Re: [PATCH v4 05/11] gdb, gdbserver: Add support of Intel shadow stack pointer register.

[Eli Zaretskii <eliz@gnu.org>]

> From: Christina Schimpe <christina.schimpe@intel.com>
> Cc: thiago.bauermann@linaro.org,
> 	eliz@gnu.org
> Date: Tue, 17 Jun 2025 12:11:41 +0000
> 
> This patch adds the user mode register PL3_SSP which is part of the
> Intel(R) Control-Flow Enforcement Technology (CET) feature for support
> of shadow stack.
> For now, only native and remote debugging support for shadow stack
> userspace on amd64 linux are covered by this patch including 64 bit and
> x32 support.  32 bit support is not covered due to missing Linux kernel
> support.
> 
> This patch requires fixing the test gdb.base/inline-frame-cycle-unwind
> which is failing in case the shadow stack pointer is unavailable.
> Such a state is possible if shadow stack is disabled for the current thread
> but supported by HW.
> 
> This test uses the Python unwinder inline-frame-cycle-unwind.py which fakes
> the cyclic stack cycle by reading the pending frame's registers and adding
> them to the unwinder:
> 
> ~~~
> for reg in pending_frame.architecture().registers("general"):
>      val = pending_frame.read_register(reg)
>      unwinder.add_saved_register(reg, val)
>      return unwinder
> ~~~
> 
> However, in case the python unwinder is used we add a register (pl3_ssp) that is
> unavailable.  This leads to a NOT_AVAILABLE_ERROR caught in
> gdb/frame-unwind.c:frame_unwind_try_unwinder and it is continued with standard
> unwinders.  This destroys the faked cyclic behavior and the stack is
> further unwinded after frame 5.
> 
> In the working scenario an error should be triggered:
> ~~~
> bt
> 0  inline_func () at /tmp/gdb.base/inline-frame-cycle-unwind.c:49^M
> 1  normal_func () at /tmp/gdb.base/inline-frame-cycle-unwind.c:32^M
> 2  0x000055555555516e in inline_func () at /tmp/gdb.base/inline-frame-cycle-unwind.c:45^M
> 3  normal_func () at /tmp/gdb.base/inline-frame-cycle-unwind.c:32^M
> 4  0x000055555555516e in inline_func () at /tmp/gdb.base/inline-frame-cycle-unwind.c:45^M
> 5  normal_func () at /tmp/gdb.base/inline-frame-cycle-unwind.c:32^M
> Backtrace stopped: previous frame identical to this frame (corrupt stack?)
> (gdb) PASS: gdb.base/inline-frame-cycle-unwind.exp: cycle at level 5: backtrace when the unwind is broken at frame 5
> ~~~
> 
> To fix the Python unwinder, we simply skip the unavailable registers.
> 
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> Reviewed-By: Eli Zaretskii <eliz@gnu.org>
> ---
>  gdb/NEWS                                      |  3 +
>  gdb/amd64-linux-nat.c                         | 17 +++++
>  gdb/amd64-linux-tdep.c                        |  1 +
>  gdb/amd64-tdep.c                              |  6 +-
>  gdb/amd64-tdep.h                              |  1 +
>  gdb/arch/amd64.c                              | 10 +++
>  gdb/arch/i386.c                               |  4 ++
>  gdb/arch/x86-linux-tdesc-features.c           |  1 +
>  gdb/doc/gdb.texinfo                           |  4 ++
>  gdb/features/Makefile                         |  2 +
>  gdb/features/i386/32bit-ssp.c                 | 14 ++++
>  gdb/features/i386/32bit-ssp.xml               | 11 +++
>  gdb/features/i386/64bit-ssp.c                 | 14 ++++
>  gdb/features/i386/64bit-ssp.xml               | 11 +++
>  gdb/i386-tdep.c                               | 22 +++++-
>  gdb/i386-tdep.h                               |  4 ++
>  gdb/nat/x86-linux-tdesc.c                     |  2 +
>  gdb/nat/x86-linux.c                           | 55 +++++++++++++++
>  gdb/nat/x86-linux.h                           |  4 ++
>  gdb/testsuite/gdb.arch/amd64-shadow-stack.c   | 22 ++++++
>  gdb/testsuite/gdb.arch/amd64-ssp.exp          | 50 +++++++++++++
>  .../gdb.base/inline-frame-cycle-unwind.py     |  4 ++
>  gdb/testsuite/lib/gdb.exp                     | 70 +++++++++++++++++++
>  gdb/x86-linux-nat.c                           | 50 +++++++++++--
>  gdb/x86-linux-nat.h                           | 11 +++
>  gdb/x86-tdep.c                                | 21 ++++++
>  gdb/x86-tdep.h                                |  9 +++
>  gdbserver/linux-x86-low.cc                    | 28 +++++++-
>  gdbsupport/x86-xstate.h                       |  5 +-
>  29 files changed, 446 insertions(+), 10 deletions(-)
>  create mode 100644 gdb/features/i386/32bit-ssp.c
>  create mode 100644 gdb/features/i386/32bit-ssp.xml
>  create mode 100644 gdb/features/i386/64bit-ssp.c
>  create mode 100644 gdb/features/i386/64bit-ssp.xml
>  create mode 100644 gdb/testsuite/gdb.arch/amd64-shadow-stack.c
>  create mode 100644 gdb/testsuite/gdb.arch/amd64-ssp.exp

The NEWS part is okay, thanks.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>