[libunwind][Android] Improve workaround for PIE zero-dlpi_addr bug

[rprichard]

Summary

The workaround added in https://reviews.llvm.org/rL299575 appears to be
working around a bug in Android JB 4.1.x and 4.2.x (API 16 and 17).

Starting in API 16, Android added support for PIE binaries, but the
dynamic linker failed to initialize dlpi_addr to the address that the
executable was loaded at. The bug was fixed in Android JB 4.3.x (API 18).

Improve the true load bias calculation:

• The code was assuming that the first segment would be the PT_PHDR segment. I think it's better to be explicit and search for PT_PHDR. (It will be almost as fast in practice.)
• It's more correct to use p_vaddr rather than p_offset. If a PIE executable is linked with a non-zero image base (e.g. lld's -Wl,--image-base=xxxx), then we must use p_vaddr here.

The "phdr->p_vaddr < image_base" condition seems unnecessary and maybe
slightly wrong. If the OS were to load a binary at an address smaller than
a vaddr in the binary, we would still want to do this workaround.

The workaround is safe when the linker bug isn't present, because it
should calculate an image_base equal to dlpi_addr. Note that with API 21
and up, this workaround should never activate for dynamically-linked
objects, because non-PIE executables aren't allowed.

Consolidate the fix into a single block of code that calculates the true
image base, and make it clear that the fix no longer applies after API 18.

See https://github.com/android/ndk/issues/505 for details.

[rprichard]

Exclude an unrelated change from the base revision.

[rprichard]

FWIW, Android's official toolchain currently only uses LLVM's libunwind for arm32, so this patch doesn't affect anything yet in the Android platform or NDK. I tested it with NDK r20 by:

• building libunwind.a with -DLIBUNWIND_TARGET_TRIPLE=i686-linux-android16, then
• building a PIE executable, linking libunwind.a in front of the libgcc.a unwinder that NDK r20 normally uses.

I verified that libunwind.a was broken on an API16 x86-32 emulator when I used a non-zero -Wl,--image-base=nnnn setting, but this patch fixes it.

(I also tried running the libunwind tests but didn't get them to run.)

[compnerd]

Thanks, this should be more robust. Nice catch with the image base being explicitly set and using p_vaddr rather than p_offset, that was something that I failed to account for. I really wish that we had a more complete test suite, but that is *way* beyond this change.

[rprichard]

(I also tried running the libunwind tests but didn't get them to run.)

FWIW, I also built libunwind for the host and was able to run ninja check-unwind successfully.

libunwind/src/AddressSpace.hpp

int found = dl_iterate_phdr(
         }
 
 #if !defined(Elf_Half)
         typedef ElfW(Half) Elf_Half;
 #endif
 #if !defined(Elf_Phdr)
         typedef ElfW(Phdr) Elf_Phdr;
 #endif
+#if !defined(Elf_Addr)
+        typedef ElfW(Addr) Elf_Addr;
+#endif
+
+        Elf_Addr image_base = pinfo->dlpi_addr;
+
+#if defined(__ANDROID__) && __ANDROID_API__ < 18
+        if (image_base == 0) {
+          // Normally, an image base of 0 indicates a non-PIE executable. On
+          // versions of Android prior to API 18, the dynamic linker reported a
+          // dlpi_addr of 0 for PIE executables. Compute the true image base
+          // using the PT_PHDR segment.
+          // See https://github.com/android/ndk/issues/505.
+          for (Elf_Half i = 0; i < pinfo->dlpi_phnum; i++) {
+            const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i];
+            if (phdr->p_type == PT_PHDR) {
+              image_base = reinterpret_cast<Elf_Addr>(pinfo->dlpi_phdr) -
+                  phdr->p_vaddr;
+              break;
+            }
+          }
+        }
+#endif
 
  #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
   #if !defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
    #error "_LIBUNWIND_SUPPORT_DWARF_UNWIND requires _LIBUNWIND_SUPPORT_DWARF_INDEX on this platform."
   #endif
         size_t object_length;
-#if defined(__ANDROID__)
-#if !defined(Elf_Addr)
-        typedef ElfW(Addr) Elf_Addr;
-#endif
-        Elf_Addr image_base =
-            pinfo->dlpi_phnum
-                ? reinterpret_cast<Elf_Addr>(pinfo->dlpi_phdr) -
-                      reinterpret_cast<const Elf_Phdr *>(pinfo->dlpi_phdr)
-                          ->p_offset
-                : 0;
-#endif
 
         for (Elf_Half i = 0; i < pinfo->dlpi_phnum; i++) {
           const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i];
           if (phdr->p_type == PT_LOAD) {
-            uintptr_t begin = pinfo->dlpi_addr + phdr->p_vaddr;
-#if defined(__ANDROID__)
-            if (pinfo->dlpi_addr == 0 && phdr->p_vaddr < image_base)
-              begin = begin + image_base;
-#endif
+            uintptr_t begin = image_base + phdr->p_vaddr;
             uintptr_t end = begin + phdr->p_memsz;
             if (cbdata->targetAddr >= begin && cbdata->targetAddr < end) {
               cbdata->sects->dso_base = begin;
               object_length = phdr->p_memsz;
               found_obj = true;
             }
           } else if (phdr->p_type == PT_GNU_EH_FRAME) {
             EHHeaderParser<LocalAddressSpace>::EHHeaderInfo hdrInfo;
-            uintptr_t eh_frame_hdr_start = pinfo->dlpi_addr + phdr->p_vaddr;
-#if defined(__ANDROID__)
-            if (pinfo->dlpi_addr == 0 && phdr->p_vaddr < image_base)
-              eh_frame_hdr_start = eh_frame_hdr_start + image_base;
-#endif
+            uintptr_t eh_frame_hdr_start = image_base + phdr->p_vaddr;
             cbdata->sects->dwarf_index_section = eh_frame_hdr_start;
             cbdata->sects->dwarf_index_section_length = phdr->p_memsz;
             found_hdr = EHHeaderParser<LocalAddressSpace>::decodeEHHdr(
                 *cbdata->addressSpace, eh_frame_hdr_start, phdr->p_memsz,
                 hdrInfo);
             if (found_hdr)
               cbdata->sects->dwarf_section = hdrInfo.eh_frame_ptr;
           }
         }
 
         if (found_obj && found_hdr) {
           cbdata->sects->dwarf_section_length = object_length;
           return true;
         } else {
           return false;
         }
  #else // defined(_LIBUNWIND_ARM_EHABI)
         for (Elf_Half i = 0; i < pinfo->dlpi_phnum; i++) {
           const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i];
           if (phdr->p_type == PT_LOAD) {
-            uintptr_t begin = pinfo->dlpi_addr + phdr->p_vaddr;
+            uintptr_t begin = image_base + phdr->p_vaddr;
             uintptr_t end = begin + phdr->p_memsz;
             if (cbdata->targetAddr >= begin && cbdata->targetAddr < end)
               found_obj = true;
           } else if (phdr->p_type == PT_ARM_EXIDX) {
-            uintptr_t exidx_start = pinfo->dlpi_addr + phdr->p_vaddr;
+            uintptr_t exidx_start = image_base + phdr->p_vaddr;
             cbdata->sects->arm_section = exidx_start;
             cbdata->sects->arm_section_length = phdr->p_memsz;
             found_hdr = true;
           }
         }
         return found_obj && found_hdr;
  #endif
       },