[libunwind] r374969 - [libunwind][Android] Improve workaround for PIE zero-dlpi_addr bug
Ryan Prichard via cfe-commits
cfe-commits at lists.llvm.org
Tue Oct 15 19:38:47 PDT 2019
Author: rprichard
Date: Tue Oct 15 19:38:47 2019
New Revision: 374969
URL: http://llvm.org/viewvc/llvm-project?rev=374969&view=rev
Log:
[libunwind][Android] Improve workaround for PIE zero-dlpi_addr bug
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.
Reviewers: mclow.lists, srhines, danalbert, compnerd
Reviewed By: compnerd
Subscribers: srhines, krytarowski, christof, libcxx-commits
Tags: #libc
Differential Revision: https://reviews.llvm.org/D68971
Modified:
libunwind/trunk/src/AddressSpace.hpp
Modified: libunwind/trunk/src/AddressSpace.hpp
URL: http://llvm.org/viewvc/llvm-project/libunwind/trunk/src/AddressSpace.hpp?rev=374969&r1=374968&r2=374969&view=diff
==============================================================================
--- libunwind/trunk/src/AddressSpace.hpp (original)
+++ libunwind/trunk/src/AddressSpace.hpp Tue Oct 15 19:38:47 2019
@@ -497,32 +497,40 @@ inline bool LocalAddressSpace::findUnwin
#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;
@@ -531,11 +539,7 @@ inline bool LocalAddressSpace::findUnwin
}
} 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(
@@ -556,12 +560,12 @@ inline bool LocalAddressSpace::findUnwin
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;
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