400000007 R_X86_64_JUMP_SLO 0000000000000000 _exit + 0
> 000000604c80  000500000007 R_X86_64_JUMP_SLO 0000000000000000 __fpending +
0
> ...
>
> (The list for .rela.plt has been truncated for brevity.)
>
> If we now run "llvm-objdump-3.1 -r /bin/true", we will get a list
of
> entries in .rela.plt section:
>
> $ llvm-objdump-3.1 -r /bin/true | grep -c JUMP_SLOT
> 37
>
> So the question is: why llvm-objdump does not list relocation entries
> in .rela.dyn section and is it possible to get at them somehow?
>
> Aleksandrs