[libunwind] Unwind through aarch64/Linux sigreturn frame

Authored by rprichard on Jan 13 2021, 4:38 PM.


[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


rprichardJan 13 2021, 4:38 PM
Restricted Project
Differential Revision
D90898: [libunwind] Unwind through aarch64/Linux sigreturn frame
rGddd03842c347: [SystemZ] Clear Available set in SystemZPostRASchedStrategy::initialize().