loader: Only extend segments if kernel supports page size migration am: c5c1d19ebb

Original change: https://android-review.googlesource.com/c/platform/bionic/+/3035335

Change-Id: Ie2eec54ba3c7307ae138787cf7f12a7e7992e941
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>

linker/linker_crt_pad_segment_test.cpp

@@ -72,13 +72,22 @@ bool GetPadSegment(const std::string& elf_path) {
 };  // anonymous namespace
 
 TEST(crt_pad_segment, note_absent) {
+  if (!page_size_migration_supported()) {
+    GTEST_SKIP() << "Kernel does not support page size migration";
+  }
   ASSERT_FALSE(GetPadSegment("no_crt_pad_segment.so"));
 }
 
 TEST(crt_pad_segment, note_present_and_enabled) {
+  if (!page_size_migration_supported()) {
+    GTEST_SKIP() << "Kernel does not support page size migration";
+  }
   ASSERT_TRUE(GetPadSegment("crt_pad_segment_enabled.so"));
 }
 
 TEST(crt_pad_segment, note_present_and_disabled) {
+  if (!page_size_migration_supported()) {
+    GTEST_SKIP() << "Kernel does not support page size migration";
+  }
   ASSERT_FALSE(GetPadSegment("crt_pad_segment_disabled.so"));
 }

linker/linker_phdr.cpp

@@ -46,6 +46,8 @@
 #include "private/CFIShadow.h" // For kLibraryAlignment
 #include "private/elf_note.h"
 
+#include <android-base/file.h>
+
 static int GetTargetElfMachine() {
 #if defined(__arm__)
   return EM_ARM;
@@ -707,8 +709,28 @@ bool ElfReader::ReserveAddressSpace(address_space_params* address_space) {
   return true;
 }
 
+/*
+ * Returns true if the kernel supports page size migration, else false.
+ */
+bool page_size_migration_supported() {
+  static bool pgsize_migration_enabled = []() {
+    std::string enabled;
+    if (!android::base::ReadFileToString("/sys/kernel/mm/pgsize_migration/enabled", &enabled)) {
+      return false;
+    }
+    return enabled.find("1") != std::string::npos;
+  }();
+  return pgsize_migration_enabled;
+}
+
 // Find the ELF note of type NT_ANDROID_TYPE_PAD_SEGMENT and check that the desc value is 1.
 bool ElfReader::ReadPadSegmentNote() {
+  if (!page_size_migration_supported()) {
+    // Don't attempt to read the note, since segment extension isn't
+    // supported; but return true so that loading can continue normally.
+    return true;
+  }
+
   // The ELF can have multiple PT_NOTE's, check them all
   for (size_t i = 0; i < phdr_num_; ++i) {
     const ElfW(Phdr)* phdr = &phdr_table_[i];

linker/linker_phdr.h

@@ -154,3 +154,5 @@ void phdr_table_get_dynamic_section(const ElfW(Phdr)* phdr_table, size_t phdr_co
 
 const char* phdr_table_get_interpreter_name(const ElfW(Phdr)* phdr_table, size_t phdr_count,
                                             ElfW(Addr) load_bias);
+
+bool page_size_migration_supported();