On Tue, Jul 18, 2017 at 8:08 PM, Brian Gerst <brge...@gmail.com> wrote: > On Tue, Jul 18, 2017 at 6:33 PM, Thomas Garnier <thgar...@google.com> wrote: >> Perpcu uses a clever design where the .percu ELF section has a virtual >> address of zero and the relocation code avoid relocating specific >> symbols. It makes the code simple and easily adaptable with or without >> SMP support. >> >> This design is incompatible with PIE because generated code always try to >> access the zero virtual address relative to the default mapping address. >> It becomes impossible when KASLR is configured to go below -2G. This >> patch solves this problem by removing the zero mapping and adapting the GS >> base to be relative to the expected address. These changes are done only >> when PIE is enabled. The original implementation is kept as-is >> by default. > > The reason the per-cpu section is zero-based on x86-64 is to > workaround GCC hardcoding the stack protector canary at %gs:40. So > this patch is incompatible with CONFIG_STACK_PROTECTOR.
Ok, that make sense. I don't want this feature to not work with CONFIG_CC_STACKPROTECTOR*. One way to fix that would be adding a GDT entry for gs so gs:40 points to the correct memory address and gs:[rip+XX] works correctly through the MSR. Given the separate discussion on mcmodel, I am going first to check if we can move from PIE to PIC with a mcmodel=small or medium that would remove the percpu change requirement. I tried before without success but I understand better percpu and other components so maybe I can make it work. Thanks a lot for the feedback. > > -- > Brian Gerst -- Thomas _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
