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+# Unwinder Support Per Android Release
+This document describes the changes in the way the libunwindstack
+unwinder works on different Android versions. It does not describe
+every change in the code made between different versions, but is
+meant to allow an app developer to know what might be supported
+on different versions. It also describes the different way an unwind
+will display on different versions of Android.
+
+## Android P
+libunwindstack was first introduced in Android P.
+
+* Supports up to and including Dwarf 4 unwinding information.
+  See http://dwarfstd.org/ for Dwarf standards.
+* Supports Arm exidx unwinding.
+* Supports the gdb JIT unwinding interface, which is how ART creates unwinding
+  information for the JIT'd Java frames.
+* Supports special frames added to represent an ART Java interpreter frame.
+  ART has marked the dex pc using cfi information that the unwinder
+  understands and handles by adding a new frame in the stacktrace.
+
+## Note
+By default, lld creates two separate maps of the elf in memory, one read-only
+and one read/executable. The libunwindstack on P and the unwinder on older
+versions of Android will not unwind properly in this case. For apps that
+target Android P or older, make sure that `-Wl,--no-rosegment` is
+included in linker arguments when using lld.
+
+## Android Q
+* Fix bug (b/109824792) that handled load bias data incorrectly when
+  FDEs use pc relative addressing in the eh\_frame\_hdr.
+  Unfortunately, this wasn't fixed correctly in Q since it assumes
+  that the bias is coming from the program header for the executable
+  load. The real fix was to use the bias from the actual section data and
+  is not completely fixed until Android R. For apps targeting Android Q,
+  if it is being compiled with the llvm linker lld, it might be necessary
+  to add the linker option `-Wl,-zseparate-code` to avoid creating an elf
+  created this way.
+* Change the way the exidx section offset is found (b/110704153). Before
+  the p\_vaddr value from the program header minus the load bias was used
+  to find the start of the exidx data. Changed to use the p\_offset since
+  it doesn't require any load bias manipulations.
+* Fix bug handling of dwarf sections without any header (b/110235461).
+  Previously, the code assumed that FDEs are non-overlapping, and the FDEs
+  are always in sorted order from low pc to high pc. Thus the code would
+  read the entire set of CIEs/FDEs and then do a binary search to find
+  the appropriate FDE for a given pc. Now the code does a sequential read
+  and stops when it finds the FDE for a pc. It also understands the
+  overlapping FDEs, so find the first FDE that matches a pc. In practice,
+  elf files with this format only ever occurs if the file was generated
+  without an eh\_frame/eh\_frame\_hdr section and only a debug\_frame. The
+  other way this has been observed is when running simpleperf to unwind since
+  sometimes there is not enough information in the eh\_frame for all points
+  in the executable. On Android P, this would result in some incorrect
+  unwinds coming from simpleperf. Nearly all crashes from Android P should
+  be correct since the eh\_frame information was enough to do the unwind
+  properly.
+* Be permissive of badly formed elf files. Previously, any detected error
+  would result in unwinds stopping even if there is enough valid information
+  to do an unwind.
+  * The code now allows program header/section header offsets to point
+    to unreadable memory. As long as the code can find the unwind tables,
+    that is good enough.
+  * The code allows program headers/section headers to be missing.
+  * Allow a symbol table section header to point to invalid symbol table
+    values.
+* Support for the linker read-only segment option (b/109657296).
+  This is a feature of lld whereby there are two sections that
+  contain elf data. The first is read-only and contains the elf header data,
+  and the second is read-execute or execute only that
+  contains the executable code from the elf. Before this, the unwinder
+  always assumed that there was only a single read-execute section that
+  contained the elf header data and the executable code.
+* Build ID information for elf objects added. This will display the
+  NT\_GNU\_BUILD\_ID note found in elf files. This information can be used
+  to identify the exact version of a shared library to help get symbol
+  information when looking at a crash.
+* Add support for displaying the soname from an apk frame. Previously,
+  a frame map name would be only the apk, but now if the shared library
+  in the apk has set a soname, the map name will be `app.apk!libexample.so`
+  instead of only `app.apk`.
+* Minimal support for Dwarf 5. This merely treats a Dwarf 5 version
+  elf file as Dwarf 4. It does not support the new dwarf ops in Dwarf 5.
+  Since the new ops are not likely to be used very often, this allows
+  continuing to unwind even when encountering Dwarf 5 elf files.
+* Fix bug in pc handling of signal frames (b/130302288). In the previous
+  version, the pc would be wrong in the signal frame. The rest of the
+  unwind was correct, only the frame in the signal handler was incorrect
+  in Android P.
+* Detect when an elf file is not readable so that a message can be
+  displayed indicating that. This can happen when an app puts the shared
+  libraries in non-standard locations that are not readable due to
+  security restrictions (selinux rules).
+
+## Android R
+* Display the offsets for Java interpreter frames. If this frame came
+  from a non-zero offset map, no offset is printed. Previously, the
+  line would look like:
+
+    #17 pc 00500d7a  GoogleCamera.apk (com.google.camera.AndroidPriorityThread.run+10)
+
+  to:
+
+    #17 pc 00500d7a  GoogleCamera.apk (offset 0x11d0000) (com.google.camera.AndroidPriorityThread.run+10)
+* Fix bug where the load bias was set from the first PT\_LOAD program
+  header that has a zero p\_offset value. Now it is set from the first
+  executable PT\_LOAD program header. This has only ever been a problem
+  for host executables compiled for the x86\_64 architecture.
+* Switched to the libc++ demangler for function names. Previously, the
+  demangler used was not complete, so some less common demangled function
+  names would not be properly demangled or the function name would not be
+  demangled at all.
+* Fix bug in load bias handling. If the unwind information in the eh\_frame
+  or eh\_frame\_hdr does not have the same bias as the executable section,
+  and uses pc relative FDEs, the unwind will be incorrect. This tends
+  to truncate unwinds since the unwinder could not find the correct unwind
+  information for a given pc.