Properly handle empty map after read-only map.

Recently, the maps for an elf in memory might show up looking like:

  f0000-f1000 0 r-- /system/lib/libc.so
  f1000-f2000 0 ---
  f2000-f3000 1000 r-x /system/lib/libc.so
  f3000-f4000 2000 rw- /system/lib/libc.so

The problem is that there is logic in the code that assumed that the
map before the execute map must be the read-only map. In the case
above, this is not true. Add a new prev_real_map that will point
to the previous map that is not one of these empty maps.

This will fix the backtraces that look like this:

  #00  pc 0000000000050d58  /apex/com.android.runtime/lib64/bionic/libc.so!libc.so (offset 0x50000) (syscall+24) (BuildId: 5252408bf30e395d49ee270b54c77ca4)

To get rid of the !libc.so and the offset value, which is not correct.

Added new unit tests to verify this.
Added new offline test which an empty map between read-only and execute
map. Before this change, the backtraces had lines like
libc.so!libc.so (offset XXX) would be present.

Bug: 148075852

Test: Ran unit tests.
Change-Id: Ie04bfc96b8f91ed885cb1e655cf1e346efe48a45

1 files changed, 14 insertions, 14 deletions

diff --git a/libunwindstack/tests/ElfCacheTest.cpp b/libunwindstack/tests/ElfCacheTest.cpp
index 573585837..5f135467d 100644
--- a/libunwindstack/tests/ElfCacheTest.cpp
+++ b/libunwindstack/tests/ElfCacheTest.cpp
@@ -78,8 +78,8 @@ void ElfCacheTest::VerifySameMap(bool cache_enabled) {
 
   uint64_t start = 0x1000;
   uint64_t end = 0x20000;
-  MapInfo info1(nullptr, start, end, 0, 0x5, tf.path);
-  MapInfo info2(nullptr, start, end, 0, 0x5, tf.path);
+  MapInfo info1(nullptr, nullptr, start, end, 0, 0x5, tf.path);
+  MapInfo info2(nullptr, nullptr, start, end, 0, 0x5, tf.path);
 
   Elf* elf1 = info1.GetElf(memory_, ARCH_ARM);
   ASSERT_TRUE(elf1->valid());
@@ -119,17 +119,17 @@ void ElfCacheTest::VerifyWithinSameMap(bool cache_enabled) {
   uint64_t start = 0x1000;
   uint64_t end = 0x20000;
   // Will have an elf at offset 0 in file.
-  MapInfo info0_1(nullptr, start, end, 0, 0x5, tf.path);
-  MapInfo info0_2(nullptr, start, end, 0, 0x5, tf.path);
+  MapInfo info0_1(nullptr, nullptr, start, end, 0, 0x5, tf.path);
+  MapInfo info0_2(nullptr, nullptr, start, end, 0, 0x5, tf.path);
   // Will have an elf at offset 0x100 in file.
-  MapInfo info100_1(nullptr, start, end, 0x100, 0x5, tf.path);
-  MapInfo info100_2(nullptr, start, end, 0x100, 0x5, tf.path);
+  MapInfo info100_1(nullptr, nullptr, start, end, 0x100, 0x5, tf.path);
+  MapInfo info100_2(nullptr, nullptr, start, end, 0x100, 0x5, tf.path);
   // Will have an elf at offset 0x200 in file.
-  MapInfo info200_1(nullptr, start, end, 0x200, 0x5, tf.path);
-  MapInfo info200_2(nullptr, start, end, 0x200, 0x5, tf.path);
+  MapInfo info200_1(nullptr, nullptr, start, end, 0x200, 0x5, tf.path);
+  MapInfo info200_2(nullptr, nullptr, start, end, 0x200, 0x5, tf.path);
   // Will have an elf at offset 0 in file.
-  MapInfo info300_1(nullptr, start, end, 0x300, 0x5, tf.path);
-  MapInfo info300_2(nullptr, start, end, 0x300, 0x5, tf.path);
+  MapInfo info300_1(nullptr, nullptr, start, end, 0x300, 0x5, tf.path);
+  MapInfo info300_2(nullptr, nullptr, start, end, 0x300, 0x5, tf.path);
 
   Elf* elf0_1 = info0_1.GetElf(memory_, ARCH_ARM);
   ASSERT_TRUE(elf0_1->valid());
@@ -216,10 +216,10 @@ void ElfCacheTest::VerifyWithinSameMapNeverReadAtZero(bool cache_enabled) {
   uint64_t start = 0x1000;
   uint64_t end = 0x20000;
   // Multiple info sections at different offsets will have non-zero elf offsets.
-  MapInfo info300_1(nullptr, start, end, 0x300, 0x5, tf.path);
-  MapInfo info300_2(nullptr, start, end, 0x300, 0x5, tf.path);
-  MapInfo info400_1(nullptr, start, end, 0x400, 0x5, tf.path);
-  MapInfo info400_2(nullptr, start, end, 0x400, 0x5, tf.path);
+  MapInfo info300_1(nullptr, nullptr, start, end, 0x300, 0x5, tf.path);
+  MapInfo info300_2(nullptr, nullptr, start, end, 0x300, 0x5, tf.path);
+  MapInfo info400_1(nullptr, nullptr, start, end, 0x400, 0x5, tf.path);
+  MapInfo info400_2(nullptr, nullptr, start, end, 0x400, 0x5, tf.path);
 
   Elf* elf300_1 = info300_1.GetElf(memory_, ARCH_ARM);
   ASSERT_TRUE(elf300_1->valid());