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.
