Re: [PATCH v7 01/41] Documentation/x86: Add CET shadow stack description

[Florian Weimer]

* szabolcs:

> syscall overhead in case of frequent stack trace collection can be
> avoided by caching (in tls) when ssp falls within the thread shadow
> stack bounds. otherwise caching does not work as the shadow stack may
> be reused (alt shadow stack or ucontext case).

Do we need to perform the system call at each page boundary only?  That
should reduce overhead to the degree that it should not matter.

> unfortunately i don't know if syscall overhead is actually a problem
> (probably not) or if backtrace across signal handlers need to work
> with alt shadow stack (i guess it should work for crash reporting).

Ideally, we would implement the backtrace function (in glibc) as just a
shadow stack copy.  But this needs to follow the chain of alternate
stacks, and it may also need some form of markup for signal handler
frames (which need program counter adjustment to reflect that a
*non-signal* frame is conceptually nested within the previous
instruction, and not the function the return address points to).  But I
think we can add support for this incrementally.

I assume there is no desire at all on the kernel side that sigaltstack
transparently allocates the shadow stack?  Because there is no
deallocation function today for sigaltstack?

Thanks,
Florian