Identify entries in a global offset table

Question

I'm attempting to write some ELF parsing logic (in C). Specifically, I'm trying to identify which entries in the GOT correspond to which functions.

I've crafted a simple program which contains references to malloc and free. Some relevant excerpts from readelf -a a.out:

Relocation section '.rela.plt' at offset 0x630 contains 2 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000003fc8  0000000100000007 R_X86_64_JUMP_SLOT     0000000000000000 free@GLIBC_2.2.5 + 0
0000000000003fd0  0000000500000007 R_X86_64_JUMP_SLOT     0000000000000000 malloc@GLIBC_2.2.5 + 0
No processor specific unwind information to decode

Symbol table '.dynsym' contains 8 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND free@GLIBC_2.2.5 (2)
     2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.34 (3)
     3: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTable
     4: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     5: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND malloc@GLIBC_2.2.5 (2)
     6: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
     7: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.2.5 (2)

I know how to use .dynstr to get the names of the symbols in .dynsym. However, how is readelf populating the symbol names in .rela.plt? I'm not seeing anything in the definitions of either Elf64_Sym or Elf64_Rel which would help. At first, I thought the st_shndx field in Elf64_Sym would be relevant but readelf is showing that value as SHN_UNDEF.

Answer 1

The information is contained in the Elf64_Rel structure. Specifically, the r_info field:

This member gives both the symbol table index with respect to which the relocation must be made and the type of relocation to apply.

The ELF64_R_SYM macro can be used to extract the offset from this field. As seen in the .rela.plt description in the OP, free, for example, has an index of 1 which corresponds to entry 1 in .dynsym.