src: use __executable_start for linux hugepages

`__executable_start` is provided by GNU's and LLVM's default linker
scripts, obviating the need to plug in a custom linker script.

The problem with our bespoke linker script is that it works with ld.bfd
but not ld.gold and cannot easily be ported because the latter linker
doesn't understand the `INSERT BEFORE` directive.

The /proc/self/maps scanner is updated to account for the fact that
there are a number of sections between `&__executable_start` and
the start of the .text section.

Fortunately, those sections are all mapped into the same memory segment
so we only need to look at the next line to find the start of our text
segment.

Fixes: nodejs#31520

PR-URL: nodejs#31547
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>

node.gyp

@@ -847,7 +847,8 @@
           ],
         }],
         [ 'OS in "linux freebsd mac" and '
-          'target_arch=="x64"', {
+          'target_arch=="x64" and '
+          'node_target_type=="executable"', {
           'defines': [ 'NODE_ENABLE_LARGE_CODE_PAGES=1' ],
           'sources': [
             'src/large_pages/node_large_page.cc',

node.gypi

@@ -317,22 +317,6 @@
       'ldflags': [ '-Wl,-z,relro',
                    '-Wl,-z,now' ]
     }],
-    [ 'OS=="linux" and '
-      'target_arch=="x64" and '
-      'llvm_version=="0.0"', {
-      'ldflags': [
-        '-Wl,-T',
-        '<!(echo "$(pwd)/src/large_pages/ld.implicit.script")',
-      ]
-    }],
-    [ 'OS=="linux" and '
-      'target_arch=="x64" and '
-      'llvm_version!="0.0"', {
-      'ldflags': [
-        '-Wl,-T',
-        '<!(echo "$(pwd)/src/large_pages/ld.implicit.script.lld")',
-      ]
-    }],
     [ 'node_use_openssl=="true"', {
       'defines': [ 'HAVE_OPENSSL=1' ],
       'conditions': [

src/large_pages/node_large_page.cc

@@ -65,7 +65,11 @@
 // Use madvise with MADV_HUGEPAGE to use Anonymous 2M Pages
 // If successful copy the code there and unmap the original region.
 
-extern char __nodetext;
+#if defined(__linux__)
+extern "C" {
+extern char __executable_start;
+}  // extern "C"
+#endif  // defined(__linux__)
 
 namespace node {
 
@@ -116,17 +120,6 @@ static struct text_region FindNodeTextRegion() {
     return nregion;
   }
 
-  std::string exename;
-  {
-      char selfexe[PATH_MAX];
-
-      size_t size = sizeof(selfexe);
-      if (uv_exepath(selfexe, &size))
-        return nregion;
-
-      exename = std::string(selfexe, size);
-  }
-
   while (std::getline(ifs, map_line)) {
     std::istringstream iss(map_line);
     iss >> std::hex >> start;
@@ -136,26 +129,42 @@ static struct text_region FindNodeTextRegion() {
     iss >> offset;
     iss >> dev;
     iss >> inode;
-    if (inode != 0) {
-      std::string pathname;
-      iss >> pathname;
-      if (pathname == exename && permission == "r-xp") {
-        uintptr_t ntext = reinterpret_cast<uintptr_t>(&__nodetext);
-        if (ntext >= start && ntext < end) {
-          char* from = reinterpret_cast<char*>(hugepage_align_up(ntext));
-          char* to = reinterpret_cast<char*>(hugepage_align_down(end));
-
-          if (from < to) {
-            size_t size = to - from;
-            nregion.found_text_region = true;
-            nregion.from = from;
-            nregion.to = to;
-            nregion.total_hugepages = size / hps;
-          }
-          break;
-        }
-      }
-    }
+
+    if (inode == 0)
+      continue;
+
+    std::string pathname;
+    iss >> pathname;
+
+    if (start != reinterpret_cast<uintptr_t>(&__executable_start))
+      continue;
+
+    // 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;
+
+    if (permission != "r-xp")
+      break;
+
+    char* from = reinterpret_cast<char*>(hugepage_align_up(start));
+    char* to = reinterpret_cast<char*>(hugepage_align_down(end));
+
+    if (from >= to)
+      break;
+
+    size_t size = to - from;
+    nregion.found_text_region = true;
+    nregion.from = from;
+    nregion.to = to;
+    nregion.total_hugepages = size / hps;
+
+    break;
   }
 
   ifs.close();
@@ -408,14 +417,12 @@ int MapStaticCodeToLargePages() {
     return -1;
   }
 
-#if defined(__linux__) || defined(__FreeBSD__)
-  if (r.from > reinterpret_cast<void*>(&MoveTextRegionToLargePages))
-    return MoveTextRegionToLargePages(r);
+#if defined(__FreeBSD__)
+  if (r.from < reinterpret_cast<void*>(&MoveTextRegionToLargePages))
+    return -1;
+#endif
 
-  return -1;
-#elif defined(__APPLE__)
   return MoveTextRegionToLargePages(r);
-#endif
 }
 
 bool IsLargePagesEnabled() {