RE: [PATCH 7/9] gdb: Provide gdbarch hook to distinguish shadow stack backtrace elements.

["Schimpe, Christina" <christina.schimpe@intel.com>]

Hi Eli, 

> -----Original Message-----
> From: Eli Zaretskii <eliz@gnu.org>
> Sent: Tuesday, September 23, 2025 1:50 PM
> To: Schimpe, Christina <christina.schimpe@intel.com>
> Cc: gdb-patches@sourceware.org
> Subject: Re: [PATCH 7/9] gdb: Provide gdbarch hook to distinguish shadow
> stack backtrace elements.
> 
> > From: Christina Schimpe <christina.schimpe@intel.com>
> > Date: Tue, 23 Sep 2025 11:18:40 +0000
> >
> > On x86 with CET there can be elements on the shadow stack which are
> > not return addresses.  In this case, we just want to print the element
> > itself in the shadow stack backtrace, but no further information.
> >
> > Provide a gdbarch hook to distinguish between return and non-return
> > addresses and use it to print the shadow stack backtrace as described
> > above.
> > ---
> >  gdb/doc/gdb.texinfo       | 19 ++++++++++++
> >  gdb/gdbarch-gen.c         | 32 ++++++++++++++++++++
> >  gdb/gdbarch-gen.h         | 15 +++++++++
> >  gdb/gdbarch.h             |  1 +
> >  gdb/gdbarch_components.py | 17 +++++++++++
> >  gdb/shadow-stack.c        | 64 +++++++++++++++++----------------------
> >  gdb/shadow-stack.h        | 37 ++++++++++++++++++++++
> >  7 files changed, 148 insertions(+), 37 deletions(-)
> 
> Thanks.
> 
> > diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo index
> > ebda4546b58..a0fde385a8e 100644
> > --- a/gdb/doc/gdb.texinfo
> > +++ b/gdb/doc/gdb.texinfo
> > @@ -8887,6 +8887,25 @@ This is how a shadow stack backtrace looks like
> on amd64:
> >  @end group
> >  @end smallexample
> >
> > +There can be elements on the shadow stack which are not return
> > +addresses, for example on x86 with the Intel Control-Flow Enforcement
> > +Technology (@xref{CET}).  In case of signals, the old shadow stack
> > +pointer is pushed
>                ^
> A cross-reference is missing here.

Thanks will fix.

> > +in a special format with bit 63 set.  For such shadow stack elements,
> > +the shadow stack frame just contains the level and the address on the
> > +shadow stack, as shown in the following example by frame 1:
> > +
> > +@smallexample
> > +@group
> > +(gdb) bt shadow 4
> > +#0  0x00007ffff7c54d90 in __restore_rt from /lib64/libc.so.6
> > +#1  0x80007ffff79fffd8
> > +#2  0x00007ffff7c54ce6 in __GI_raise at ../sysdeps/posix/raise.c:27
> > +#3  0x000000000040115d in main at /tmp/amd64-shadow-stack-
> signal.c:32
> > +(More shadow stack frames follow...)
> > +@end group
> > +@end smallexample
> 
> Would it make sense to show something like "<signal caught>", instead of a
> frame with only an address?

Yeah, this is a good idea, I wondered about a similar thing actually but wanted to discuss
the general direction for handling those specific elements on the shadow stack first.
Maybe there are more options on other architectures that we have to consider.
Let's wait for more feedback on this, I added Thiago again in cc here.

Christina

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