PIE self-relocation in Rust

pie_reloc.rs

/// Processes all relocations in the ELF dynamic section
unsafe fn process_pie_relocations(auxv: &mut AuxVec) {
    // Locate the PT_DYNAMIC segment in the program headers. If we don't have
    // a .dynamic section then it means that we are a static executable and
    // don't need any relocations.
    let pt_dynamic = auxv
        .phdr
        .iter()
        .filter(|p| p.p_type == linux::PT_DYNAMIC)
        .next();

    // Calculate our load offset and the address of _DYNAMIC.
    let mut dynamic = if let Some(pt_dynamic) = pt_dynamic {
        // PIE executables have __executable_start at link address 0.
        #[cfg(target_arch = "aarch64")]
        asm!("adrp {}, __executable_start", out(reg) auxv.load_offset, options(pure, nomem, nostack, preserves_flags));
        #[cfg(target_arch = "riscv64")]
        asm!("lla {}, __executable_start", out(reg) auxv.load_offset, options(pure, nomem, nostack));
        pt_dynamic.p_vaddr.wrapping_add(auxv.load_offset as u64) as *const linux::Elf64_Dyn
    } else {
        // Static executables have a load offset of 0.
        return;
    };

    // Find the relocation arrays
    let mut rela = (ptr::null(), 0);
    loop {
        match (*dynamic).d_tag as u32 {
            linux::DT_NULL => break,
            linux::DT_RELA => {
                rela.0 = auxv.load_offset.wrapping_add((*dynamic).d_val as usize)
                    as *const linux::Elf64_Rela
            }
            linux::DT_RELASZ => {
                rela.1 = (*dynamic).d_val as usize / mem::size_of::<linux::Elf64_Rela>()
            }
            linux::DT_RELAENT => {
                if (*dynamic).d_val as usize != mem::size_of::<linux::Elf64_Rela>() {
                    early_abort()
                }
            }
            // The linker should only generate RELA entries
            linux::DT_RELSZ => early_abort(),
            _ => {}
        }
        dynamic = dynamic.add(1);
    }

    // Process the relocations. We only support R_*_RELATIVE relocations.
    if !rela.0.is_null() {
        for r in slice::from_raw_parts(rela.0, rela.1) {
            // Due to a linker bug, we may see stray R_RISCV_NONE relocations.
            // https://sourceware.org/bugzilla/show_bug.cgi?id=24673
            if cfg!(target_arch = "riscv64") && linux::ELF64_R_TYPE(r.r_info) == linux::R_RISCV_NONE
            {
                continue;
            }

            #[cfg(target_arch = "aarch64")]
            let r_relative = linux::R_AARCH64_RELATIVE;
            #[cfg(target_arch = "riscv64")]
            let r_relative = linux::R_RISCV_RELATIVE;
            if linux::ELF64_R_TYPE(r.r_info) != r_relative {
                early_abort();
            }
            let ptr = auxv.load_offset.wrapping_add(r.r_offset as usize) as *mut usize;
            *ptr = auxv.load_offset.wrapping_add(r.r_addend as usize);
        }
    }
}