----- 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. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
