On Fri, Apr 8, 2016 at 6:50 AM, Dmitry Safonov <dsafonov@xxxxxxxxxxxxx> wrote: > On 04/07/2016 05:39 PM, Andy Lutomirski wrote: >> >> For 32-bit, the vdso *must* exist in memory at the address that the >> kernel thinks it's at. Even if you had a pure 32-bit restore stub, >> you would still need vdso remap, because there's a chance the vdso >> could land at an unusable address, say one page off from where you >> want it. You couldn't map a wrapper because there wouldn't be any >> space for it without moving the real vdso out of the way. >> >> Remember, you *cannot* mremap() the 32-bit vdso because you will >> crash. It works by luck for 64-bit, but it's plausible that we'd want >> to change that some day. (I have awful patches that speed a bunch of >> things up at the cost of a vdso trampoline for 64-bit code and a bunch >> of other hacks. Those patches will never go in for real, but >> something else might want the ability to use 64-bit vdso trampolines.) > > Hello again, > what do you think about attached patch? > I think it should fix landing problem for i386 vdso mremap. > It does not touch fast syscall path, so there should be no > speed regression. >>> >>> I did remapping for vdso as blob for native x86_64 task differs >>> to compatible task. So it's just changing blobs, address value >>> is there for convenience - I may omit it and just remap >>> different vdso blob at the same place where was previous vdso. >>> I'm not sure, why do we need possibility to map 64-bit vdso blob >>> on native 32-bit builds? >> >> That would fail, but I think the API should exist. But a native >> 32-bit program should be able to remap the 32-bit vdso. >> >> IOW, I think you should be able to do, roughly: >> >> map_new_vdso(VDSO_32BIT, addr); >> >> on any kernel. >> >> Am I making sense? > > I will still work for this interface - just wanted that > usuall mremap to work on vdso mappings. For this thing: + /* Fixing userspace landing - look at do_fast_syscall_32 */ + if (current_thread_info()->status & TS_COMPAT) + regs->ip = (unsigned long)current->mm->context.vdso + + vdso_image_32.sym_int80_landing_pad; Either check that ip was where you expected it or simply remove this code -- user programs that are mremapping the vdso are already playing with fire and can just use int $0x80 to do it. Other than that, it looks generally sane. The .mremap hook didn't exist last time I looked at this :) The main downside of your approach is that it doesn't allow switching between the 32-bit, 64-bit, and x32 images. Also, it requires awareness of how vvar and vdso line up, whereas a dedicated API could do the whole thing. > > Thanks, > Dmitry. -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kselftest" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
