RE: Shadow stack backtrace command name

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

Hi all, 

Thanks a lot for your feedback and ideas. Please see my comments below.

> >> A shadow stack is a second stack for a program introduced in the
> >> Intel (R) Control-Flow Enforcement Technology (CET).  The shadow
> >> stack is used for control transfer operations to store the return addresses.
> >
> > One question I had is when, as a gdb user, would I want to see this
> > information?

> I think the most common shadow stack error a GDB user would encounter
> would be when the inferior is returning from a function and gets a SIGSEGV
> because the return address is wrong (e.g., because a buffer overflow wrote over
> it).
> 
> There are other possibilities, for example a program can create different shadow
> stacks and switch between them (e.g., when it implements userspace-level
> threading) so some error could happen during that process. E.g., in AArch64's
> Guarded Control Stacks, there needs to be a special "cap" value at the end of the
> incoming stack and a SIGSEGV is generated if that's not the case. In this case I
> think the user would want to be able to direct the shadow stack backtrace
> command to print the backtrace of that other stack, instead of the currently
> active one.

Yes, I agree here. Common use cases would be debugging of stack corruption
or more complex scenarios that cause a control protection exception, as Thiago
points out.

> Another example would be trying to write to a mapped shadow stack that is
> read-only. That also causes a SIGSEGV. Though not sure if the shadow stack
> backtrace is relevant in this scenario.
> 
> >> It is configurable using "print symbol-filename" and COUNT.
> >> The command can be called by the following names:
> >> - "info shadow-stack bt", "info shadow-stack backtrace"
> >
> > Like others in the thread, I'm -1 on "info" as a prefix.
> > I liked "bt -shadow", but I was also wondering if the information
> > should just be integrated into the ordinary backtrace when
> > available... that's why I'm wondering when I'd want to see this.
> 
> In my first example, it would be useful if the regular backtrace output noted
> where it differs from the shadow stack. Though I think a way to see the shadow
> stack backtrace by itself would still be useful.

I like the idea to combine the outputs as the traces can be easily compared by
the user in this way. 

However, based on the use cases that I am aware of, I am not sure if the user
wants to always see the shadow stack bt in the ordinary bt output (if shadow
stack is enabled).

In addition, I wonder how the output would look like. Dependent on the source
code of the debugged program, the frames of the ordinary bt command can be
very long already. 

Christina

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