fusl/ldso/dlstart.c

#include <stddef.h>
#include "dynlink.h"

#ifndef START
#define START "_dlstart"
#endif

#include "crt_arch.h"

#ifndef GETFUNCSYM
#define GETFUNCSYM(fp, sym, got)                                   \
  do {                                                             \
    __attribute__((__visibility__("hidden"))) void sym();          \
    static void (*static_func_ptr)() = sym;                        \
    __asm__ __volatile__("" : "+m"(static_func_ptr) : : "memory"); \
    *(fp) = static_func_ptr;                                       \
  } while (0)
#endif

__attribute__((__visibility__("hidden"))) void _dlstart_c(size_t* sp,
                                                          size_t* dynv) {
  size_t i, aux[AUX_CNT], dyn[DYN_CNT];
  size_t *rel, rel_size, base;

  int argc = *sp;
  char** argv = (void*)(sp + 1);

  for (i = argc + 1; argv[i]; i++)
    ;
  size_t* auxv = (void*)(argv + i + 1);

  for (i = 0; i < AUX_CNT; i++)
    aux[i] = 0;
  for (i = 0; auxv[i]; i += 2)
    if (auxv[i] < AUX_CNT)
      aux[auxv[i]] = auxv[i + 1];

#if DL_FDPIC
  struct fdpic_loadseg *segs, fakeseg;
  size_t j;
  if (dynv) {
    /* crt_arch.h entry point asm is responsible for reserving
     * space and moving the extra fdpic arguments to the stack
     * vector where they are easily accessible from C. */
    segs = ((struct fdpic_loadmap*)(sp[-1] ? sp[-1] : sp[-2]))->segs;
  } else {
    /* If dynv is null, the entry point was started from loader
     * that is not fdpic-aware. We can assume normal fixed-
     * displacement ELF loading was performed, but when ldso was
     * run as a command, finding the Ehdr is a heursitic: we
     * have to assume Phdrs start in the first 4k of the file. */
    base = aux[AT_BASE];
    if (!base)
      base = aux[AT_PHDR] & -4096;
    segs = &fakeseg;
    segs[0].addr = base;
    segs[0].p_vaddr = 0;
    segs[0].p_memsz = -1;
    Ehdr* eh = (void*)base;
    Phdr* ph = (void*)(base + eh->e_phoff);
    size_t phnum = eh->e_phnum;
    size_t phent = eh->e_phentsize;
    while (phnum-- && ph->p_type != PT_DYNAMIC)
      ph = (void*)((size_t)ph + phent);
    dynv = (void*)(base + ph->p_vaddr);
  }
#endif

  for (i = 0; i < DYN_CNT; i++)
    dyn[i] = 0;
  for (i = 0; dynv[i]; i += 2)
    if (dynv[i] < DYN_CNT)
      dyn[dynv[i]] = dynv[i + 1];

#if DL_FDPIC
  for (i = 0; i < DYN_CNT; i++) {
    if (i == DT_RELASZ || i == DT_RELSZ)
      continue;
    if (!dyn[i])
      continue;
    for (j = 0; dyn[i] - segs[j].p_vaddr >= segs[j].p_memsz; j++)
      ;
    dyn[i] += segs[j].addr - segs[j].p_vaddr;
  }
  base = 0;

  const Sym* syms = (void*)dyn[DT_SYMTAB];

  rel = (void*)dyn[DT_RELA];
  rel_size = dyn[DT_RELASZ];
  for (; rel_size; rel += 3, rel_size -= 3 * sizeof(size_t)) {
    if (!IS_RELATIVE(rel[1], syms))
      continue;
    for (j = 0; rel[0] - segs[j].p_vaddr >= segs[j].p_memsz; j++)
      ;
    size_t* rel_addr = (void*)(rel[0] + segs[j].addr - segs[j].p_vaddr);
    if (R_TYPE(rel[1]) == REL_FUNCDESC_VAL) {
      *rel_addr += segs[rel_addr[1]].addr - segs[rel_addr[1]].p_vaddr +
                   syms[R_SYM(rel[1])].st_value;
      rel_addr[1] = dyn[DT_PLTGOT];
    } else {
      size_t val = syms[R_SYM(rel[1])].st_value;
      for (j = 0; val - segs[j].p_vaddr >= segs[j].p_memsz; j++)
        ;
      *rel_addr = rel[2] + segs[j].addr - segs[j].p_vaddr + val;
    }
  }
#else
  /* If the dynamic linker is invoked as a command, its load
   * address is not available in the aux vector. Instead, compute
   * the load address as the difference between &_DYNAMIC and the
   * virtual address in the PT_DYNAMIC program header. */
  base = aux[AT_BASE];
  if (!base) {
    size_t phnum = aux[AT_PHNUM];
    size_t phentsize = aux[AT_PHENT];
    Phdr* ph = (void*)aux[AT_PHDR];
    for (i = phnum; i--; ph = (void*)((char*)ph + phentsize)) {
      if (ph->p_type == PT_DYNAMIC) {
        base = (size_t)dynv - ph->p_vaddr;
        break;
      }
    }
  }

  /* MIPS uses an ugly packed form for GOT relocations. Since we
   * can't make function calls yet and the code is tiny anyway,
   * it's simply inlined here. */
  if (NEED_MIPS_GOT_RELOCS) {
    size_t local_cnt = 0;
    size_t* got = (void*)(base + dyn[DT_PLTGOT]);
    for (i = 0; dynv[i]; i += 2)
      if (dynv[i] == DT_MIPS_LOCAL_GOTNO)
        local_cnt = dynv[i + 1];
    for (i = 0; i < local_cnt; i++)
      got[i] += base;
  }

  rel = (void*)(base + dyn[DT_REL]);
  rel_size = dyn[DT_RELSZ];
  for (; rel_size; rel += 2, rel_size -= 2 * sizeof(size_t)) {
    if (!IS_RELATIVE(rel[1], 0))
      continue;
    size_t* rel_addr = (void*)(base + rel[0]);
    *rel_addr += base;
  }

  rel = (void*)(base + dyn[DT_RELA]);
  rel_size = dyn[DT_RELASZ];
  for (; rel_size; rel += 3, rel_size -= 3 * sizeof(size_t)) {
    if (!IS_RELATIVE(rel[1], 0))
      continue;
    size_t* rel_addr = (void*)(base + rel[0]);
    *rel_addr = base + rel[2];
  }
#endif

  stage2_func dls2;
  GETFUNCSYM(&dls2, __dls2, base + dyn[DT_PLTGOT]);
  dls2((void*)base, sp);
}