Linux ELF files: Which byte will differ for static and dynamic ELF programs?

Question

I'm working with linux elf files.

I want to detect, if the given elf program is statically linked (full static link, ldd says "not a dynamic executable") or dynamically linked. The ELF is for embedded Linux, so I can't just run it or use ldd utility.

I want to do this entirely in my program, by reading and checking some bytes. I want not to depend on file utility or on libelf, binutils, etc.

Which bytes will be different?

Answer 1

How about using ldd.c from μClibc? It should be fairly easy to strip out any unwanted dependencies / checks if you want. I think this is a smarter approach than trying to figure out all the corner cases from reading man 5 elf, though FWIW it looks to be just checking for a PT_INTERP program header as you suspect in the comments.

Update: There's a few more checks. I've tried to extract the relevant parts, but I can't be sure if I've missed anything so check for yourself. The code checks 32-bit and 64-bit x86 ELF files. It assumes a little-endian architecture.

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
#include <ctype.h>
#include <inttypes.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>

#include <elf.h>

int main(int argc, char* argv[])
{
    const char* fname = argv[0];
    if (argc >= 2) fname = argv[1];

    int fd;
    struct stat st;
    void *mapping;

    if ((fd = open(fname, O_RDONLY)) == -1) {
        perror(fname);
        return 1;
    }

    if (fstat(fd, &st)) {
        perror("fstat");
        close(fd);
        return 1;
    }

    if ((mapping = mmap(NULL, st.st_size, PROT_READ, MAP_SHARED, fd, 0)) == MAP_FAILED) {
        perror("mmap");
        close(fd);
        return 1;
    }
    const Elf32_Ehdr* eh = mapping;

    if (st.st_size < (off_t)sizeof(Elf32_Ehdr) ||
        eh->e_ident[EI_MAG0] != ELFMAG0 || 
        eh->e_ident[EI_MAG1] != ELFMAG1 || 
        eh->e_ident[EI_MAG2] != ELFMAG2 || 
        eh->e_ident[EI_MAG3] != ELFMAG3 ||
        eh->e_ident[EI_VERSION] != EV_CURRENT) {
        printf("Not a valid ELF file\n");
        return 0;
    }

    if (eh->e_type != ET_EXEC && eh->e_type != ET_DYN) {
        printf("Not executable or shared object\n");
        return 0;
    }

    int is_dynamic = 0;

    // change as appropriate, but remember that byteswapping might be needed in some cases
    if (eh->e_ident[EI_CLASS] == ELFCLASS32 && eh->e_ident[EI_DATA] == ELFDATA2LSB && eh->e_machine == EM_386) {
        uint16_t ph_cnt;
        for (ph_cnt = 0; ph_cnt < eh->e_phnum; ph_cnt++) {
            const Elf32_Phdr* ph = (const Elf32_Phdr*)((const uint8_t*)mapping + eh->e_phoff + ph_cnt * eh->e_phentsize);
            if (ph->p_type == PT_DYNAMIC || ph->p_type == PT_INTERP) {
                is_dynamic = 1;
            }
        }
    } else if (eh->e_ident[EI_CLASS] == ELFCLASS64 && eh->e_ident[EI_DATA] == ELFDATA2LSB && eh->e_machine == EM_X86_64) {
        const Elf64_Ehdr* eh = mapping;
        uint16_t ph_cnt;
        for (ph_cnt = 0; ph_cnt < eh->e_phnum; ph_cnt++) {
            const Elf64_Phdr* ph = (const Elf64_Phdr*)((const uint8_t*)mapping + eh->e_phoff + ph_cnt * eh->e_phentsize);
            if (ph->p_type == PT_DYNAMIC || ph->p_type == PT_INTERP) {
                is_dynamic = 1;
            }
        }
    } else {
        printf("Unsupported architecture\n");
        return 0;
    }

    munmap(mapping, st.st_size);
    close(fd);
    printf("%s: %sdynamic\n", fname, is_dynamic?"":"not ");
    return 0;
}