From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lf0-f70.google.com (mail-lf0-f70.google.com [209.85.215.70]) by kanga.kvack.org (Postfix) with ESMTP id 59B606B025E for ; Mon, 16 May 2016 09:55:29 -0400 (EDT) Received: by mail-lf0-f70.google.com with SMTP id y84so100436799lfc.3 for ; Mon, 16 May 2016 06:55:29 -0700 (PDT) Received: from mail-wm0-x243.google.com (mail-wm0-x243.google.com. [2a00:1450:400c:c09::243]) by mx.google.com with ESMTPS id w2si38813821wjs.32.2016.05.16.06.55.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 16 May 2016 06:55:27 -0700 (PDT) Received: by mail-wm0-x243.google.com with SMTP id r12so18332068wme.0 for ; Mon, 16 May 2016 06:55:26 -0700 (PDT) Date: Mon, 16 May 2016 15:55:22 +0200 From: Ingo Molnar Subject: Re: [PATCHv8 resend 1/2] x86/vdso: add mremap hook to vm_special_mapping Message-ID: <20160516135522.GB14452@gmail.com> References: <1462886951-23376-1-git-send-email-dsafonov@virtuozzo.com> <79f9fe67-a343-43b8-0933-a79461900c1b@virtuozzo.com> <20160516105429.GA20440@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Dmitry Safonov Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, luto@amacapital.net, tglx@linutronix.de, hpa@zytor.com, x86@kernel.org, akpm@linux-foundation.org, linux-mm@kvack.org, 0x7f454c46@gmail.com * Dmitry Safonov wrote: > On 05/16/2016 01:54 PM, Ingo Molnar wrote: > > > >* Dmitry Safonov wrote: > > > >>On 05/10/2016 04:29 PM, Dmitry Safonov wrote: > >>>Add possibility for userspace 32-bit applications to move > >>>vdso mapping. Previously, when userspace app called > >>>mremap for vdso, in return path it would land on previous > >>>address of vdso page, resulting in segmentation violation. > >>>Now it lands fine and returns to userspace with remapped vdso. > >>>This will also fix context.vdso pointer for 64-bit, which does not > >>>affect the user of vdso after mremap by now, but this may change. > >>> > >>>As suggested by Andy, return EINVAL for mremap that splits vdso image. > >>> > >>>Renamed and moved text_mapping structure declaration inside > >>>map_vdso, as it used only there and now it complement > >>>vvar_mapping variable. > >>> > >>>There is still problem for remapping vdso in glibc applications: > >>>linker relocates addresses for syscalls on vdso page, so > >>>you need to relink with the new addresses. Or the next syscall > >>>through glibc may fail: > >>> Program received signal SIGSEGV, Segmentation fault. > >>> #0 0xf7fd9b80 in __kernel_vsyscall () > >>> #1 0xf7ec8238 in _exit () from /usr/lib32/libc.so.6 > >>> > >>>Signed-off-by: Dmitry Safonov > >>>Acked-by: Andy Lutomirski > >>>--- > >>>v8: add WARN_ON_ONCE on current->mm != new_vma->vm_mm > >>>v7: build fix > >>>v6: moved vdso_image_32 check and fixup code into vdso_fix_landing function > >>> with ifdefs around > >>>v5: as Andy suggested, add a check that new_vma->vm_mm and current->mm are > >>> the same, also check not only in_ia32_syscall() but image == &vdso_image_32 > >>>v4: drop __maybe_unused & use image from mm->context instead vdso_image_32 > >>>v3: as Andy suggested, return EINVAL in case of splitting vdso blob on mremap; > >>> used is_ia32_task instead of ifdefs > >>>v2: added __maybe_unused for pt_regs in vdso_mremap > >> > >>Ping? > > > >There's no 0/2 boilerplate explaining the background of the changes - why do you > >want to mremap() the vDSO? > > Thanks for the answer. > > Well, one could move vdso vma before this patch, but doing fast > syscalls through it will not work because of code relying on > mm->context.vdso pointer. > So all this code is just fixup for that pointer on moving. > (Also adds preventing for splitting vdso vma). > As Andy notted, vDSO mremap for !i386 tasks also worked only by a chance > before this patch. > > I need to move vdso vma in CRIU - on restore we need to choose it's > position: > - if vDSO blob of restoring application is the same as the kernel has, > we need to move it on the same place; > - if it differs, we need to choose place that wasn't tooken by other > vma of restoring application and add jump trampolines to it from the > place of vDSO in restoring application. > And CRIU code now relies on possibility on x86_64 to mremap vDSO. > Without this patch that may be broken in future. > And as I work on C/R of compatible 32-bit applications on x86_64, > I need this to work also for 32-bit vDSO. Which does not work, > because of pointer mentioned above. Ok, this looks useful - please add this information to the changelog (with typos fixed). Thanks, Ingo -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org