src: use _init as the start of large pages

The assumption that the .text section starts at the mapping following
the one that contains `__executable_start` is not valid on 64-bit
Ubuntu 18.04 where the whole code resides in one mapping.

OTOH, The symbol `_init` is usually located at the beginning of the
.text section and it does not need the assumption that the next
mapping has the .text section. Thus, we use the symbol, if available,
to perform the mapping on Ubuntu 18.04.

We also rename the section into which we place the remapping code to
`lpstub`. This causes the linker to produce symbols `__start_lpstub`
and `__stop_lpstub`, the latter of which we do not use. Still,
`__start_lpstub` helps us find the end of the .text section because on
Ubuntu 18.04 this section is inserted before the end of the sole
mapping, so we use `__start_lpstub` as the end instead of the end of
the mapping.

src/large_pages/node_large_page.cc

@@ -70,8 +70,10 @@
 // If successful copy the code there and unmap the original region.
 
 #if defined(__linux__)
+#include <dlfcn.h>  // For retrieving _init at runtime
 extern "C" {
 extern char __executable_start;
+extern char __start_lpstub;
 }  // extern "C"
 #endif  // defined(__linux__)
 
@@ -106,6 +108,20 @@ inline uintptr_t hugepage_align_down(uintptr_t addr) {
   return ((addr) & ~((hps) - 1));
 }
 
+#if defined(__linux__)
+inline uintptr_t RetrieveInitOffset() {
+  uintptr_t init_offset = 0;
+  void* dlhandle = dlopen(nullptr, RTLD_NOW | RTLD_NOLOAD);
+  if (dlhandle != nullptr) {
+    init_offset = reinterpret_cast<uintptr_t>(dlsym(dlhandle, "_init"));
+    if (dlclose(dlhandle) != 0) {
+      PrintWarning("Failed to dlclose() self after retrieving _init");
+    }
+  }
+  return init_offset;
+}
+#endif
+
 // The format of the maps file is the following
 // address           perms offset  dev   inode       pathname
 // 00400000-00452000 r-xp 00000000 08:02 173521      /usr/bin/dbus-daemon
@@ -121,6 +137,7 @@ struct text_region FindNodeTextRegion() {
   std::string dev;
   char dash;
   uintptr_t start, end, offset, inode;
+  uintptr_t init_offset = RetrieveInitOffset();
 
   ifs.open("/proc/self/maps");
   if (!ifs) {
@@ -147,15 +164,31 @@ struct text_region FindNodeTextRegion() {
     if (start != reinterpret_cast<uintptr_t>(&__executable_start))
       continue;
 
-    // The next line is our .text section.
-    if (!std::getline(ifs, map_line))
-      break;
+    // On Ubuntu 18.04 the binary gets loaded into a single mapping. So, before
+    // we make the assumption that the next mapping contains the .text section
+    // we check if this mapping contains the symbol `_init` -- which, on
+    // Ubuntu 18.04 it does. If so, we calculate `start` and `end` from this
+    // mapping and take into account that we must exclude the section `lpstub`
+    // from the returned range, because `lpstub` contains the code responsible
+    // for re-mapping the .text section, and we don't want it re-mapping itself
+    // as it's doing that, because that will cause the process to crash.
+    if (init_offset != 0 && init_offset >= start && init_offset < end) {
+      uintptr_t lpstub_start = reinterpret_cast<uintptr_t>(&__start_lpstub);
+      if (lpstub_start > start && lpstub_start <= end) {
+        end = lpstub_start;
+      }
+      start = init_offset;
+    } else {
+      // The next line is our .text section.
+      if (!std::getline(ifs, map_line))
+        break;
 
-    iss = std::istringstream(map_line);
-    iss >> std::hex >> start;
-    iss >> dash;
-    iss >> std::hex >> end;
-    iss >> permission;
+      iss = std::istringstream(map_line);
+      iss >> std::hex >> start;
+      iss >> dash;
+      iss >> std::hex >> end;
+      iss >> permission;
+    }
 
     if (permission != "r-xp")
       break;
@@ -318,7 +351,7 @@ static bool IsSuperPagesEnabled() {
 // d. If successful copy the code there and unmap the original region
 int
 #if !defined(__APPLE__)
-__attribute__((__section__(".lpstub")))
+__attribute__((__section__("lpstub")))
 #else
 __attribute__((__section__("__TEXT,__lpstub")))
 #endif