[Bug libc/23004] New: unwind_stop doesn't work with alternate signal stack and shadow stack

["hjl.tools at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=23004

            Bug ID: 23004
           Summary: unwind_stop doesn't work with alternate signal stack
                    and shadow stack
           Product: glibc
           Version: 2.28
               URL: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85086
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: hjl.tools at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---
            Target: x86_64-*-*, i?86-*-*

When alternate signal stack is used with thread cancellation,
_Unwind_Resume fails when it tries to unwind shadow stack from signal
handler on alternate signal stack.  The issue is that signal handler on
alternate signal stack uses a separate shadow stack and we must switch
to the original shadow stack to unwind it. But frame count will be wrong
in this case.  For thread cancellation, there is no need to unwind shadow
stack since it will long jump back and exit.

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