Re: [PATCHv8 resend 1/2] x86/vdso: add mremap hook to vm_special_mapping

[Ingo Molnar <mingo@kernel.org>]

* Dmitry Safonov <dsafonov@virtuozzo.com> wrote:

> On 05/16/2016 01:54 PM, Ingo Molnar wrote:
> >
> >* Dmitry Safonov <dsafonov@virtuozzo.com> 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 <dsafonov@virtuozzo.com>
> >>>Acked-by: Andy Lutomirski <luto@kernel.org>
> >>>---
> >>>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