Enable BTI in bionic linker

This patch adds support to load BTI-enabled objects.

According to the ABI, BTI is recorded in the .note.gnu.property section.
The new parser evaluates the property section, if exists.
It searches for .note section with NT_GNU_PROPERTY_TYPE_0.
Once found it tries to find GNU_PROPERTY_AARCH64_FEATURE_1_AND.
The results are cached.

The main change in linker is when protection of loaded ranges gets
applied. When BTI is requested and the platform also supports it
the prot flags have to be amended with PROT_BTI for executable ranges.
Failing to add PROT_BTI flag would disable BTI protection.
Moreover, adding the new PROT flag for shared objects without BTI
compatibility would break applications.

Kernel does not add PROT_BTI to a loaded ELF which has interpreter.
Linker handles this case too.

Test: 1. Flame boots
      2. Tested on FVP with BTI enabled

Change-Id: Iafdf223b74c6e75d9f17ca90500e6fe42c4c1218

11 files changed, 811 insertions, 13 deletions

diff --git a/linker/Android.bp b/linker/Android.bp
index a51b73fe4..6c59cffdd 100644
--- a/linker/Android.bp
+++ b/linker/Android.bp
@@ -169,6 +169,7 @@ filegroup {
         "linker_namespaces.cpp",
         "linker_logger.cpp",
         "linker_mapped_file_fragment.cpp",
+        "linker_note_gnu_property.cpp",
         "linker_phdr.cpp",
         "linker_relocate.cpp",
         "linker_sdk_versions.cpp",
@@ -447,6 +448,7 @@ cc_test {
         "linker_block_allocator_test.cpp",
         "linker_config_test.cpp",
         "linked_list_test.cpp",
+        "linker_note_gnu_property_test.cpp",
         "linker_sleb128_test.cpp",
         "linker_utils_test.cpp",
         "linker_gnu_hash_test.cpp",
@@ -455,6 +457,7 @@ cc_test {
         "linker_block_allocator.cpp",
         "linker_config.cpp",
         "linker_debug.cpp",
+        "linker_note_gnu_property.cpp",
         "linker_test_globals.cpp",
         "linker_utils.cpp",
     ],
diff --git a/linker/linker.cpp b/linker/linker.cpp
index 302e4b3f9..77f754cf4 100644
--- a/linker/linker.cpp
+++ b/linker/linker.cpp
@@ -3141,6 +3141,14 @@ bool soinfo::prelink_image() {
         // resolves everything eagerly, so these can be ignored.
         break;
 
+#if defined(__aarch64__)
+      case DT_AARCH64_BTI_PLT:
+      case DT_AARCH64_PAC_PLT:
+      case DT_AARCH64_VARIANT_PCS:
+        // Ignored: AArch64 processor-specific dynamic array tags.
+        break;
+#endif
+
       default:
         if (!relocating_linker) {
           const char* tag_name;
diff --git a/linker/linker.h b/linker/linker.h
index 3e851daf9..e1775fb66 100644
--- a/linker/linker.h
+++ b/linker/linker.h
@@ -181,3 +181,9 @@ struct address_space_params {
 int get_application_target_sdk_version();
 ElfW(Versym) find_verdef_version_index(const soinfo* si, const version_info* vi);
 bool validate_verdef_section(const soinfo* si);
+
+struct platform_properties {
+#if defined(__aarch64__)
+  bool bti_supported = false;
+#endif
+};
diff --git a/linker/linker_globals.cpp b/linker/linker_globals.cpp
index 31da02cfe..4a17d0918 100644
--- a/linker/linker_globals.cpp
+++ b/linker/linker_globals.cpp
@@ -40,6 +40,8 @@ android_namespace_t g_default_namespace;
 
 std::unordered_map<uintptr_t, soinfo*> g_soinfo_handles_map;
 
+platform_properties g_platform_properties;
+
 static char __linker_dl_err_buf[768];
 
 char* linker_get_error_buffer() {
diff --git a/linker/linker_globals.h b/linker/linker_globals.h
index 83cedcab4..09986290c 100644
--- a/linker/linker_globals.h
+++ b/linker/linker_globals.h
@@ -79,11 +79,14 @@ extern char** g_envp;
 
 struct soinfo;
 struct android_namespace_t;
+struct platform_properties;
 
 extern android_namespace_t g_default_namespace;
 
 extern std::unordered_map<uintptr_t, soinfo*> g_soinfo_handles_map;
 
+extern platform_properties g_platform_properties;
+
 // Error buffer "variable"
 char* linker_get_error_buffer();
 size_t linker_get_error_buffer_size();
diff --git a/linker/linker_main.cpp b/linker/linker_main.cpp
index 41bb4baee..aad8f6f6f 100644
--- a/linker/linker_main.cpp
+++ b/linker/linker_main.cpp
@@ -297,6 +297,13 @@ static ExecutableInfo load_executable(const char* orig_path) {
   return result;
 }
 
+static void platform_properties_init() {
+#if defined(__aarch64__)
+  const unsigned long hwcap2 = getauxval(AT_HWCAP2);
+  g_platform_properties.bti_supported = (hwcap2 & HWCAP2_BTI) != 0;
+#endif
+}
+
 static ElfW(Addr) linker_main(KernelArgumentBlock& args, const char* exe_to_load) {
   ProtectedDataGuard guard;
 
@@ -311,6 +318,9 @@ static ElfW(Addr) linker_main(KernelArgumentBlock& args, const char* exe_to_load
   // Initialize system properties
   __system_properties_init(); // may use 'environ'
 
+  // Initialize platform properties.
+  platform_properties_init();
+
   // Register the debuggerd signal handler.
   linker_debuggerd_init();
 
@@ -381,6 +391,20 @@ static ElfW(Addr) linker_main(KernelArgumentBlock& args, const char* exe_to_load
   solinker->set_realpath(interp);
   init_link_map_head(*solinker);
 
+#if defined(__aarch64__)
+  if (exe_to_load == nullptr) {
+    // Kernel does not add PROT_BTI to executable pages of the loaded ELF.
+    // Apply appropriate protections here if it is needed.
+    auto note_gnu_property = GnuPropertySection(somain);
+    if (note_gnu_property.IsBTICompatible() &&
+        (phdr_table_protect_segments(somain->phdr, somain->phnum, somain->load_bias,
+                                     &note_gnu_property) < 0)) {
+      __linker_error("error: can't protect segments for \"%s\": %s", exe_info.path.c_str(),
+                     strerror(errno));
+    }
+  }
+#endif
+
   // Register the main executable and the linker upfront to have
   // gdb aware of them before loading the rest of the dependency
   // tree.
diff --git a/linker/linker_note_gnu_property.cpp b/linker/linker_note_gnu_property.cpp
new file mode 100644
index 000000000..be1aebc88
--- /dev/null
+++ b/linker/linker_note_gnu_property.cpp
@@ -0,0 +1,186 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include "linker_note_gnu_property.h"
+
+#include <elf.h>
+#include <link.h>
+
+#include "linker.h"
+#include "linker_debug.h"
+#include "linker_globals.h"
+#include "linker_soinfo.h"
+
+GnuPropertySection::GnuPropertySection(const soinfo* si)
+    : GnuPropertySection(si->phdr, si->phnum, si->load_bias, si->get_realpath()) {}
+
+GnuPropertySection::GnuPropertySection(const ElfW(Phdr)* phdr, size_t phdr_count,
+                                       const ElfW(Addr) load_bias, const char* name) {
+  // Try to find PT_GNU_PROPERTY segment.
+  auto note_gnu_property = FindSegment(phdr, phdr_count, load_bias, name);
+  // Perform some validity checks.
+  if (note_gnu_property && SanityCheck(note_gnu_property, name)) {
+    // Parse section.
+    Parse(note_gnu_property, name);
+  }
+}
+
+const ElfW(NhdrGNUProperty)* GnuPropertySection::FindSegment(const ElfW(Phdr)* phdr,
+                                                             size_t phdr_count,
+                                                             const ElfW(Addr) load_bias,
+                                                             const char* name) const {
+  // According to Linux gABI extension this segment should contain
+  // .note.gnu.property section only.
+  if (phdr != nullptr) {
+    for (size_t i = 0; i < phdr_count; ++i) {
+      if (phdr[i].p_type != PT_GNU_PROPERTY) {
+        continue;
+      }
+
+      TRACE("\"%s\" PT_GNU_PROPERTY: found at segment index %zu", name, i);
+
+      // Check segment size.
+      if (phdr[i].p_memsz < sizeof(ElfW(NhdrGNUProperty))) {
+        DL_ERR_AND_LOG(
+            "\"%s\" PT_GNU_PROPERTY segment is too small. Segment "
+            "size is %zu, minimum is %zu.",
+            name, static_cast<size_t>(phdr[i].p_memsz), sizeof(ElfW(NhdrGNUProperty)));
+        return nullptr;
+      }
+
+      // PT_GNU_PROPERTY contains .note.gnu.property which has SHF_ALLOC
+      // attribute, therefore it is loaded.
+      auto note_nhdr = reinterpret_cast<ElfW(NhdrGNUProperty)*>(load_bias + phdr[i].p_vaddr);
+
+      // Check that the n_descsz <= p_memsz
+      if ((phdr[i].p_memsz - sizeof(ElfW(NhdrGNUProperty))) < note_nhdr->nhdr.n_descsz) {
+        DL_ERR_AND_LOG(
+            "\"%s\" PT_GNU_PROPERTY segment p_memsz (%zu) is too small for note n_descsz (%zu).",
+            name, static_cast<size_t>(phdr[i].p_memsz),
+            static_cast<size_t>(note_nhdr->nhdr.n_descsz));
+        return nullptr;
+      }
+
+      return note_nhdr;
+    }
+  }
+
+  TRACE("\"%s\" PT_GNU_PROPERTY: not found", name);
+  return nullptr;
+}
+
+bool GnuPropertySection::SanityCheck(const ElfW(NhdrGNUProperty)* note_nhdr,
+                                     const char* name) const {
+  // Check .note section type
+  if (note_nhdr->nhdr.n_type != NT_GNU_PROPERTY_TYPE_0) {
+    DL_ERR_AND_LOG("\"%s\" .note.gnu.property: unexpected note type. Expected %u, got %u.", name,
+                   NT_GNU_PROPERTY_TYPE_0, note_nhdr->nhdr.n_type);
+    return false;
+  }
+
+  if (note_nhdr->nhdr.n_namesz != 4) {
+    DL_ERR_AND_LOG("\"%s\" .note.gnu.property: unexpected name size. Expected 4, got %u.", name,
+                   note_nhdr->nhdr.n_namesz);
+    return false;
+  }
+
+  if (strncmp(note_nhdr->n_name, "GNU", 4) != 0) {
+    DL_ERR_AND_LOG("\"%s\" .note.gnu.property: unexpected name. Expected 'GNU', got '%s'.", name,
+                   note_nhdr->n_name);
+    return false;
+  }
+
+  return true;
+}
+
+bool GnuPropertySection::Parse(const ElfW(NhdrGNUProperty)* note_nhdr, const char* name) {
+  // The total length of the program property array is in _bytes_.
+  ElfW(Word) offset = 0;
+  while (offset < note_nhdr->nhdr.n_descsz) {
+    DEBUG("\"%s\" .note.gnu.property: processing at offset 0x%x", name, offset);
+
+    // At least the "header" part must fit.
+    // The ABI doesn't say that pr_datasz can't be 0.
+    if ((note_nhdr->nhdr.n_descsz - offset) < sizeof(ElfW(Prop))) {
+      DL_ERR_AND_LOG(
+          "\"%s\" .note.gnu.property: no more space left for a "
+          "Program Property Note header.",
+          name);
+      return false;
+    }
+
+    // Loop on program property array.
+    const ElfW(Prop)* property = reinterpret_cast<const ElfW(Prop)*>(&note_nhdr->n_desc[offset]);
+    const ElfW(Word) property_size =
+        align_up(sizeof(ElfW(Prop)) + property->pr_datasz, sizeof(ElfW(Addr)));
+    if ((note_nhdr->nhdr.n_descsz - offset) < property_size) {
+      DL_ERR_AND_LOG(
+          "\"%s\" .note.gnu.property: property descriptor size is "
+          "invalid. Expected at least %u bytes, got %u.",
+          name, property_size, note_nhdr->nhdr.n_descsz - offset);
+      return false;
+    }
+
+    // Cache found properties.
+    switch (property->pr_type) {
+#if defined(__aarch64__)
+      case GNU_PROPERTY_AARCH64_FEATURE_1_AND: {
+        if (property->pr_datasz != 4) {
+          DL_ERR_AND_LOG(
+              "\"%s\" .note.gnu.property: property descriptor size is "
+              "invalid. Expected %u bytes for GNU_PROPERTY_AARCH64_FEATURE_1_AND, got %u.",
+              name, 4, property->pr_datasz);
+          return false;
+        }
+
+        const ElfW(Word) flags = *reinterpret_cast<const ElfW(Word)*>(&property->pr_data[0]);
+        properties_.bti_compatible = (flags & GNU_PROPERTY_AARCH64_FEATURE_1_BTI) != 0;
+        if (properties_.bti_compatible) {
+          INFO("[ BTI compatible: \"%s\" ]", name);
+        }
+        break;
+      }
+#endif
+      default:
+        DEBUG("\"%s\" .note.gnu.property: found property pr_type %u pr_datasz 0x%x", name,
+              property->pr_type, property->pr_datasz);
+        break;
+    }
+
+    // Move offset, this should be safe to add because of previous checks.
+    offset += property_size;
+  }
+
+  return true;
+}
+
+#if defined(__aarch64__)
+bool GnuPropertySection::IsBTICompatible() const {
+  return (g_platform_properties.bti_supported && properties_.bti_compatible);
+}
+#endif
diff --git a/linker/linker_note_gnu_property.h b/linker/linker_note_gnu_property.h
new file mode 100644
index 000000000..b8b4ef792
--- /dev/null
+++ b/linker/linker_note_gnu_property.h
@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <elf.h>
+#include <link.h>
+
+#include "linker_soinfo.h"
+
+// The Elf* structures below are derived from the document
+// Linux Extensions to gABI (https://github.com/hjl-tools/linux-abi/wiki).
+// Essentially, these types would be defined in <elf.h>, but this is not
+// the case at the moment.
+
+struct Elf32_Prop {
+  Elf32_Word pr_type;
+  Elf32_Word pr_datasz;
+  char pr_data[0];
+};
+
+// On 32-bit machines this should be 4-byte aligned.
+struct Elf32_NhdrGNUProperty {
+  Elf32_Nhdr nhdr;
+  char n_name[4];
+  char n_desc[0];
+};
+
+struct Elf64_Prop {
+  Elf64_Word pr_type;
+  Elf64_Word pr_datasz;
+  char pr_data[0];
+};
+
+// On 64-bit machines this should be 8-byte aligned.
+struct Elf64_NhdrGNUProperty {
+  Elf64_Nhdr nhdr;
+  char n_name[4];
+  char n_desc[0];
+};
+
+struct ElfProgramProperty {
+#if defined(__aarch64__)
+  bool bti_compatible = false;
+#endif
+};
+
+// Representation of the .note.gnu.property section found in the segment
+// with p_type = PT_GNU_PROPERTY.
+class GnuPropertySection {
+ public:
+  GnuPropertySection(){};
+  explicit GnuPropertySection(const soinfo* si);
+  GnuPropertySection(const ElfW(Phdr)* phdr, size_t phdr_count, const ElfW(Addr) load_bias,
+                     const char* name);
+
+#if defined(__aarch64__)
+  bool IsBTICompatible() const;
+#endif
+
+ private:
+  const ElfW(NhdrGNUProperty)* FindSegment(const ElfW(Phdr)* phdr, size_t phdr_count,
+                                           const ElfW(Addr) load_bias, const char* name) const;
+  bool SanityCheck(const ElfW(NhdrGNUProperty)* note_nhdr, const char* name) const;
+  bool Parse(const ElfW(NhdrGNUProperty)* note_nhdr, const char* name);
+
+  ElfProgramProperty properties_ __unused;
+};
diff --git a/linker/linker_note_gnu_property_test.cpp b/linker/linker_note_gnu_property_test.cpp
new file mode 100644
index 000000000..41fc47bc2
--- /dev/null
+++ b/linker/linker_note_gnu_property_test.cpp
@@ -0,0 +1,435 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <stdlib.h>
+#include <iostream>
+#include <sstream>
+#include <string>
+
+#include <gtest/gtest.h>
+
+#include "linker.h"
+#include "linker_globals.h"
+#include "linker_note_gnu_property.h"
+#include "platform/bionic/macros.h"
+
+#define SONAME "test_so"
+
+static char error_buffer[1024];
+
+char* linker_get_error_buffer() {
+  return error_buffer;
+}
+
+size_t linker_get_error_buffer_size() {
+  return std::size(error_buffer);
+}
+
+static void reset_error_buffer() {
+  error_buffer[0] = '\0';
+}
+
+platform_properties g_platform_properties {
+#if defined(__aarch64__)
+  // Assume "hardware" supports Armv8.5-A BTI.
+  .bti_supported = true
+#endif
+};
+
+// Helper macro to make the test cleaner.
+#define PHDR_WITH_NOTE_GNU_PROPERTY(__prop)                                   \
+  reset_error_buffer();                                                       \
+  ElfW(Phdr) phdrs[] = {                                                      \
+      {.p_type = PT_LOAD},                                                    \
+      {                                                                       \
+          .p_type = PT_GNU_PROPERTY,                                          \
+          .p_vaddr = reinterpret_cast<ElfW(Addr)>(__prop),                    \
+          .p_memsz = sizeof(ElfW(NhdrGNUProperty)) + (__prop)->nhdr.n_descsz, \
+      },                                                                      \
+      {.p_type = PT_NULL},                                                    \
+  };                                                                          \
+  auto note = GnuPropertySection(&phdrs[0], std::size(phdrs), 0, SONAME)
+
+// Helper to check for no error message.
+#define ASSERT_NO_ERROR_MSG() ASSERT_STREQ(error_buffer, "")
+
+// Helper to check expected error message.
+#define ASSERT_ERROR_MSG_EQ(__expected) ASSERT_STREQ(error_buffer, "\"" SONAME "\" " __expected)
+
+static void test_bti_not_supported(GnuPropertySection& note __unused) {
+#if defined(__aarch64__)
+  ASSERT_FALSE(note.IsBTICompatible());
+#endif
+}
+
+#if defined(__aarch64__)
+static void test_bti_supported(GnuPropertySection& note __unused) {
+  ASSERT_TRUE(note.IsBTICompatible());
+}
+#endif
+
+// Helper class to build a well-formed .note.gnu.property section.
+class GnuPropertySectionBuilder {
+ public:
+  GnuPropertySectionBuilder() {
+    note = reinterpret_cast<ElfW(NhdrGNUProperty)*>(&section[0]);
+    note->nhdr.n_namesz = 4;
+    note->nhdr.n_descsz = 0;
+    note->nhdr.n_type = NT_GNU_PROPERTY_TYPE_0;
+    memcpy(note->n_name, "GNU", 4);
+  }
+
+  template <typename T>
+  bool push(ElfW(Word) pr_type, ElfW(Word) pr_datasz, const T* pr_data) {
+    // Must be aligned.
+    const uintptr_t addition = align_up(pr_datasz, sizeof(ElfW(Addr)));
+    if ((offset() + addition) > kMaxSectionSize) {
+      return false;
+    }
+    ++entries;
+    ElfW(Prop)* prop = reinterpret_cast<ElfW(Prop)*>(&section[offset()]);
+    // Header
+    prop->pr_type = pr_type;
+    prop->pr_datasz = pr_datasz;
+    step(2 * sizeof(ElfW(Word)));
+    // Data
+    memcpy(&section[offset()], reinterpret_cast<const void*>(pr_data), pr_datasz);
+    step(pr_datasz);
+    // Padding
+    memset(&section[offset()], 0xAA, addition - pr_datasz);
+    step(addition - pr_datasz);
+    return true;
+  }
+
+  ElfW(NhdrGNUProperty)* data() const { return note; }
+
+  void dump() const {
+    std::cout << ".note.gnu.property\n";
+    dump_member("n_namesz", note->nhdr.n_namesz);
+    dump_member("n_descsz", note->nhdr.n_descsz);
+    dump_member("n_type  ", note->nhdr.n_type);
+    dump_member("n_name  ", note->n_name);
+    dump_member("entries ", entries);
+    if (entries > 0) {
+      std::cout << "    raw data:";
+      const uintptr_t offset = note->nhdr.n_descsz + 16;
+      for (uintptr_t offs = 16; offs < offset; ++offs) {
+        std::cout << std::hex;
+        if ((offs % 8) == 0) {
+          std::cout << "\n   ";
+        }
+        auto value = static_cast<unsigned>(section[offs]);
+        std::cout << " ";
+        if (value < 0x10) {
+          std::cout << "0";
+        }
+        std::cout << static_cast<unsigned>(section[offs]);
+      }
+      std::cout << std::dec << "\n";
+    }
+  }
+
+  void corrupt_n_descsz(ElfW(Word) n_descsz) { note->nhdr.n_descsz = n_descsz; }
+
+ private:
+  template <typename T>
+  void dump_member(const char* name, T value) const {
+    std::cout << "  " << name << " " << value << "\n";
+  }
+
+  ElfW(Word) offset() const { return note->nhdr.n_descsz + 16; }
+
+  template <typename T>
+  void step(T value) {
+    note->nhdr.n_descsz += static_cast<ElfW(Word)>(value);
+  }
+
+  static const size_t kMaxSectionSize = 1024;
+
+  alignas(8) uint8_t section[kMaxSectionSize];
+  ElfW(NhdrGNUProperty)* note;
+  size_t entries = 0;
+};
+
+// Tests that the default constructed instance does not report support
+// for Armv8.5-A BTI.
+TEST(note_gnu_property, default) {
+  GnuPropertySection note;
+  test_bti_not_supported(note);
+  ASSERT_NO_ERROR_MSG();
+}
+
+// Tests that an instance without valid phdr pointer does not report
+// support for Armv8.5-A BTI.
+TEST(note_gnu_property, phdr_null) {
+  auto note = GnuPropertySection(nullptr, 0, 0, SONAME);
+  test_bti_not_supported(note);
+  ASSERT_NO_ERROR_MSG();
+}
+
+// Tests that an instance without finding PT_GNU_PROPERTY does not
+// report support for Armv8.5-A BTI.
+TEST(note_gnu_property, no_pt_gnu_property) {
+  ElfW(Phdr) phdrs[] = {
+      {.p_type = PT_LOAD},
+      {.p_type = PT_NULL},
+  };
+
+  reset_error_buffer();
+  auto note = GnuPropertySection(&phdrs[0], std::size(phdrs), 0, SONAME);
+  test_bti_not_supported(note);
+  ASSERT_NO_ERROR_MSG();
+}
+
+// Tests the validity check for invalid PT_GNU_PROPERTY size.
+TEST(note_gnu_property, pt_gnu_property_bad_size) {
+  ElfW(Phdr) phdrs[] = {
+      {.p_type = PT_LOAD},
+      {
+          .p_type = PT_GNU_PROPERTY,
+          .p_vaddr = 0,
+          .p_memsz = sizeof(ElfW(NhdrGNUProperty)) - 1,  // Invalid
+      },
+      {.p_type = PT_NULL},
+  };
+
+  reset_error_buffer();
+  auto note = GnuPropertySection(&phdrs[0], std::size(phdrs), 0, SONAME);
+  test_bti_not_supported(note);
+  ASSERT_ERROR_MSG_EQ("PT_GNU_PROPERTY segment is too small. Segment size is 15, minimum is 16.");
+}
+
+// Tests that advertised n_descsz should still fit into p_memsz.
+TEST(note_gnu_property, pt_gnu_property_too_small) {
+  ElfW(NhdrGNUProperty) prop = {
+      .nhdr = {.n_namesz = PT_GNU_PROPERTY, .n_descsz = 1, .n_type = NT_GNU_PROPERTY_TYPE_0},
+      .n_name = "GNU",
+  };
+  ElfW(Phdr) phdrs[] = {
+      {
+          .p_type = PT_GNU_PROPERTY,
+          .p_vaddr = reinterpret_cast<ElfW(Addr)>(&prop),
+          .p_memsz = sizeof(ElfW(NhdrGNUProperty)),  // Off by one
+      },
+  };
+
+  reset_error_buffer();
+  auto note = GnuPropertySection(&phdrs[0], std::size(phdrs), 0, SONAME);
+  test_bti_not_supported(note);
+  ASSERT_ERROR_MSG_EQ("PT_GNU_PROPERTY segment p_memsz (16) is too small for note n_descsz (1).");
+}
+
+// Tests the validity check for invalid .note.gnu.property type.
+TEST(note_gnu_property, pt_gnu_property_bad_type) {
+  ElfW(NhdrGNUProperty) prop = {
+      .nhdr =
+          {
+              .n_namesz = 4,
+              .n_descsz = 0,
+              .n_type = NT_GNU_PROPERTY_TYPE_0 - 1  // Invalid
+          },
+      .n_name = "GNU",
+  };
+  PHDR_WITH_NOTE_GNU_PROPERTY(&prop);
+  test_bti_not_supported(note);
+  ASSERT_ERROR_MSG_EQ(".note.gnu.property: unexpected note type. Expected 5, got 4.");
+}
+
+// Tests the validity check for invalid .note.gnu.property name size.
+TEST(note_gnu_property, pt_gnu_property_bad_namesz) {
+  ElfW(NhdrGNUProperty) prop = {
+      .nhdr = {.n_namesz = 3,  // Invalid
+               .n_descsz = 0,
+               .n_type = NT_GNU_PROPERTY_TYPE_0},
+      .n_name = "GNU",
+  };
+  PHDR_WITH_NOTE_GNU_PROPERTY(&prop);
+  test_bti_not_supported(note);
+  ASSERT_ERROR_MSG_EQ(".note.gnu.property: unexpected name size. Expected 4, got 3.");
+}
+
+// Tests the validity check for invalid .note.gnu.property name.
+TEST(note_gnu_property, pt_gnu_property_bad_name) {
+  ElfW(NhdrGNUProperty) prop = {
+      .nhdr = {.n_namesz = 4, .n_descsz = 0, .n_type = NT_GNU_PROPERTY_TYPE_0},
+      .n_name = "ABC",  // Invalid
+  };
+  PHDR_WITH_NOTE_GNU_PROPERTY(&prop);
+  test_bti_not_supported(note);
+  ASSERT_ERROR_MSG_EQ(".note.gnu.property: unexpected name. Expected 'GNU', got 'ABC'.");
+}
+
+// Tests the validity check for not enough space for a Program Property header.
+TEST(note_gnu_property, pt_gnu_property_pphdr_no_space) {
+  ElfW(NhdrGNUProperty) prop = {
+      .nhdr = {.n_namesz = 4,
+               .n_descsz = 7,  // Invalid
+               .n_type = NT_GNU_PROPERTY_TYPE_0},
+      .n_name = "GNU",
+  };
+  PHDR_WITH_NOTE_GNU_PROPERTY(&prop);
+  test_bti_not_supported(note);
+  ASSERT_ERROR_MSG_EQ(".note.gnu.property: no more space left for a Program Property Note header.");
+}
+
+// Tests an empty .note.gnu.property.
+TEST(note_gnu_property, pt_gnu_property_no_data) {
+  GnuPropertySectionBuilder prop;
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  test_bti_not_supported(note);
+  ASSERT_NO_ERROR_MSG();
+}
+
+// Tests a .note.gnu.property section with elements with pr_datasz = 0.
+TEST(note_gnu_property, pt_gnu_property_no_prop) {
+  GnuPropertySectionBuilder prop;
+  ASSERT_TRUE(prop.push(1, 0, (void*)nullptr));
+  ASSERT_TRUE(prop.push(2, 0, (void*)nullptr));
+  ASSERT_TRUE(prop.push(3, 0, (void*)nullptr));
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  test_bti_not_supported(note);
+  ASSERT_NO_ERROR_MSG();
+}
+
+// Tests that GNU_PROPERTY_AARCH64_FEATURE_1_AND must have pr_datasz = 4.
+TEST(note_gnu_property, pt_gnu_property_bad_pr_datasz) {
+#if defined(__aarch64__)
+  GnuPropertySectionBuilder prop;
+  ElfW(Word) pr_data[] = {GNU_PROPERTY_AARCH64_FEATURE_1_BTI, 0, 0};
+  ASSERT_TRUE(prop.push(GNU_PROPERTY_AARCH64_FEATURE_1_AND, 12, &pr_data));
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  test_bti_not_supported(note);
+  ASSERT_ERROR_MSG_EQ(
+      ".note.gnu.property: property descriptor size is invalid. Expected 4 bytes for "
+      "GNU_PROPERTY_AARCH64_FEATURE_1_AND, got 12.");
+#else
+  GTEST_SKIP() << "BTI is not supported on this architecture.";
+#endif
+}
+
+// Tests a .note.gnu.property section with only GNU_PROPERTY_AARCH64_FEATURE_1_BTI property array.
+TEST(note_gnu_property, pt_gnu_property_ok_1) {
+#if defined(__aarch64__)
+  GnuPropertySectionBuilder prop;
+  ElfW(Word) pr_data[] = {GNU_PROPERTY_AARCH64_FEATURE_1_BTI};
+  ASSERT_TRUE(prop.push(GNU_PROPERTY_AARCH64_FEATURE_1_AND, sizeof(pr_data), &pr_data));
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  ASSERT_NO_ERROR_MSG();
+  test_bti_supported(note);
+#else
+  GTEST_SKIP() << "BTI is not supported on this architecture.";
+#endif
+}
+
+// Tests a .note.gnu.property section with only GNU_PROPERTY_AARCH64_FEATURE_1_BTI property array.
+TEST(note_gnu_property, pt_gnu_property_ok_2) {
+#if defined(__aarch64__)
+  GnuPropertySectionBuilder prop;
+  ElfW(Word) pr_data[] = {static_cast<ElfW(Word)>(~GNU_PROPERTY_AARCH64_FEATURE_1_BTI)};
+  ASSERT_TRUE(prop.push(GNU_PROPERTY_AARCH64_FEATURE_1_AND, sizeof(pr_data), &pr_data));
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  ASSERT_NO_ERROR_MSG();
+  test_bti_not_supported(note);
+#else
+  GTEST_SKIP() << "BTI is not supported on this architecture.";
+#endif
+}
+
+// Tests a .note.gnu.property section with more property arrays.
+TEST(note_gnu_property, pt_gnu_property_ok_3) {
+#if defined(__aarch64__)
+  GnuPropertySectionBuilder prop;
+
+  ElfW(Word) pr_data_0[8] = {0xCD};
+  ASSERT_TRUE(prop.push(1, 4, &pr_data_0));
+  ASSERT_TRUE(prop.push(2, 3, &pr_data_0));
+  ASSERT_TRUE(prop.push(3, 8, &pr_data_0));
+
+  ElfW(Word) pr_data[] = {GNU_PROPERTY_AARCH64_FEATURE_1_BTI};
+  ASSERT_TRUE(prop.push(GNU_PROPERTY_AARCH64_FEATURE_1_AND, sizeof(pr_data), &pr_data));
+
+  ASSERT_TRUE(prop.push(4, 1, &pr_data_0));
+
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  ASSERT_NO_ERROR_MSG();
+  test_bti_supported(note);
+#else
+  GTEST_SKIP() << "BTI is not supported on this architecture.";
+#endif
+}
+
+// Tests a .note.gnu.property but with bad property descriptor size.
+TEST(note_gnu_property, pt_gnu_property_bad_n_descsz) {
+#if defined(__aarch64__)
+  GnuPropertySectionBuilder prop;
+  ElfW(Word) pr_data[] = {GNU_PROPERTY_AARCH64_FEATURE_1_BTI};
+  ASSERT_TRUE(prop.push(GNU_PROPERTY_AARCH64_FEATURE_1_AND, sizeof(pr_data), &pr_data));
+
+  ElfW(Word) n_descsz;
+  if (sizeof(ElfW(Addr)) == 4) {
+    n_descsz = 11;
+  } else {
+    n_descsz = 15;
+  }
+
+  prop.corrupt_n_descsz(n_descsz);
+
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  if (sizeof(ElfW(Addr)) == 4) {
+    ASSERT_ERROR_MSG_EQ(
+        ".note.gnu.property: property descriptor size is invalid. Expected at least 12 bytes, got "
+        "11.");
+  } else {
+    ASSERT_ERROR_MSG_EQ(
+        ".note.gnu.property: property descriptor size is invalid. Expected at least 16 bytes, got "
+        "15.");
+  }
+  test_bti_not_supported(note);
+#else
+  GTEST_SKIP() << "BTI is not supported on this architecture.";
+#endif
+}
+
+// Tests if platform support is missing.
+TEST(note_gnu_property, no_platform_support) {
+#if defined(__aarch64__)
+  auto bti_supported_orig = g_platform_properties.bti_supported;
+  g_platform_properties.bti_supported = false;
+
+  GnuPropertySectionBuilder prop;
+  ElfW(Word) pr_data[] = {GNU_PROPERTY_AARCH64_FEATURE_1_BTI};
+  ASSERT_TRUE(prop.push(GNU_PROPERTY_AARCH64_FEATURE_1_AND, sizeof(pr_data), &pr_data));
+  PHDR_WITH_NOTE_GNU_PROPERTY(prop.data());
+  ASSERT_NO_ERROR_MSG();
+  test_bti_not_supported(note);
+
+  g_platform_properties.bti_supported = bti_supported_orig;
+#else
+  GTEST_SKIP() << "BTI is not supported on this architecture.";
+#endif
+}
diff --git a/linker/linker_phdr.cpp b/linker/linker_phdr.cpp
index 1e8909457..9b1b99ffd 100644
--- a/linker/linker_phdr.cpp
+++ b/linker/linker_phdr.cpp
@@ -169,8 +169,16 @@ bool ElfReader::Load(address_space_params* address_space) {
   if (did_load_) {
     return true;
   }
-  if (ReserveAddressSpace(address_space) && LoadSegments() && FindPhdr()) {
+  if (ReserveAddressSpace(address_space) && LoadSegments() && FindPhdr() &&
+      FindGnuPropertySection()) {
     did_load_ = true;
+#if defined(__aarch64__)
+    // For Armv8.5-A loaded executable segments may require PROT_BTI.
+    if (note_gnu_property_.IsBTICompatible()) {
+      did_load_ = (phdr_table_protect_segments(phdr_table_, phdr_num_, load_bias_,
+                                               &note_gnu_property_) == 0);
+    }
+#endif
   }
 
   return did_load_;
@@ -748,15 +756,21 @@ static int _phdr_table_set_load_prot(const ElfW(Phdr)* phdr_table, size_t phdr_c
     ElfW(Addr) seg_page_start = PAGE_START(phdr->p_vaddr) + load_bias;
     ElfW(Addr) seg_page_end   = PAGE_END(phdr->p_vaddr + phdr->p_memsz) + load_bias;
 
-    int prot = PFLAGS_TO_PROT(phdr->p_flags);
-    if ((extra_prot_flags & PROT_WRITE) != 0) {
+    int prot = PFLAGS_TO_PROT(phdr->p_flags) | extra_prot_flags;
+    if ((prot & PROT_WRITE) != 0) {
       // make sure we're never simultaneously writable / executable
       prot &= ~PROT_EXEC;
     }
+#if defined(__aarch64__)
+    if ((prot & PROT_EXEC) == 0) {
+      // Though it is not specified don't add PROT_BTI if segment is not
+      // executable.
+      prot &= ~PROT_BTI;
+    }
+#endif
 
-    int ret = mprotect(reinterpret_cast<void*>(seg_page_start),
-                       seg_page_end - seg_page_start,
-                       prot | extra_prot_flags);
+    int ret =
+        mprotect(reinterpret_cast<void*>(seg_page_start), seg_page_end - seg_page_start, prot);
     if (ret < 0) {
       return -1;
     }
@@ -768,16 +782,26 @@ static int _phdr_table_set_load_prot(const ElfW(Phdr)* phdr_table, size_t phdr_c
  * You should only call this after phdr_table_unprotect_segments and
  * applying all relocations.
  *
+ * AArch64: also called from linker_main and ElfReader::Load to apply
+ *     PROT_BTI for loaded main so and other so-s.
+ *
  * Input:
  *   phdr_table  -> program header table
  *   phdr_count  -> number of entries in tables
  *   load_bias   -> load bias
+ *   prop        -> GnuPropertySection or nullptr
  * Return:
  *   0 on error, -1 on failure (error code in errno).
  */
-int phdr_table_protect_segments(const ElfW(Phdr)* phdr_table,
-                                size_t phdr_count, ElfW(Addr) load_bias) {
-  return _phdr_table_set_load_prot(phdr_table, phdr_count, load_bias, 0);
+int phdr_table_protect_segments(const ElfW(Phdr)* phdr_table, size_t phdr_count,
+                                ElfW(Addr) load_bias, const GnuPropertySection* prop __unused) {
+  int prot = 0;
+#if defined(__aarch64__)
+  if ((prop != nullptr) && prop->IsBTICompatible()) {
+    prot |= PROT_BTI;
+  }
+#endif
+  return _phdr_table_set_load_prot(phdr_table, phdr_count, load_bias, prot);
 }
 
 /* Change the protection of all loaded segments in memory to writable.
@@ -1081,7 +1105,7 @@ void phdr_table_get_dynamic_section(const ElfW(Phdr)* phdr_table, size_t phdr_co
  * Return:
  *   pointer to the program interpreter string.
  */
-const char* phdr_table_get_interpreter_name(const ElfW(Phdr) * phdr_table, size_t phdr_count,
+const char* phdr_table_get_interpreter_name(const ElfW(Phdr)* phdr_table, size_t phdr_count,
                                             ElfW(Addr) load_bias) {
   for (size_t i = 0; i<phdr_count; ++i) {
     const ElfW(Phdr)& phdr = phdr_table[i];
@@ -1124,6 +1148,15 @@ bool ElfReader::FindPhdr() {
   return false;
 }
 
+// Tries to find .note.gnu.property section.
+// It is not considered an error if such section is missing.
+bool ElfReader::FindGnuPropertySection() {
+#if defined(__aarch64__)
+  note_gnu_property_ = GnuPropertySection(phdr_table_, phdr_num_, load_start(), name_.c_str());
+#endif
+  return true;
+}
+
 // Ensures that our program header is actually within a loadable
 // segment. This should help catch badly-formed ELF files that
 // would cause the linker to crash later when trying to access it.
diff --git a/linker/linker_phdr.h b/linker/linker_phdr.h
index 4cb48f5d7..548dc51dd 100644
--- a/linker/linker_phdr.h
+++ b/linker/linker_phdr.h
@@ -37,6 +37,7 @@
 
 #include "linker.h"
 #include "linker_mapped_file_fragment.h"
+#include "linker_note_gnu_property.h"
 
 class ElfReader {
  public:
@@ -67,6 +68,7 @@ class ElfReader {
   bool ReserveAddressSpace(address_space_params* address_space);
   bool LoadSegments();
   bool FindPhdr();
+  bool FindGnuPropertySection();
   bool CheckPhdr(ElfW(Addr));
   bool CheckFileRange(ElfW(Addr) offset, size_t size, size_t alignment);
 
@@ -110,13 +112,16 @@ class ElfReader {
 
   // Is map owned by the caller
   bool mapped_by_caller_;
+
+  // Only used by AArch64 at the moment.
+  GnuPropertySection note_gnu_property_ __unused;
 };
 
 size_t phdr_table_get_load_size(const ElfW(Phdr)* phdr_table, size_t phdr_count,
                                 ElfW(Addr)* min_vaddr = nullptr, ElfW(Addr)* max_vaddr = nullptr);
 
-int phdr_table_protect_segments(const ElfW(Phdr)* phdr_table,
-                                size_t phdr_count, ElfW(Addr) load_bias);
+int phdr_table_protect_segments(const ElfW(Phdr)* phdr_table, size_t phdr_count,
+                                ElfW(Addr) load_bias, const GnuPropertySection* prop = nullptr);
 
 int phdr_table_unprotect_segments(const ElfW(Phdr)* phdr_table, size_t phdr_count,
                                   ElfW(Addr) load_bias);
@@ -139,5 +144,5 @@ void phdr_table_get_dynamic_section(const ElfW(Phdr)* phdr_table, size_t phdr_co
                                     ElfW(Addr) load_bias, ElfW(Dyn)** dynamic,
                                     ElfW(Word)* dynamic_flags);
 
-const char* phdr_table_get_interpreter_name(const ElfW(Phdr) * phdr_table, size_t phdr_count,
+const char* phdr_table_get_interpreter_name(const ElfW(Phdr)* phdr_table, size_t phdr_count,
                                             ElfW(Addr) load_bias);