Re: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension

[Christian Brauner]

On Wed, Jan 12, 2022 at 09:47:29AM -0500, Mathieu Desnoyers wrote:
> ----- On Jan 12, 2022, at 3:46 AM, Christian Brauner christian.brauner@xxxxxxxxxx wrote:
> 
> > On Tue, Jan 11, 2022 at 12:43:05PM -0500, Mathieu Desnoyers wrote:
> [...]
> >> >> + *
> >> >> + * rseq critical sections defined with the RSEQ_CS_FLAG_ABORT_AT_IP flag
> >> >> + * have the following behavior on abort: when the stack grows down: the
> >> >> + * stack pointer is decremented to skip the redzone, and decremented of
> >> >> + * the pointer size.  The aborted address (abort-at-ip) is stored at
> >> >> + * this stack pointer location.  The user-space abort handler needs to
> >> >> + * pop the abort-at-ip address from the stack, and add the redzone size
> >> >> + * to the stack pointer.
> >> >> + *
> >> >> + * TODO: describe stack grows up.
> >> > 
> >> > Is this intentional or did you forget? :)
> >> 
> >> Since I did not implement abort-at-ip on stack-grows-up architectures, I felt
> >> it would be too early to describe the algorithm. I can simply remove the TODO
> >> altogether and we'll take care of it when we get there ? If I had to try to
> >> wordsmith it, it would look like e.g.:
> >> 
> >>  *                                    [...] When the stack grows up: the
> >>  * stack pointer is incremented to skip the redzone, and incremented of
> >>  * the pointer size.  The aborted address (abort-at-ip) is stored immediately
> >>  * under this stack pointer location.  The user-space abort handler needs to
> >>  * pop the abort-at-ip address from the stack, and subtract the redzone size
> >>  * from the stack pointer.
> >> 
> >> [ Please let me know if I got somehow confused in my understanding of stack
> >> grows
> >> up architectures. ]
> >> 
> >> I'm also unsure whether any of the stack grows up architecture have redzones ?
> > 
> > I don't think so? From when I last touched that piece of arch code when
> > massaging copy_thread() I only remember parisc as having an upwards
> > growing stack.
> > 
> >> From a quick grep for redzone in Linux arch/, only openrisc, powerpc64 and
> > > x86-64 appear to have redzones.
> 
> I figured it was kind of silly to special-case arch-agnostic comments for stack
> grows up/down, how about the following instead ?
> 
>  * rseq critical sections defined with the RSEQ_CS_FLAG_ABORT_AT_IP flag
>  * have the following behavior on abort: the stack pointer is adjusted to
>  * skip over the redzone [*], and the aborted address (abort-at-ip) is pushed
>  * at this stack pointer location.  The user-space abort handler needs to pop
>  * the abort-at-ip address from the stack, and adjust the stack pointer to skip
>  * back over the redzone.
>  *
>  * [*] The openrisc, powerpc64 and x86-64 architectures define a "redzone" as a
>  *     stack area beyond the stack pointer which can be used by the compiler
>  *     to store local variables in leaf functions.

Sounds good to me.