Messages in this thread | | | Subject | Re: [PATCH 1/2] x86/arch_prctl: add ARCH_SET_{COMPAT,NATIVE} to change compatible mode | From | Dmitry Safonov <> | Date | Fri, 8 Apr 2016 19:18:57 +0300 |
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On 04/08/2016 06:56 PM, Andy Lutomirski wrote: > On Fri, Apr 8, 2016 at 6:50 AM, Dmitry Safonov <dsafonov@virtuozzo.com> wrote: >> 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. > 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 And if it's not there - return error? > 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. Yes, I'm working on it. This patch will only allow moving vdso image with general mremap - so I could use arch_prctl for that API, as for native i386 one may move vdso with mremap and cannot map any other vdso blobs. Does it sound fine?
So, I have some difficulties with removing TIF_IA32 flag: it's checked by perf for interpreting stack frames/instructions and may be checked out of syscall executing (when tracing page fault events, for example). I doubt, is it sane to remove TS_COMPAT instead, leaving TIF_IA32, as for some cases we need to know if task is compatible outside of syscall's path? And the comment in asm/syscall.h says: > * TIF_IA32 tasks should always have TS_COMPAT set at > * system call time. that means, that TS_COMPAT is always set on TIF_IA32, so is meaningless. What do you think?
Thanks, Dmitry.
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