path: root/reloc.c

/* reloc.c - MemTest-86  Version 3.3
 *
 * Released under version 2 of the Gnu Public License.
 * By Eric Biederman
 */

#include "stddef.h"
#include "stdint.h"
#include "elf.h"

#define __ELF_NATIVE_CLASS 32
#define ELF_MACHINE_NO_RELA 1

/* We use this macro to refer to ELF types independent of the native wordsize.
   `ElfW(TYPE)' is used in place of `Elf32_TYPE' or `Elf64_TYPE'.  */
#define ElfW(type)	_ElfW (Elf, __ELF_NATIVE_CLASS, type)
#define _ElfW(e,w,t)	_ElfW_1 (e, w, _##t)
#define _ElfW_1(e,w,t)	e##w##t
#define ELFW(type)	_ElfW (ELF, __ELF_NATIVE_CLASS, type)

static inline void __attribute__ ((unused))
fail_assert(unsigned line_num, unsigned code) {
    // Put line num in low bits of %edx and a sentinel in the high bits.
    line_num |= 0xEEEC0000;  // eek!
    asm __volatile__ ("movl %%edx, %%edx"
                      : "=d"(line_num), "=c" (code)
                      : "d" (line_num), "c"  (code));

    // Now observe %edx in a virtual machine's debugger.
    // Have a nice day.
    while(1) {}
}

// TODO(jcoiner): It would be nice to try to print a message here,
// even though we don't have function calls yet, maybe we can write
// to the screen before going into our infinite loop.
#define assert(expr, code) do {                        \
        if (!(expr)) { fail_assert(__LINE__, code); }  \
    } while(0)

/* This #define produces dynamic linking inline functions for
   bootstrap relocation instead of general-purpose relocation.  */
#define RTLD_BOOTSTRAP

struct link_map {
    ElfW(Addr) l_addr;  /* Current load address */
    ElfW(Addr) ll_addr; /* Last load address */
    ElfW(Dyn)  *l_ld;
    /* Indexed pointers to dynamic section.
       [0,DT_NUM) are indexed by the processor-independent tags.
       [DT_NUM,DT_NUM+DT_PROCNUM) are indexed by the tag minus DT_LOPROC.
       [DT_NUM+DT_PROCNUM,
        DT_NUM+DT_PROCNUM+DT_EXTRANUM)
       are indexed by DT_EXTRATAGIDX(tagvalue) (see <elf.h>).  */
    ElfW(Dyn)  *l_info[DT_NUM + DT_PROCNUM + DT_EXTRANUM];
};

/* Return the link-time address of _DYNAMIC.  Conveniently, this is the
   first element of the GOT.  This must be inlined in a function which
   uses global data.  */
static inline Elf32_Addr __attribute__ ((unused))
elf_machine_dynamic(void) {
    register Elf32_Addr *got asm ("%ebx");
    return *got;
}

/* Return the run-time load address of the shared object.  */
static inline Elf32_Addr __attribute__ ((unused))
elf_machine_load_address(void) {
    Elf32_Addr addr;
    asm volatile ("leal _start@GOTOFF(%%ebx), %0\n"
                  : "=r" (addr) : : "cc");
    return addr;
}

/* Perform the relocation specified by RELOC and SYM (which is fully resolved).
   MAP is the object containing the reloc.  */
static inline void
elf_machine_rel(struct link_map *map, const Elf32_Rel *reloc,
                const Elf32_Sym *sym, Elf32_Addr *const reloc_addr) {
    Elf32_Addr ls_addr, s_addr;
    Elf32_Addr value;
    if (ELF32_R_TYPE (reloc->r_info) == R_386_RELATIVE)
    {
        *reloc_addr += map->l_addr - map->ll_addr;
        return;
    }
    if (ELF32_R_TYPE(reloc->r_info) == R_386_NONE) {
        return;
    }
    value = sym->st_value;
    /* Every section except the undefined section has a base of map->l_addr */
    ls_addr = sym->st_shndx == SHN_UNDEF ? 0 : map->ll_addr;
    s_addr = sym->st_shndx == SHN_UNDEF ? 0 : map->l_addr;

    switch (ELF32_R_TYPE (reloc->r_info))
    {
    case R_386_COPY: {
        /* Roll memcpy by hand as we don't have function calls yet. */
        unsigned char *dest, *src;
        long i;
        dest = (unsigned char *)reloc_addr;
        src = (unsigned char *)(value + s_addr);
        for(i = 0; i < sym->st_size; i++) {
            dest[i] = src[i];
        }
	break;
    }
    case R_386_GLOB_DAT:
        *reloc_addr = s_addr + value;
        break;
    case R_386_JMP_SLOT:
        *reloc_addr = s_addr + value;
        break;
    case R_386_32:
        if (map->ll_addr == 0) {
            *reloc_addr += value;
        }
        *reloc_addr += s_addr - ls_addr;
        break;
    case R_386_PC32:
        if (map->ll_addr == 0) {
            *reloc_addr += value - reloc->r_offset;
        }
        *reloc_addr += (s_addr - map->l_addr) - (ls_addr - map->ll_addr);
        break;
    default:
        assert (! "unexpected dynamic reloc type", 0);
        break;
    }
}

/* Read the dynamic section at DYN and fill in INFO with indices DT_*.  */

static inline void __attribute__ ((unused))
elf_get_dynamic_info(ElfW(Dyn) *dyn, ElfW(Addr) l_addr,
                     ElfW(Dyn) *info[DT_NUM + DT_PROCNUM + DT_EXTRANUM]) {
    if (! dyn)
        return;

    while (dyn->d_tag != DT_NULL) {
        if (dyn->d_tag < DT_NUM)
            info[dyn->d_tag] = dyn;
        else if (dyn->d_tag >= DT_LOPROC &&
                 dyn->d_tag < DT_LOPROC + DT_PROCNUM)
            info[dyn->d_tag - DT_LOPROC + DT_NUM] = dyn;
        else if ((Elf32_Word) DT_EXTRATAGIDX (dyn->d_tag) < DT_EXTRANUM)
            info[DT_EXTRATAGIDX (dyn->d_tag) + DT_NUM + DT_PROCNUM] = dyn;
#if 0
        else {
            // JPC this is failing, BOZO!
            // Not surprising, there are a bunch of sections we don't
            // handle and which modern ELF relocation code in glibc
            // will handle and relocate. Bah.
            assert (! "bad dynamic tag", dyn->d_tag);
        }
#endif
        ++dyn;
    }

    if (info[DT_PLTGOT] != NULL) 
        info[DT_PLTGOT]->d_un.d_ptr += l_addr;
    if (info[DT_STRTAB] != NULL)
        info[DT_STRTAB]->d_un.d_ptr += l_addr;
    if (info[DT_SYMTAB] != NULL)
        info[DT_SYMTAB]->d_un.d_ptr += l_addr;
#if ! ELF_MACHINE_NO_RELA
    if (info[DT_RELA] != NULL) {
        assert (info[DT_RELAENT]->d_un.d_val == sizeof (ElfW(Rela)), 0);
        info[DT_RELA]->d_un.d_ptr += l_addr;
    }
#endif
#if ! ELF_MACHINE_NO_REL
    if (info[DT_REL] != NULL) {
        assert (info[DT_RELENT]->d_un.d_val == sizeof (ElfW(Rel)), 0);
        info[DT_REL]->d_un.d_ptr += l_addr;
    }
#endif
    if (info[DT_PLTREL] != NULL) {
#if ELF_MACHINE_NO_RELA
        assert (info[DT_PLTREL]->d_un.d_val == DT_REL, 0);
#elif ELF_MACHINE_NO_REL
        assert (info[DT_PLTREL]->d_un.d_val == DT_RELA, 0);
#else
        assert (info[DT_PLTREL]->d_un.d_val == DT_REL
                || info[DT_PLTREL]->d_un.d_val == DT_RELA, 0);
#endif
    }
    if (info[DT_JMPREL] != NULL)
        info[DT_JMPREL]->d_un.d_ptr += l_addr;
}

/* Perform the relocations in MAP on the running program image as specified
   by RELTAG, SZTAG.  If LAZY is nonzero, this is the first pass on PLT
   relocations; they should be set up to call _dl_runtime_resolve, rather
   than fully resolved now.  */

static inline void
elf_dynamic_do_rel(struct link_map *map,
                   ElfW(Addr) reladdr, ElfW(Addr) relsize) {
    const ElfW(Rel) *r = (const void *) reladdr;
    const ElfW(Rel) *end = (const void *) (reladdr + relsize);

    const ElfW(Sym) *const symtab =
        (const void *) map->l_info[DT_SYMTAB]->d_un.d_ptr;

    for (; r < end; ++r) {
        elf_machine_rel (map, r, &symtab[ELFW(R_SYM) (r->r_info)],
                         (void *) (map->l_addr + r->r_offset));
    }
}

void _dl_start(void) {
    static Elf32_Addr last_load_address = 0;
    struct link_map map;
    size_t cnt;

    /* Partly clean the `map' structure up.  Don't use `memset'
       since it might nor be built in or inlined and we cannot make function
       calls at this point.  */
    for (cnt = 0; cnt < sizeof(map.l_info) / sizeof(map.l_info[0]); ++cnt) {
        map.l_info[cnt] = 0;
    }

    /* Get the last load address */
    map.ll_addr = last_load_address;

    /* Figure out the run-time load address of the dynamic linker itself.  */
    last_load_address = map.l_addr = elf_machine_load_address();
	
    /* Read our own dynamic section and fill in the info array.  */
    map.l_ld = (void *)map.l_addr + elf_machine_dynamic();

    elf_get_dynamic_info (map.l_ld, map.l_addr - map.ll_addr, map.l_info);

    /* Relocate ourselves so we can do normal function calls and
     * data access using the global offset table.  
     */
#if !ELF_MACHINE_NO_REL
    elf_dynamic_do_rel(&map, 
                       map.l_info[DT_REL]->d_un.d_ptr,
                       map.l_info[DT_RELSZ]->d_un.d_val);
    if (map.l_info[DT_PLTREL]->d_un.d_val == DT_REL) {
        elf_dynamic_do_rel(&map, 
                           map.l_info[DT_JMPREL]->d_un.d_ptr,
                           map.l_info[DT_PLTRELSZ]->d_un.d_val);
    }
#endif

#if !ELF_MACHINE_NO_RELA
    elf_dynamic_do_rela(&map, 
                        map.l_info[DT_RELA]->d_un.d_ptr,
                        map.l_info[DT_RELASZ]->d_un.d_val);
    if (map.l_info[DT_PLTREL]->d_un.d_val == DT_RELA) {
        elf_dynamic_do_rela(&map, 
                            map.l_info[DT_JMPREL]->d_un.d_ptr,
                            map.l_info[DT_PLTRELSZ]->d_un.d_val);
    }
#endif

    /* Now life is sane; we can call functions and access global data.
       Set up to use the operating system facilities, and find out from
       the operating system's program loader where to find the program
       header table in core.  Put the rest of _dl_start into a separate
       function, that way the compiler cannot put accesses to the GOT
       before ELF_DYNAMIC_RELOCATE.  */
    return;
}