commit 377f0b9138e132029fdb8c96682a76044c4ffa03
author Kalesh Singh <kaleshsingh@google.com> Wed Jan 31 20:23:39 2024 -0800
committer Kalesh Singh <kaleshsingh@google.com> Tue Feb 06 17:59:01 2024 -0800
tree a08a2e773ac0322249f6a633b712bd2e97317201
parent f0050fb015c37803d727b88ca203dd88b9619b2c

bionic: Introduce ElfReader::ReadPadSegmentNote()

ReadPadSegmentNote() finds the elf note of type
NT_ANDROID_TYPE_PAD_SEGMENT and checks that the desc value
is 1, to decided whether the LOAD segment mappings should
be extended (padded) to avoid gaps.

Cache the result of this operation in ElfReader and soinfo
for use in the subsequent patch which handles the extension
of the segment mappings.

Test: atest -c linker-unit-tests [Later patch]
Test: m && launch_cvd
Bug: 316403210
Change-Id: I32c05cce741d221c3f92835ea09d932c40bdf8b1
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

linker/linker_phdr.cpp

diff --git a/linker/linker_phdr.cpp b/linker/linker_phdr.cpp
index 0ad0fd5..1ea79f0 100644
--- a/linker/linker_phdr.cpp
+++ b/linker/linker_phdr.cpp
@@ -42,7 +42,9 @@
 #include "linker_debug.h"
 #include "linker_utils.h"
 
+#include "private/bionic_asm_note.h"
 #include "private/CFIShadow.h" // For kLibraryAlignment
+#include "private/elf_note.h"
 
 static int GetTargetElfMachine() {
 #if defined(__arm__)
@@ -163,7 +165,8 @@
       VerifyElfHeader() &&
       ReadProgramHeaders() &&
       ReadSectionHeaders() &&
-      ReadDynamicSection()) {
+      ReadDynamicSection() &&
+      ReadPadSegmentNote()) {
     did_read_ = true;
   }
 
@@ -694,6 +697,46 @@
   return true;
 }
 
+// Find the ELF note of type NT_ANDROID_TYPE_PAD_SEGMENT and check that the desc value is 1.
+bool ElfReader::ReadPadSegmentNote() {
+  // 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];
+
+    if (phdr->p_type != PT_NOTE) {
+      continue;
+    }
+
+    // note_fragment is scoped to within the loop so that there is
+    // at most 1 PT_NOTE mapped at anytime during this search.
+    MappedFileFragment note_fragment;
+    if (!note_fragment.Map(fd_, file_offset_, phdr->p_offset, phdr->p_memsz)) {
+      DL_ERR("\"%s\" note mmap failed: %s", name_.c_str(), strerror(errno));
+      return false;
+    }
+
+    const ElfW(Nhdr)* note_hdr = nullptr;
+    const char* note_desc = nullptr;
+    if (!__get_elf_note(NT_ANDROID_TYPE_PAD_SEGMENT, "Android",
+                        reinterpret_cast<ElfW(Addr)>(note_fragment.data()),
+                        phdr, &note_hdr, &note_desc)) {
+      continue;
+    }
+
+    if (note_hdr->n_descsz != sizeof(ElfW(Word))) {
+      DL_ERR("\"%s\" NT_ANDROID_TYPE_PAD_SEGMENT note has unexpected n_descsz: %u",
+             name_.c_str(), reinterpret_cast<unsigned int>(note_hdr->n_descsz));
+      return false;
+    }
+
+    // 1 == enabled, 0 == disabled
+    should_pad_segments_ = *reinterpret_cast<const ElfW(Word)*>(note_desc) == 1;
+    return true;
+  }
+
+  return true;
+}
+
 bool ElfReader::LoadSegments() {
   for (size_t i = 0; i < phdr_num_; ++i) {
     const ElfW(Phdr)* phdr = &phdr_table_[i];