Loading a Static ELF

Static executable: all symbols resolved; no shared libraries (except potentially compiled in).

Loading process:

int load_elf(int fd) {
    ElfHeader hdr;
    pread(fd, &hdr, sizeof hdr, 0);

    // Validate
    if (memcmp(hdr.ident, "\x7fELF", 4) != 0) return -1;
    if (hdr.ident[4] != 2) return -1;  // require 64-bit

    // Map all PT_LOAD segments
    for (int i = 0; i < hdr.phnum; i++) {
        ProgramHeader ph;
        pread(fd, &ph, sizeof ph, hdr.phoff + i * hdr.phentsize);
        if (ph.type != PT_LOAD) continue;

        int prot = 0;
        if (ph.flags & PF_R) prot |= PROT_READ;
        if (ph.flags & PF_W) prot |= PROT_WRITE;
        if (ph.flags & PF_X) prot |= PROT_EXEC;

        void *addr = mmap((void*)ph.vaddr, ph.memsz,
                          prot, MAP_PRIVATE | MAP_FIXED,
                          fd, ph.offset & ~(PAGE_SIZE - 1));
        if (addr == MAP_FAILED) return -1;

        // Zero BSS (if memsz > filesz)
        if (ph.memsz > ph.filesz) {
            memset((char*)ph.vaddr + ph.filesz, 0, ph.memsz - ph.filesz);
        }
    }

    // Set up stack
    void *stack = mmap(NULL, STACK_SIZE,
                       PROT_READ | PROT_WRITE,
                       MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    // Push argv, envp, auxv onto stack
    setup_initial_stack(stack, argv, envp);

    // Jump to entry
    ((void(*)(void))hdr.entry)();
    return 0;
}

Stack layout (System V ABI):

high addr
[NULL]
[envp[N], envp[N-1], ..., envp[0]]
[argv[N], argv[N-1], ..., argv[0]]
[argc]
low addr <- rsp

Auxiliary vector (auxv): kernel-provided info (page size, hwcap flags, random bytes, executable path).

Entry point (_start):

1. Sets up TLS (thread-local storage).
2. Calls __libc_start_main:
   • Run constructors.
   • Call main(argc, argv).
   • Run destructors.
   • exit() with main's return value.

Static binaries: large, but no runtime deps. Go binaries are static by default.