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[libunwind] Unwind through aarch64/Linux sigreturn frame

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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.

Reviewed By: danielkiss, compnerd, #libunwind

Differential Revision: https://reviews.llvm.org/D90898
This commit is contained in:
Ryan Prichard 2021-01-13 16:38:36 -08:00
parent ddd03842c3
commit c82deed676
4 changed files with 116 additions and 12 deletions
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3
libunwind/include/__libunwind_config.h
View file

@ -27,6 +27,9 @@
#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

110
libunwind/src/UnwindCursor.hpp
View file

@ -925,6 +925,25 @@ private:
}
#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,
@ -1179,6 +1198,9 @@ private:
unw_proc_info_t _info;
bool _unwindInfoMissing;
bool _isSignalFrame;
#if defined(_LIBUNWIND_TARGET_LINUX) && defined(_LIBUNWIND_TARGET_AARCH64)
bool _isSigReturn = false;
#endif
};
@ -1873,7 +1895,11 @@ bool UnwindCursor<A, R>::getInfoFromSEH(pint_t pc) {
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.
@ -1971,10 +1997,77 @@ void UnwindCursor<A, R>::setInfoBasedOnIPRegister(bool isReturnAddress) {
}
#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.
@ -1983,20 +2076,27 @@ int UnwindCursor<A, R>::step() {
// 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();
result = this->stepWithCompactEncoding();
#elif defined(_LIBUNWIND_SUPPORT_SEH_UNWIND)
result = this->stepWithSEHData();
result = this->stepWithSEHData();
#elif defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
result = this->stepWithDwarfFDE();
result = this->stepWithDwarfFDE();
#elif defined(_LIBUNWIND_ARM_EHABI)
result = this->stepWithEHABI();
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) {

9
libunwind/test/signal_unwind.pass.cpp
View file

@ -8,7 +8,7 @@
//===----------------------------------------------------------------------===//
// 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>
@ -23,10 +23,11 @@
_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;

6
libunwind/test/unwind_leaffunction.pass.cpp
View file

@ -8,7 +8,7 @@
//===----------------------------------------------------------------------===//
// Ensure that leaf function can be unwund.
// REQUIRES: x86_64-linux
// REQUIRES: linux && (target-aarch64 || target-x86_64)
#include <assert.h>
#include <dlfcn.h>
@ -23,10 +23,10 @@
_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;