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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <elf.h>
/* The Android NDK headers define those */
#undef Elf_Ehdr
#undef Elf_Addr
#if defined(__LP64__)
# define Elf_Ehdr Elf64_Ehdr
# define Elf_Addr Elf64_Addr
# define Elf_Ehdr Elf32_Ehdr
# define Elf_Addr Elf32_Addr
// On ARM, PC-relative function calls have a limit in how far they can jump,
// which might not be enough for e.g. The easy way out would be
// to use the long_call attribute, which forces the compiler to generate code
// that can call anywhere, but clang doesn't support the attribute yet
// (, and while the command-line
// equivalent does exist, it's currently broken
// ( So we create a manual
// trampoline, corresponding to the code GCC generates with long_call.
#ifdef __arm__
__attribute__((section(".text._init_trampoline"), naked)) int init_trampoline(
int argc, char** argv, char** env) {
__asm__ __volatile__(
// thumb doesn't allow to use r12/ip with ldr, and thus would require an
// additional push/pop to save/restore the modified register, which would
// also change the call into a blx. It's simpler to switch to arm.
" ldr ip, .LADDR\n"
" add ip, pc, ip\n"
" bx ip\n"
" .word real_original_init-(.LAFTER+8)\n");
extern __attribute__((visibility("hidden"))) void original_init(int argc,
char** argv,
char** env);
extern __attribute__((visibility("hidden"))) Elf_Addr relhack[];
extern __attribute__((visibility("hidden"))) Elf_Ehdr elf_header;
extern __attribute__((visibility("hidden"))) int (*mprotect_cb)(void* addr,
size_t len,
int prot);
extern __attribute__((visibility("hidden"))) long (*sysconf_cb)(int name);
extern __attribute__((visibility("hidden"))) char relro_start[];
extern __attribute__((visibility("hidden"))) char relro_end[];
static inline __attribute__((always_inline)) void do_relocations(void) {
Elf_Addr* ptr;
for (Elf_Addr* entry = relhack; *entry; entry++) {
if ((*entry & 1) == 0) {
ptr = (Elf_Addr*)((intptr_t)&elf_header + *entry);
*ptr += (intptr_t)&elf_header;
} else {
size_t remaining = (8 * sizeof(Elf_Addr) - 1);
Elf_Addr bits = *entry;
do {
bits >>= 1;
if (bits & 1) {
*ptr += (intptr_t)&elf_header;
} while (bits);
ptr += remaining;
__attribute__((section(".text._init_noinit"))) int init_noinit(int argc,
char** argv,
char** env) {
return 0;
__attribute__((section(".text._init"))) int init(int argc, char** argv,
char** env) {
original_init(argc, argv, env);
// Ensure there is no tail-call optimization, avoiding the use of the
// B.W instruction in Thumb for the call above.
return 0;
static inline __attribute__((always_inline)) void do_relocations_with_relro(
void) {
long page_size = sysconf_cb(_SC_PAGESIZE);
uintptr_t aligned_relro_start = ((uintptr_t)relro_start) & ~(page_size - 1);
// The relro segment may not end at a page boundary. If that's the case, the
// remainder of the page needs to stay read-write, so the last page is never
// set read-only. Thus the aligned relro end is page-rounded down.
uintptr_t aligned_relro_end = ((uintptr_t)relro_end) & ~(page_size - 1);
// By the time the injected code runs, the relro segment is read-only. But
// we want to apply relocations in it, so we set it r/w first. We'll restore
// it to read-only in relro_post.
aligned_relro_end - aligned_relro_start, PROT_READ | PROT_WRITE);
aligned_relro_end - aligned_relro_start, PROT_READ);
// mprotect_cb and sysconf_cb are allocated in .bss, so we need to restore
// them to a NULL value.
mprotect_cb = NULL;
sysconf_cb = NULL;
__attribute__((section(".text._init_noinit_relro"))) int init_noinit_relro(
int argc, char** argv, char** env) {
return 0;
__attribute__((section(".text._init_relro"))) int init_relro(int argc,
char** argv,
char** env) {
original_init(argc, argv, env);
return 0;