Understanding section headers ELF

Question

static inline Elf32_Shdr *elf_sheader(Elf32_Ehdr *hdr) {
    return (Elf32_Shdr *)((int)hdr + hdr->e_shoff);
}

static inline Elf32_Shdr *elf_section(Elf32_Ehdr *hdr, int idx) {
    return &elf_sheader(hdr)[idx];
}

Okay the first function here returns a pointer to a elf section header by using hdr_shoff because that is the offset to first section header . Now the second function is used to access more section headers ( if there be any ) just by using array indexing .

static inline char *elf_str_table(Elf32_Ehdr *hdr) {
    if(hdr->e_shstrndx == SHN_UNDEF) return NULL;
    return (char *)hdr + elf_section(hdr, hdr->e_shstrndx)->sh_offset;
}

static inline char *elf_lookup_string(Elf32_Ehdr *hdr, int offset) {
    char *strtab = elf_str_table(hdr);
    if(strtab == NULL) return NULL;
    return strtab + offset;
}

I am having problems with the above two function used for accessing section names . e->shstrndx is the index of the string table . So in elf_str_table we first check it against SHN_UNDEF . But in the return I don't understand that hdr->e_shstrndx is the index to a string table , how is that index added to the starting address of the elf_section header giving another elf section header ( as we are using it access sh_offset ) . My confusion is that e->shstrndx is an index to a string table but how is it that this index along with elf_section returning a pointer to struct Elf32_Shdr ?

Reference : http://wiki.osdev.org/ELF_Tutorial#Accessing_Section_Headers

Answer 1

You said yourself that elf_section returns a section header based on an index.

e_shstrndx is the index of the section header that contains the offset of the section header string table.

So, you use e_shstrndx as a parameter for elf_section to get that section header :

Elf32_Shdr* shstr = elf_section(hdr, hdr->e_shstrndx);

Then get the offset from that section header :

int strtab_offset = shstr->sh_offset;

And use it to get the actual string table :

char* strtab = (char*) hdr + strtab_offset;

From this string table, you can then get the names of sections based on their offset :

char* str = strtab + offset;