Allow sharing the RELRO section via a file.

Add flags and a file descriptor to android_dlopen_ext() to allow writing
the RELRO section of the loaded library to a file after relocation
processing, and to allow mapping identical pages from the file over the
top of relocated memory in another process. Explicitly comparing the
pages is required in case a page contains a reference to a symbol
defined in another library loaded at a random base address.

Bug: 13005501
Change-Id: Ibb5b2d384edfaa5acf3e97a5f8b6115c10497a1e

linker/linker_phdr.cpp

@@ -31,6 +31,9 @@
 #include <errno.h>
 #include <machine/exec.h>
 #include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
 #include "linker.h"
 #include "linker_debug.h"
@@ -523,6 +526,129 @@ int phdr_table_protect_gnu_relro(const ElfW(Phdr)* phdr_table, size_t phdr_count
   return _phdr_table_set_gnu_relro_prot(phdr_table, phdr_count, load_bias, PROT_READ);
 }
 
+/* Serialize the GNU relro segments to the given file descriptor. This can be
+ * performed after relocations to allow another process to later share the
+ * relocated segment, if it was loaded at the same address.
+ *
+ * Input:
+ *   phdr_table  -> program header table
+ *   phdr_count  -> number of entries in tables
+ *   load_bias   -> load bias
+ *   fd          -> writable file descriptor to use
+ * Return:
+ *   0 on error, -1 on failure (error code in errno).
+ */
+int phdr_table_serialize_gnu_relro(const ElfW(Phdr)* phdr_table, size_t phdr_count, ElfW(Addr) load_bias,
+                                   int fd) {
+  const ElfW(Phdr)* phdr = phdr_table;
+  const ElfW(Phdr)* phdr_limit = phdr + phdr_count;
+  ssize_t file_offset = 0;
+
+  for (phdr = phdr_table; phdr < phdr_limit; phdr++) {
+    if (phdr->p_type != PT_GNU_RELRO) {
+      continue;
+    }
+
+    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;
+    ssize_t size = seg_page_end - seg_page_start;
+
+    ssize_t written = TEMP_FAILURE_RETRY(write(fd, reinterpret_cast<void*>(seg_page_start), size));
+    if (written != size) {
+      return -1;
+    }
+    void* map = mmap(reinterpret_cast<void*>(seg_page_start), size, PROT_READ,
+                     MAP_PRIVATE|MAP_FIXED, fd, file_offset);
+    if (map == MAP_FAILED) {
+      return -1;
+    }
+    file_offset += size;
+  }
+  return 0;
+}
+
+/* Where possible, replace the GNU relro segments with mappings of the given
+ * file descriptor. This can be performed after relocations to allow a file
+ * previously created by phdr_table_serialize_gnu_relro in another process to
+ * replace the dirty relocated pages, saving memory, if it was loaded at the
+ * same address. We have to compare the data before we map over it, since some
+ * parts of the relro segment may not be identical due to other libraries in
+ * the process being loaded at different addresses.
+ *
+ * Input:
+ *   phdr_table  -> program header table
+ *   phdr_count  -> number of entries in tables
+ *   load_bias   -> load bias
+ *   fd          -> readable file descriptor to use
+ * Return:
+ *   0 on error, -1 on failure (error code in errno).
+ */
+int phdr_table_map_gnu_relro(const ElfW(Phdr)* phdr_table, size_t phdr_count, ElfW(Addr) load_bias,
+                             int fd) {
+  // Map the file at a temporary location so we can compare its contents.
+  struct stat file_stat;
+  if (TEMP_FAILURE_RETRY(fstat(fd, &file_stat)) != 0) {
+    return -1;
+  }
+  off_t file_size = file_stat.st_size;
+  void* temp_mapping = mmap(NULL, file_size, PROT_READ, MAP_PRIVATE, fd, 0);
+  if (temp_mapping == MAP_FAILED) {
+    return -1;
+  }
+  size_t file_offset = 0;
+
+  // Iterate over the relro segments and compare/remap the pages.
+  const ElfW(Phdr)* phdr = phdr_table;
+  const ElfW(Phdr)* phdr_limit = phdr + phdr_count;
+
+  for (phdr = phdr_table; phdr < phdr_limit; phdr++) {
+    if (phdr->p_type != PT_GNU_RELRO) {
+      continue;
+    }
+
+    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;
+
+    char* file_base = static_cast<char*>(temp_mapping) + file_offset;
+    char* mem_base = reinterpret_cast<char*>(seg_page_start);
+    size_t match_offset = 0;
+    size_t size = seg_page_end - seg_page_start;
+
+    while (match_offset < size) {
+      // Skip over dissimilar pages.
+      while (match_offset < size &&
+             memcmp(mem_base + match_offset, file_base + match_offset, PAGE_SIZE) != 0) {
+        match_offset += PAGE_SIZE;
+      }
+
+      // Count similar pages.
+      size_t mismatch_offset = match_offset;
+      while (mismatch_offset < size &&
+             memcmp(mem_base + mismatch_offset, file_base + mismatch_offset, PAGE_SIZE) == 0) {
+        mismatch_offset += PAGE_SIZE;
+      }
+
+      // Map over similar pages.
+      if (mismatch_offset > match_offset) {
+        void* map = mmap(mem_base + match_offset, mismatch_offset - match_offset,
+                         PROT_READ, MAP_PRIVATE|MAP_FIXED, fd, match_offset);
+        if (map == MAP_FAILED) {
+          munmap(temp_mapping, file_size);
+          return -1;
+        }
+      }
+
+      match_offset = mismatch_offset;
+    }
+
+    // Add to the base file offset in case there are multiple relro segments.
+    file_offset += size;
+  }
+  munmap(temp_mapping, file_size);
+  return 0;
+}
+
+
 #if defined(__arm__)
 
 #  ifndef PT_ARM_EXIDX