Meaning of ELF64_R_SYM(rela->r_info)

[Peng Yu]

I am trying to understand this better. I change the c source code a bit.

What is "a" in ".rodata + a"? What the offset refers to? I don't quite
follow the document. Of the two "puts - 4", could anybody explain how
r_offset is computed?

"r_offset indicates the location at which the relocation should be
applied. For a relocatable file, this is the offset, in bytes, from
the beginning of the section to the beginning of the storage unit
being relocated."

What is - 4 in "puts - 4" and "s - 4"? Why are they all "-4"?

$ cat a.c
#include <stdio.h>
const char s[] = "Hello World2!";
int main() {
  puts("Hello World!");
  puts(s);
}
$ readelf -rW a.o

Relocation section '.rela.text' at offset 0x248 contains 4 entries:
    Offset             Info             Type               Symbol's
Value  Symbol's Name + Addend
0000000000000007  0000000500000002 R_X86_64_PC32
0000000000000000 .rodata + a
000000000000000c  0000000c00000004 R_X86_64_PLT32
0000000000000000 puts - 4
0000000000000013  0000000900000002 R_X86_64_PC32          0000000000000000 s - 4
0000000000000018  0000000c00000004 R_X86_64_PLT32
0000000000000000 puts - 4

Relocation section '.rela.eh_frame' at offset 0x2a8 contains 1 entry:
    Offset             Info             Type               Symbol's
Value  Symbol's Name + Addend
0000000000000020  0000000200000002 R_X86_64_PC32
0000000000000000 .text + 0
$ readelf -s a.o

Symbol table '.symtab' contains 13 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS a.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT    7
     7: 0000000000000000     0 SECTION LOCAL  DEFAULT    8
     8: 0000000000000000     0 SECTION LOCAL  DEFAULT    6
     9: 0000000000000000    14 OBJECT  GLOBAL DEFAULT    5 s
    10: 0000000000000000    35 FUNC    GLOBAL DEFAULT    1 main
    11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_
    12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND puts

On 4/21/21, Nick Clifton <nickc@redhat.com> wrote:
> Hi Peng,
>
>> I see ELF64_R_SYM(rela->r_info) are 00000005 and 0000000b
>> respectively. I don't find what they refer to.
>
> You have already discovered this:
>
>> "r_info contains both a symbol table index and a relocation type. The
>> symbol table index identifies the symbol whose value should be used in
>> the relocation."
>
> So a value of 00000005 refers to the 5th entry in the symbol table
> associated with the relocation section that contains the reloc.
> Similarly 0000000b refers to the 11th entry in the symbol table.
>
>
>>      Offset             Info             Type               Symbol's
>> Value  Symbol's Name + Addend
>> 0000000000000007  0000000500000002 R_X86_64_PC32
>> 0000000000000000 .rodata - 4
>> 000000000000000c  0000000b00000004 R_X86_64_PLT32
>> 0000000000000000 puts - 4
>
> So readelf has decoded symbol index 5 and displayed ".rodata" and symbol
> index 11 and displayed "puts".
>
>
>> But I checked the symbol table and I don't think 00000005 and 0000000b
>> have anything to do with this table.
>
> But they do:
>
>>       5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5
>
> Symbol 5 is a section symbol, so its name is the section name, ie .rodata.
>
>>      11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND puts
>
> And here is the "puts" symbol.
>
>
>> Also for 00000002 and 00000004 (the lower half of Info), they just
>> refer to R_X86_64_PC32 and R_X86_64_PLT32. Why showing the same info
>> twice in the output of readelf -rW? Why not just show it once?
>
> It is done this way in order to help debug problems.  The raw value of
> the info field is displayed so that users can see the actual value, and
> then the decoded data (symbol name + relocation name + offset) is
> displayed so that the value can be easily interpreted.  Users *could*
> interpret the raw value themselves, but it can be a chore, and it is
> simple for readelf to perform that task.
>
> Cheers
>    Nick
>
> PS. A very recent update to readelf has changed the display of section
> symbols in the symbol table. so that they now show the section name,
> instead of just a blank space.
>
>


-- 
Regards,
Peng