[libunwind] Don't use dl_iterate_phdr if __ANDROID_API__ < 21

[thomasanderson]

Summary

On ARM, dl_iterate_phdr is only implemented in the Android NDK version 21 or later:
https://android.googlesource.com/platform/prebuilts/ndk/+/dev/platform/sysroot/usr/include/link.h#55

[danalbert]

Doesn't this just turn a build failure into a runtime failure? There are no more fallback implementations after this one. After this change the function will just return false and it will fail to find any unwind sections.

[thomasanderson]

Oh yeah, that's correct. But some Android systems don't have dl_iterate_phdr in their libc, so I'm not sure how to get libc++/libc++abi/libunwind working with them.

[thomasanderson]

Add fallback to dl_unwind_find_exidx

[thomasanderson]

danalbert@ Turns out those versions of the ndk have dl_unwind_find_exidx on ARM, which does what we want -- added this in the latest patch set

[danalbert]

But some Android systems don't have dl_iterate_phdr in their libc, so I'm not sure how to get libc++/libc++abi/libunwind working with them.

The NDK is on an older version of libunwind from before dl_iterate_phdr was required. Looks like that changed with https://reviews.llvm.org/D28082, which removed essentially the block of code you've now added. You're putting it back in such a way that it only affects Android targets that can't use dl_iterate_phdr, so LGTM. Thanks for fixing this!

src/AddressSpace.hpp

     }
   }
   return false;
-#elif defined(_LIBUNWIND_ARM_EHABI) || defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
+#elif (defined(_LIBUNWIND_ARM_EHABI) &&                                        \
+       !(defined(__BIONIC__) && (__ANDROID_API__ < 21))) ||                    \
+    defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
   struct dl_iterate_cb_data {
     LocalAddressSpace *addressSpace;
     UnwindInfoSections *sects;