1. Add a configure option, --enable-cet, to compile libunwind with
"-mshstk -fcf-protection" and link with -Wl,-z,cet-report=error.
CET is always enabled if -fcf-protection is on by default.
2. Add a frames field to struct cursor and update unw_step to cont stack
frames to pop.
3. Update x86_64_sigreturn to pop 4 shadow stack frames.
4. Update x86_64_local_resume to pop the same number of shadow stack
frames as the regular stack frames.
5. Update _Ux86_64_setcontext to pop 2 shadow stack frames.
There are no failures with
$ ./configure --enable-cet
$ make -j12
$ GLIBC_TUNABLES=glibc.cpu.hwcaps=SHSTK make check
on Linux when shadow stack is enabled.
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
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