ciabatta/loader/loader-self-reloc.c

// Stage 2 of the dynamic loader
// the purpose of this module is to relocate itself
// and jump to the main body of the loader.

// Note that loader-entry.asm doesn't "jump" into
// the entry point of this module, but rather
// "falls-through". Which means the sections
// of this module and the module above must be
// contiguous, which we do by doing linker magic

// This also means that this module CANNOT have
// any externally-defined functions (static functions
// are fine with gcc and clang (I think))

#include <cia-def.h>
#include <bin/elf.h>
#include <sys/syscall.h>
#include <stdarg.h>
#include "loader.h"

extern void loader_entry(Loader_Info *ld_info);

void _dlstart_reloc_c(u64 *sp, Elf64_Dyn *dynv) {
    _dbg_print_string("Entered dynamic loader\n");
    // Find argc, argv in stack
    int argc = *sp;
    char **argv = (void *)(sp+1);
    // Skip over environment
    char **envp = argv+argc+1;
    int envc = 0;
    for(int i = 0; envp[i]; i += 1) {
        envc += 1;
    }
    // Load aux vector, which is responsible
    // for holding information for the loader such as
    // the location of phdrs or its load address
    u64 aux[AUX_CNT];
    Elf64_Aux *auxv = (Elf64_Aux *)(envp + envc + 1);
    for(int i = 0; i < AUX_CNT; ++i) {
        aux[i] = 0;
    }
    for(int i = 0; auxv[i].a_type != AT_NULL; ++i) {
        int aux_type = auxv[i].a_type;
        if(aux_type < AUX_CNT) {
            aux[aux_type] = auxv[i].a_val;
        }
    }
    // Read the _DYNAMIC vector
    u64 dyn[DYN_CNT];
    for(int i = 0; i < DYN_CNT; ++i) {
        dyn[i] = 0;
    }
    _dbg_printf("Dynamic section contents:\n");
    for(int i = 0; dynv[i].d_tag != DT_NULL; ++i) {
        u64 dyn_tag = dynv[i].d_tag;
        _dbg_printf("  %x: %x\n", dyn_tag, dynv[i].d_un.d_val);
        if(dyn_tag < DYN_CNT) {
            dyn[dyn_tag] = dynv[i].d_un.d_val;
        }
    }
    // Find the dynamic section
    if(dynv == NULL) {
        _dbg_printf("_DYNAMIC section wasn't found in dynamic loader\n");
    }
    // If the dynamic linker is not invoked as a command
    // It's load address can't be found in the aux vector
    // Therefore we compute the base as the difference
    // between &_DYNAMIC and it's p_vaddr
    u8 *base = (void *)aux[AT_BASE];
    if(base == NULL) {
        _dbg_printf("trying to find base\n");
        u8 *phdrs = (void *)aux[AT_PHDR];
        u64 ph_ent = aux[AT_PHENT];
        u64 ph_num = aux[AT_PHNUM];
        for(int i = 1; i < ph_num; ++i) {
            u64 ph_off = i * ph_ent;
            Elf64_Phdr *ph = (void *)(phdrs + ph_off);
            if(ph->p_type == PT_DYNAMIC) {
                _dbg_printf("_DYNAMIC vaddr: %x\n", ph->p_vaddr);
                base = (void *)(dynv - ph->p_vaddr);
            }
        }
    }
    _dbg_printf("ld-cia.so base: %x\n", base);
    _dbg_printf("ld-cia.so _dynamic: %x\n", dynv);
    Elf64_Sym *symtab = symtab = (void *)(base + dyn[DT_SYMTAB]);
    if(symtab == NULL) {
        _dbg_printf("ERROR: No .dynsym found\n");
    }
    // Use memory fences, to MAKE SURE the compiler won't reorder code and
    // accidentally use relocations when they are not ready. The code before
    // this point is carefully written to avoid generating relocations.
    _mfence();
    // Apply relocations
    if(dyn[DT_REL] != 0) {
        _dbg_printf("REL Relocations found. This part isn't implemented\n");
        u8 *rel_ents = (void *)(base + dyn[DT_REL]);
        u64 rel_ent = dyn[DT_RELENT];
        u64 rel_size = dyn[DT_RELSZ];
        u64 rel_offs = 0;
        while(rel_offs < rel_size) {
            Elf64_Rel *rel = (void *)(rel_ents + rel_offs);
            u64 offs = rel->r_offset;
            u32 sym = ELF64_R_SYM(rel->r_info);
            u32 type = ELF64_R_TYPE(rel->r_info);
            _dbg_printf("  %d @ %d (%d)\n", sym, offs, type);
            // TODO: if needed
            rel_offs += rel_ent;
        }
    }
    if(dyn[DT_RELA] != 0) {
        _dbg_printf("RELA:\n");
        u8 *rela_ents = (void *)(base + dyn[DT_RELA]);
        u64 rela_ent = dyn[DT_RELAENT];
        u64 rela_size = dyn[DT_RELASZ];
        u64 rela_offs = 0;
        while(rela_offs < rela_size) {
            Elf64_Rela *rela = (void *)(rela_ents + rela_offs);
            u64 reloc_offs = rela->r_offset;
            u64 addend = rela->r_addend;
            u32 sym_idx = ELF64_R_SYM(rela->r_info);
            u32 type = ELF64_R_TYPE(rela->r_info);
            _dbg_printf("  %x+%d, @%x (%d)\n", sym_idx, addend, reloc_offs, type);
            if(type == R_X86_64_GLOB_DAT) {
                Elf64_Sym *sym = &symtab[sym_idx];
                void *sym_addr = (void *)(base + sym->st_value);
                void **reloc_addr = (void *)(base + reloc_offs);
                _dbg_printf("  -> %x\n", sym_addr);
                *reloc_addr = sym_addr;
            }
            else if(type == R_X86_64_RELATIVE) {
                void *addr = (void *)(base + addend);
                void **reloc_addr = (void *)(base + reloc_offs);
                *reloc_addr = addr;
                _dbg_printf("  -> %x\n", addr);
            }
            else {
                printf("ERROR: unhandled relocation type: %d\n", type);
                sys_exit(1);
            }
            rela_offs += rela_ent;
        }
    }
    if(dyn[DT_PLTGOT] != 0) {
        _dbg_printf("PLT relocations found\n");
        if(dyn[DT_PLTREL] == DT_REL) {
            _dbg_printf("  PLT relocations use relocations of type REL\n");
            printf("ERROR: .plt relocations of type REL not implemented\n");
            sys_exit(1);
        }
        else if(dyn[DT_PLTREL] == DT_RELA) {
            _dbg_printf("  PLT relocations use relocations of type RELA\n");
        }
        void *plt = (void *)(base + dyn[DT_PLTGOT]);
        void *rela_plt = (void *)(base + dyn[DT_JMPREL]);
        u64 rela_ent = sizeof(Elf64_Rela);
        u64 rela_size = dyn[DT_PLTRELSZ];
        u64 rela_offs = 0;
        while(rela_offs < rela_size) {
            Elf64_Rela *rela = (void *)(rela_plt + rela_offs);
            u64 reloc_offs = rela->r_offset;
            u64 addend = rela->r_addend;
            u32 sym_idx = ELF64_R_SYM(rela->r_info);
            u32 type = ELF64_R_TYPE(rela->r_info);
            _dbg_printf("  %x+%d, @%x (%d)\n", sym_idx, addend, reloc_offs, type);
            if(type == R_X86_64_JUMP_SLOT) {
                Elf64_Sym *sym = &symtab[sym_idx];
                void *sym_addr = (void *)(base + sym->st_value);
                void **reloc_addr = (void *)(base + reloc_offs);
                _dbg_printf("  -> resolving with %x\n", sym_addr);
                *reloc_addr = sym_addr;
            }
            rela_offs += rela_ent;
        }
        
    }
    _mfence();
    Loader_Info ld_info;
    ld_info.sp = sp;
    ld_info.ldso_base = base;
    ld_info.dyn = dyn;
    ld_info.aux = aux;
    _dbg_printf("Self-relocation finished. Entering the loader\n");
    loader_entry(&ld_info);
    sys_exit(0);
}