On June 19, 2017 8:00:19 PM GMT+02:00, Richard Biener <richard.guent...@gmail.com> wrote: >On June 19, 2017 7:29:32 PM GMT+02:00, Jakub Jelinek <ja...@redhat.com> >wrote: >>On Mon, Jun 19, 2017 at 11:07:06AM -0600, Jeff Law wrote: >>> After much poking around I concluded that we really need to >implement >>> allocation and probing via a "moving sp" strategy. Probing into >>> unallocated areas runs afoul of valgrind, so that's a non-starter. >>> >>> Allocating stack space, then probing the pages within the space is >>> vulnerable to async signal delivery between the allocation point and >>the >>> probe point. If that occurs the signal handler could end up running >>on >>> a stack that has collided with the heap. >>> >>> Ideally we would allocate and probe a page as an atomic unit (which >>is >>> feasible on PPC). Alternatively, due to ISA restrictions, allocate >a >>> page, then probe the page as distinct instructions. The latter >still >>> has a race, but we'd have to take the async signal in a single >>> instruction window. >> >>And if the allocation is only a page at a time, the single insn race >>window >>can be mitigated in the kernel (probe (read-only is fine) the word at >>the >>stack when setting up a signal frame for async signal). >> >>> So, time to open the discussion to questions & comments. >>> >>> I've got patches I need to cleanup and post for comments that >>implement >>> this for x86, ppc, aarch64 and s390. x86 and ppc are IMHO in good >>> shape. THere's an unhandled case for s390. I've got evaluation >>still >>> to do on aarch64. >> >>In the patches Jeff is going to post, we have (at least for >>-fasynchronous-unwind-tables which is on by default on e.g. x86) >>precise unwind info even with the new stack check mode. >>ira.c currently has: >> /* We need the frame pointer to catch stack overflow exceptions >if >> the stack pointer is moving (as for the alloca case just above). >*/ >> || (STACK_CHECK_MOVING_SP >> && flag_stack_check >> && flag_exceptions >> && cfun->can_throw_non_call_exceptions) >>For alloca we have a frame pointer for other reasons, the question is >>if we really need this hunk even if we provided proper unwind info >>even for the Ada -fstack-check mode. Or, if we provide proper unwind >>info >>for -fasynchronous-unwind-tables, if the above could not be also >>&& !flag_asynchronous_unwind_tables. Eric, what exactly is the reason >>for the above, is it just lack of proper CFI notes, or something >>different? >> >>Also, on i?86 orq $0, (%rsp) or orl $0, (%esp) is used to probe stack, >>while it is shorter, is it actually faster or as slow as movq $0, >>(%rsp) >>or movl $0, (%esp) ? > >It at least has the chance of bypassing all of the store queue in CPUs >and thus cause no cacheline allocation or trigger prefetching. > >Not sure if any of that is done though. > >Performance counters might tell. > >Otherwise incrementing SP by 4095 and then pushing al would work as >well (and be similarly short as the or).
Oh, and using push intelligently with first bumping to SP & 4096-1 + 4095 would solve the signal atomicity as well. Might be larger and somewhat interfere with CPUs stack engine. Who knows... Richard. >Richard. > >Richard. > >> Jakub
