[libunwind] Unwind through aarch64/Linux sigreturn frame

[rprichard]

Summary

An AArch64 sigreturn trampoline frame can't currently be described
in a DWARF .eh_frame section, because the AArch64 DWARF spec currently
doesn't define a constant for the PC register. (PC and LR may need to
be restored to different values.)

Instead, use the same technique as libgcc or github.com/libunwind and
detect the sigreturn frame by looking for the sigreturn instructions:

mov x8, #0x8b
svc #0x0

If a sigreturn frame is detected, libunwind restores all the GPRs by
assuming that sp points at an rt_sigframe Linux kernel struct. This
behavior is a fallback mode that is only used if there is no ordinary
unwind info for sigreturn.

If libunwind can't find unwind info for a PC, it assumes that the PC is
readable, and would crash if it isn't. This could happen if:

• The PC points at a function compiled without unwind info, and which is part of an execute-only mapping (e.g. using -Wl,--execute-only).
• The PC is invalid and happens to point to unreadable or unmapped memory.

In the tests, ignore a failed dladdr call so that the tests can run on
user-mode qemu for AArch64, which uses a stack-allocated trampoline
instead of a vDSO.

[compnerd]

Could you add a bit of a comment explaining why the XOM case is not protected against? libunwind can be used for generating backtraces during regular execution (e.g. profiling) and crashing if the XOM is in use seems unfortunate. We could check the memory mapping and just give up in the XOM case.

libunwind/include/__libunwind_config.h

31

What do you think of having an else clause which sets this to 0 to allow using the macro as a condition rather than checking the definedness of it?

libunwind/src/UnwindCursor.hpp

940

What do you think of Registers instead of R2?

2020

It really would be amazing to have a link to the VDSO code here to make it easier to understand the check - I figured it was that this is the implementation, but had to chase through the files.

[danielkiss]

libunwind/src/UnwindCursor.hpp

2020

VDSO is an elf, so we could do something like this:

void* handle = dlopen("linux-vdso.so.1", RTLD_LAZY);
const pint_t ptr = (const pint_t)dlsym(handle, "__kernel_rt_sigreturn");
if( pc == ptr ) ...

This could help with the XOM issue too.

[rprichard]

In D90898#2379387, @compnerd wrote:

Could you add a bit of a comment explaining why the XOM case is not protected against?

I think I can expand on the comment.

libunwind can be used for generating backtraces during regular execution (e.g. profiling) and crashing if the XOM is in use seems unfortunate.

For profiling, I assume the unwind info needs to be accurate at every instruction? I know that's definitely not the case for AArch64, but I thought it's still only mostly-accurate on other archs? I don't *really* know that unwinding through a signal handler is important for Android, but I don't know how to rule it out either, and we do have a test for it. I think the test predated Android's libunwindstack unwinder. Maybe someone could be using _Unwind_Backtrace in a segfault handler, and that probably avoids the async-unwind-info problem most of the time. But I also don't think that's how third-party Android crash dumpers work.

It would only crash on an XOM function if that function also doesn't have unwind info.

We could check the memory mapping and just give up in the XOM case.

That could work. Some similar approaches:

• process_vm_readv
• pipe+write
• open /proc/self/mem

They all have the drawback that /proc might be unavailable or that SELinux/seccomp/etc might restrict some system calls.

libunwind/include/__libunwind_config.h

31

D31078 was a previous attempt to establish a different macro convention for booleans (each macro is either undefined or defined to empty), but it seems to have left a lot of the macros defined to 1 anyway. e.g. _LIBUNWIND_TARGET_I386 was changed from 1 to empty, but none of the other target macros were affected.

I don't currently have a opinion on what the convention should be, so I was trying to match what I thought the convention was (boolean macros are undefined or defined to 1, then checked with defined(...)).

libunwind/src/UnwindCursor.hpp

940

Both R and Registers would be in scope and would be the names of Registers_* classes, but that seems OK to me.

2020

I think the vDSO is the most canonical implementation -- Bionic uses it, and it looks like glibc also does. It's not universal, though. e.g.:

• musl provides its own restore function on all architectures (and doesn't provide DWARF/EHABI unwind info for any of them).
• The user-mode qemu trampoline doesn't come from a vDSO. When I skimming through the code, I see something that writes the trampoline onto the stack.

FWIW, Bionic provides its own restorer function on arm/x86/x86_64 and only uses the kernel's restorer for arm64. This patch is sufficient to fix unwinding through a signal handler for Bionic, but not other targets. e.g. The situations above, but I think x86(-32) with glibc is also affected if there is no vDSO.

I think I can add a link to the vDSO, probably to:
https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/vdso/sigreturn.S

2020

It looks like dlopen("linux-vdso.so.1", ...) does find the vDSO with Bionic and glibc, but not with musl. (I wondered about Bionic, because Bionic's dl_iterate_phdr reports the name of the vDSO as [vdso] rather than linux-vdso.so.1. However, when I tested, it looks like Bionic's dlopen can open linux-vdso.so but not [vdso].)

This approach might have trouble with static linking? For statically-linked Bionic programs, a special limited dl_iterate_phdr is provided in libc.a, but there is no dlopen linked in by default. There is a stub dlopen in libdl.a, but the stub just returns nullptr. For glibc, dlopen also requires linking with -ldl, and then ld.bfd/ld.gold apparently print a warning for static executables:

/tmp/test-f61321.o:test.c:function main: warning: Using 'dlopen' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking

One approach I thought of: change findUnwindSections so that it reports a readable range of memory containing the lookup PC. I think this would break with qemu user-mode emulation but would work in most other configurations. (e.g. A static musl executable doesn't look like it shows the vDSO via dl_iterate_phdr, but that's OK if musl provides its own sigreturn function.)

[rprichard]

Update comments.

[rprichard]

That could work. Some similar approaches:

• process_vm_readv
• pipe+write
• open /proc/self/mem

FWIW, Bionic doesn't expose the process_vm_readv syscall wrapper function until API 23 (M, Android 6.0). The first version of Android to support AArch64 is API 21 (L, Android 5.0). Maybe libunwind could call syscall instead.

Also, with respect to execute-only memory (XOM), there are a couple of mitigating factors:

• As long as the function has unwind info, libunwind won't try to read the memory at the PC.
• Execute-only memory broke another security mitigation, PAN (see here and here), so it only shipped in Android 10. It was removed from Android 11 and from the upstream Linux kernel. Even on Android 10, I think it was only supported on sufficiently-new devices (e.g. not the Pixel 2), and at least for the Pixel 3, the kernel doesn't seem to enforce XOM anymore as of Android 10 QPR3. e.g. I still see --x pages, but it's now possible to read them w/o segfaulting. The first page I linked mentioned backporting a 4.9 kernel patch disabling XOM. Maybe the feature would be resurrected eventually, though.

[danielkiss]

unwind_leaffunction.pass.cpp could be enable too for aarch64.

LGTM.

[rprichard]

Enable unwind_leaffunction.pass.cpp for AArch64 Linux.

[rprichard]

Can I get a code-owner to review this?

[compnerd]

In D90898#2384775, @rprichard wrote:

In D90898#2379387, @compnerd wrote:

Could you add a bit of a comment explaining why the XOM case is not protected against?

I think I can expand on the comment.

libunwind can be used for generating backtraces during regular execution (e.g. profiling) and crashing if the XOM is in use seems unfortunate.

For profiling, I assume the unwind info needs to be accurate at every instruction? I know that's definitely not the case for AArch64, but I thought it's still only mostly-accurate on other archs? I don't *really* know that unwinding through a signal handler is important for Android, but I don't know how to rule it out either, and we do have a test for it. I think the test predated Android's libunwindstack unwinder. Maybe someone could be using _Unwind_Backtrace in a segfault handler, and that probably avoids the async-unwind-info problem most of the time. But I also don't think that's how third-party Android crash dumpers work.

Sure, though, libunwind is generic and is used outside of android as well, including other AArch64 targets, so we do need to consider those use cases while making changes.

[rprichard]

In D90898#2496725, @compnerd wrote:

In D90898#2384775, @rprichard wrote:

In D90898#2379387, @compnerd wrote:

Could you add a bit of a comment explaining why the XOM case is not protected against?

I think I can expand on the comment.

libunwind can be used for generating backtraces during regular execution (e.g. profiling) and crashing if the XOM is in use seems unfortunate.

For profiling, I assume the unwind info needs to be accurate at every instruction? I know that's definitely not the case for AArch64, but I thought it's still only mostly-accurate on other archs? I don't *really* know that unwinding through a signal handler is important for Android, but I don't know how to rule it out either, and we do have a test for it. I think the test predated Android's libunwindstack unwinder. Maybe someone could be using _Unwind_Backtrace in a segfault handler, and that probably avoids the async-unwind-info problem most of the time. But I also don't think that's how third-party Android crash dumpers work.

Sure, though, libunwind is generic and is used outside of android as well, including other AArch64 targets, so we do need to consider those use cases while making changes.

Makes sense. Thanks for the review!

libunwind/include/__libunwind_config.h

 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_ARM       287
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_OR1K      32
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_MIPS      65
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_SPARC     31
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_HEXAGON   34
 #define _LIBUNWIND_HIGHEST_DWARF_REGISTER_RISCV     64
 
 #if defined(_LIBUNWIND_IS_NATIVE_ONLY)
+# if defined(__linux__)
+#  define _LIBUNWIND_TARGET_LINUX 1
+# endif
 # if defined(__i386__)
 #  define _LIBUNWIND_TARGET_I386
 #  define _LIBUNWIND_CONTEXT_SIZE 8
 #  define _LIBUNWIND_CURSOR_SIZE 15
 #  define _LIBUNWIND_HIGHEST_DWARF_REGISTER _LIBUNWIND_HIGHEST_DWARF_REGISTER_X86
 # elif defined(__x86_64__)
 #  define _LIBUNWIND_TARGET_X86_64 1
 #  if defined(_WIN64)

libunwind/src/UnwindCursor.hpp

const uint32_t *ehtp =
                          &off, &len);
     if (_Unwind_VRS_Interpret((_Unwind_Context *)this, ehtp, off, len) !=
             _URC_CONTINUE_UNWIND)
       return UNW_STEP_END;
     return UNW_STEP_SUCCESS;
   }
 #endif
 
+#if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
+  bool setInfoForSigReturn() {
+    R dummy;
+    return setInfoForSigReturn(dummy);
+  }
+  int stepThroughSigReturn() {
+    R dummy;
+    return stepThroughSigReturn(dummy);
+  }
+  bool setInfoForSigReturn(Registers_arm64 &);
+  int stepThroughSigReturn(Registers_arm64 &);
+  template <typename Registers> bool setInfoForSigReturn(Registers &) {
+    return false;
+  }
+  template <typename Registers> int stepThroughSigReturn(Registers &) {
+    return UNW_STEP_END;
+  }
+#endif
+
 #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
   bool getInfoFromFdeCie(const typename CFI_Parser<A>::FDE_Info &fdeInfo,
                          const typename CFI_Parser<A>::CIE_Info &cieInfo,
                          pint_t pc, uintptr_t dso_base);
   bool getInfoFromDwarfSection(pint_t pc, const UnwindInfoSections &sects,
                                             uint32_t fdeSectionOffsetHint=0);
   int stepWithDwarfFDE() {
     return DwarfInstructions<A, R>::stepWithDwarf(_addressSpace,

 #endif // defined(_LIBUNWIND_SUPPORT_SEH_UNWIND)
 
 
   A               &_addressSpace;
   R                _registers;
   unw_proc_info_t  _info;
   bool             _unwindInfoMissing;
   bool             _isSignalFrame;
+#if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
+  bool             _isSigReturn = false;
+#endif
 };
 
 
 template <typename A, typename R>
 UnwindCursor<A, R>::UnwindCursor(unw_context_t *context, A &as)
     : _addressSpace(as), _registers(context), _unwindInfoMissing(false),
       _isSignalFrame(false) {
   static_assert((check_fit<UnwindCursor<A, R>, unw_cursor_t>::does_fit),
#endif
   setLastPC(pc);
   return true;
 }
 #endif
 
 
 template <typename A, typename R>
 void UnwindCursor<A, R>::setInfoBasedOnIPRegister(bool isReturnAddress) {
-  pint_t pc = (pint_t)this->getReg(UNW_REG_IP);
+#if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
+  _isSigReturn = false;
+#endif
+
+  pint_t pc = static_cast<pint_t>(this->getReg(UNW_REG_IP));
 #if defined(_LIBUNWIND_ARM_EHABI)
   // Remove the thumb bit so the IP represents the actual instruction address.
   // This matches the behaviour of _Unwind_GetIP on arm.
   pc &= (pint_t)~0x1;
 #endif
 
   // Exit early if at the top of the stack.
   if (pc == 0) {
if (!CFI_Parser<A>::decodeFDE(_addressSpace, fde, &fdeInfo, &cieInfo)) {
       // Double check this FDE is for a function that includes the pc.
       if ((fdeInfo.pcStart <= pc) && (pc < fdeInfo.pcEnd))
         if (getInfoFromFdeCie(fdeInfo, cieInfo, pc, 0))
           return;
     }
   }
 #endif // #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
 
+#if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
+  if (setInfoForSigReturn())
+    return;
+#endif
+
   // no unwind info, flag that we can't reliably unwind
   _unwindInfoMissing = true;
 }
 
+#if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
+template <typename A, typename R>
+bool UnwindCursor<A, R>::setInfoForSigReturn(Registers_arm64 &) {
+  // Look for the sigreturn trampoline. The trampoline's body is two
+  // specific instructions (see below). Typically the trampoline comes from the
+  // vDSO[1] (i.e. the __kernel_rt_sigreturn function). A libc might provide its
+  // own restorer function, though, or user-mode QEMU might write a trampoline
+  // onto the stack.
+  //
+  // This special code path is a fallback that is only used if the trampoline
+  // lacks proper (e.g. DWARF) unwind info. On AArch64, a new DWARF register
+  // constant for the PC needs to be defined before DWARF can handle a signal
+  // trampoline. This code may segfault if the target PC is unreadable, e.g.:
+  //  - The PC points at a function compiled without unwind info, and which is
+  //    part of an execute-only mapping (e.g. using -Wl,--execute-only).
+  //  - The PC is invalid and happens to point to unreadable or unmapped memory.
+  //
+  // [1] https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/vdso/sigreturn.S
+  const pint_t pc = static_cast<pint_t>(this->getReg(UNW_REG_IP));
+  // Look for instructions: mov x8, #0x8b; svc #0x0
+  if (_addressSpace.get32(pc) == 0xd2801168 &&
+      _addressSpace.get32(pc + 4) == 0xd4000001) {
+    _info = {};
+    _isSigReturn = true;
+    return true;
+  }
+  return false;
+}
+
+template <typename A, typename R>
+int UnwindCursor<A, R>::stepThroughSigReturn(Registers_arm64 &) {
+  // In the signal trampoline frame, sp points to an rt_sigframe[1], which is:
+  //  - 128-byte siginfo struct
+  //  - ucontext struct:
+  //     - 8-byte long (uc_flags)
+  //     - 8-byte pointer (uc_link)
+  //     - 24-byte stack_t
+  //     - 128-byte signal set
+  //     - 8 bytes of padding because sigcontext has 16-byte alignment
+  //     - sigcontext/mcontext_t
+  // [1] https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/signal.c
+  const pint_t kOffsetSpToSigcontext = (128 + 8 + 8 + 24 + 128 + 8); // 304
+
+  // Offsets from sigcontext to each register.
+  const pint_t kOffsetGprs = 8; // offset to "__u64 regs[31]" field
+  const pint_t kOffsetSp = 256; // offset to "__u64 sp" field
+  const pint_t kOffsetPc = 264; // offset to "__u64 pc" field
+
+  pint_t sigctx = _registers.getSP() + kOffsetSpToSigcontext;
+
+  for (int i = 0; i <= 30; ++i) {
+    uint64_t value = _addressSpace.get64(sigctx + kOffsetGprs +
+                                         static_cast<pint_t>(i * 8));
+    _registers.setRegister(UNW_ARM64_X0 + i, value);
+  }
+  _registers.setSP(_addressSpace.get64(sigctx + kOffsetSp));
+  _registers.setIP(_addressSpace.get64(sigctx + kOffsetPc));
+  _isSignalFrame = true;
+  return UNW_STEP_SUCCESS;
+}
+#endif // defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
+
 template <typename A, typename R>
 int UnwindCursor<A, R>::step() {
   // Bottom of stack is defined is when unwind info cannot be found.
   if (_unwindInfoMissing)
     return UNW_STEP_END;
 
   // Use unwinding info to modify register set as if function returned.
   int result;
+#if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
+  if (_isSigReturn) {
+    result = this->stepThroughSigReturn();
+  } else
+#endif
+  {
 #if defined(_LIBUNWIND_SUPPORT_COMPACT_UNWIND)
     result = this->stepWithCompactEncoding();
 #elif defined(_LIBUNWIND_SUPPORT_SEH_UNWIND)
     result = this->stepWithSEHData();
 #elif defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
     result = this->stepWithDwarfFDE();
 #elif defined(_LIBUNWIND_ARM_EHABI)
     result = this->stepWithEHABI();
 #else
   #error Need _LIBUNWIND_SUPPORT_COMPACT_UNWIND or \
               _LIBUNWIND_SUPPORT_SEH_UNWIND or \
               _LIBUNWIND_SUPPORT_DWARF_UNWIND or \
               _LIBUNWIND_ARM_EHABI
 #endif
+  }
 
   // update info based on new PC
   if (result == UNW_STEP_SUCCESS) {
     this->setInfoBasedOnIPRegister(true);
     if (_unwindInfoMissing)
       return UNW_STEP_END;
   }
 

libunwind/test/signal_unwind.pass.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Ensure that the unwinder can cope with the signal handler.
-// REQUIRES: x86_64-linux
+// REQUIRES: linux && (target-aarch64 || target-x86_64)
 
 #include <assert.h>
 #include <dlfcn.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <unwind.h>
 
 _Unwind_Reason_Code frame_handler(struct _Unwind_Context* ctx, void* arg) {
   (void)arg;
   Dl_info info = { 0, 0, 0, 0 };
-  assert(dladdr((void*)_Unwind_GetIP(ctx), &info));
 
-  // Unwind util the main is reached, above frames deeped on the platfrom and architecture.
-  if(info.dli_sname && !strcmp("main", info.dli_sname)) {
+  // Unwind util the main is reached, above frames depend on the platform and
+  // architecture.
+  if (dladdr(reinterpret_cast<void *>(_Unwind_GetIP(ctx)), &info) &&
+      info.dli_sname && !strcmp("main", info.dli_sname)) {
     _Exit(0);
   }
   return _URC_NO_REASON;
 }
 
 void signal_handler(int signum) {
   (void)signum;
   _Unwind_Backtrace(frame_handler, NULL);
   _Exit(-1);
 }
 
 int main(int, char**) {
   signal(SIGUSR1, signal_handler);
   kill(getpid(), SIGUSR1);
   return -2;
 }

libunwind/test/unwind_leaffunction.pass.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // Ensure that leaf function can be unwund.
-// REQUIRES: x86_64-linux
+// REQUIRES: linux && (target-aarch64 || target-x86_64)
 
 #include <assert.h>
 #include <dlfcn.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include <unwind.h>
 
 _Unwind_Reason_Code frame_handler(struct _Unwind_Context* ctx, void* arg) {
   (void)arg;
   Dl_info info = { 0, 0, 0, 0 };
-  assert(dladdr((void*)_Unwind_GetIP(ctx), &info));
 
   // Unwind util the main is reached, above frames deeped on the platfrom and architecture.
-  if(info.dli_sname && !strcmp("main", info.dli_sname)) {
+  if (dladdr(reinterpret_cast<void *>(_Unwind_GetIP(ctx)), &info) &&
+      info.dli_sname && !strcmp("main", info.dli_sname)) {
     _Exit(0);
   }
   return _URC_NO_REASON;
 }
 
 void signal_handler(int signum) {
   (void)signum;
   _Unwind_Backtrace(frame_handler, NULL);
   _Exit(-1);
 }
 
 int* faultyPointer = NULL;
 
 __attribute__((noinline)) void crashing_leaf_func(void) {
   *faultyPointer = 0;
 }
 
 int main(int, char**) {
   signal(SIGSEGV, signal_handler);
   crashing_leaf_func();
   return -2;
 }
 No newline at end of file