1 files changed, 136 insertions, 0 deletions
diff --git a/build/unix/elfhack/inject.c b/build/unix/elfhack/inject.c
new file mode 100644
index 0000000000..9e8399a1d1
--- /dev/null
+++ b/build/unix/elfhack/inject.c
@@ -0,0 +1,136 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+#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
+#else
+#  define Elf_Ehdr Elf32_Ehdr
+#  define Elf_Addr Elf32_Addr
+#endif
+
+// On ARM, PC-relative function calls have a limit in how far they can jump,
+// which might not be enough for e.g. libxul.so. 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
+// (https://bugs.llvm.org/show_bug.cgi?id=40623), and while the command-line
+// equivalent does exist, it's currently broken
+// (https://bugs.llvm.org/show_bug.cgi?id=40624). 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.
+      ".arm\n"
+      "  ldr ip, .LADDR\n"
+      ".LAFTER:\n"
+      "  add ip, pc, ip\n"
+      "  bx ip\n"
+      ".LADDR:\n"
+      "  .word real_original_init-(.LAFTER+8)\n");
+}
+#endif
+
+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;
+        remaining--;
+        ptr++;
+        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) {
+  do_relocations();
+  return 0;
+}
+
+__attribute__((section(".text._init"))) int init(int argc, char** argv,
+                                                 char** env) {
+  do_relocations();
+  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.
+  mprotect_cb((void*)aligned_relro_start,
+              aligned_relro_end - aligned_relro_start, PROT_READ | PROT_WRITE);
+
+  do_relocations();
+
+  mprotect_cb((void*)aligned_relro_start,
+              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) {
+  do_relocations_with_relro();
+  return 0;
+}
+
+__attribute__((section(".text._init_relro"))) int init_relro(int argc,
+                                                             char** argv,
+                                                             char** env) {
+  do_relocations_with_relro();
+  original_init(argc, argv, env);
+  return 0;
+}