[all-commits] [llvm/llvm-project] c82dee: [libunwind] Unwind through aarch64/Linux sigreturn...

[Ryan Prichard via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: c82deed6764cbc63966374baf9721331901ca958
      https://github.com/llvm/llvm-project/commit/c82deed6764cbc63966374baf9721331901ca958
  Author: Ryan Prichard <rprichard at google.com>
  Date:   2021-01-13 (Wed, 13 Jan 2021)

  Changed paths:
    M libunwind/include/__libunwind_config.h
    M libunwind/src/UnwindCursor.hpp
    M libunwind/test/signal_unwind.pass.cpp
    M libunwind/test/unwind_leaffunction.pass.cpp

  Log Message:
  -----------
  [libunwind] Unwind through aarch64/Linux sigreturn frame

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