Retrieve symbol name of RELA in ELF executable

Question

I am trying to retrieve the symbol name of RELA jump slots found in an ELF executable. Using libElf, I have managed to retrieve the address of the RELA but still trying to figure out how to get the symbol(-name).

According to How can I get the symbol name in struct "Elf64_Rela", the symbol is stored in the DYNSYM section which I tried to query but without satisfying results.

Here is what I've got so far (a little dirty though):

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <libelf.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <gelf.h>

#define LIB_PATH "/tmp/a.out"

int main(void)
{
    int fd = open(LIB_PATH, O_RDONLY);
    Elf *elf;
    Elf_Scn *scn;
    Elf_Data *rela_data = NULL;
    Elf_Data *sym_data = NULL;
    GElf_Shdr shdr_rela;
    GElf_Shdr shdr_sym;
    GElf_Rela rela;
    GElf_Sym sym;
    int count = 0;

    elf_version(EV_CURRENT);
    elf = elf_begin(fd, ELF_C_READ, NULL);
    if (!elf)
            fprintf(stderr, "ERROR: %s\n", elf_errmsg(elf_errno()));
    for (scn = elf_getscn(elf, 0); scn; scn = elf_nextscn(elf, scn))
    {
        gelf_getshdr(scn, &shdr_sym);
        if (shdr_sym.sh_type == SHT_DYNSYM)
            break;
    }
    sym_data = elf_getdata(scn, sym_data);
    for (scn = elf_getscn(elf, 0); scn; scn = elf_nextscn(elf, scn))
    {
        gelf_getshdr(scn, &shdr_rela);
        if (shdr_rela.sh_type == SHT_RELA)
            break;
    }
    scn = elf_nextscn(elf, scn);
    gelf_getshdr(scn, &shdr_rela);
    rela_data = elf_getdata(scn, rela_data);
    for (unsigned int it = 0; it < (shdr_rela.sh_size / shdr_rela.sh_entsize); ++it)
    {
        gelf_getrela(rela_data, it, &rela);
        gelf_getsym(sym_data, GELF_R_SYM(rela.r_info), &sym);
        printf("[%-8p]: (%d) %s\n", rela.r_offset, GELF_R_SYM(rela.r_info), elf_strptr(elf, shdr_rela.sh_link, sym.st_shndx));
    }
    elf_end(elf);
    close(fd);
    return 0;
}

How can I retrieve the name of the RELA present in my ELF executable ?

score 0 · Answer 1 · answered Sep 19 '22 at 01:35

There are two incorrect arguments in the call to elf_strptr(). To find the symbol names, use:

elf_strptr(elf, shdr_sym.sh_link, sym.st_name)

Explanation: the names are in the dynsym section (shdr_sym), not the rela section, and the index of the name is identified by the field st_name not st_shndx.

This answer is obviously too late to be useful for the OP, but may be of interest to others trying to do the same.

Retrieve symbol name of RELA in ELF executable

1 Answers1