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[PATCHv10 0/2] mremap vDSO for 32-bit

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  [PATCHv10 0/2] mremap vDSO for 32-bit Dmitry Safonov <dsafonov@virtuozzo.com> - 2016-06-28 13:40 +0200
    Re: [PATCHv10 1/2] x86/vdso: add mremap hook to vm_special_mapping Andy Lutomirski <luto@amacapital.net> - 2016-07-06 16:10 +0200
    Re: [PATCHv10 2/2] selftest/x86: add mremap vdso test Ingo Molnar <mingo@kernel.org> - 2016-07-08 14:20 +0200

#1432901 — [PATCHv10 0/2] mremap vDSO for 32-bit

The first patch adds support of mremapping 32-bit vDSO.
One could move vDSO vma before this patch, but fast syscalls
on moved vDSO hasn't been working. It's because of code that
relies 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, 64-bit vDSO mremap also has worked only by a chance
before this patches.
The second patch adds a test for the new functionality.

I need possibility to move vDSO in CRIU - on restore we need
to choose vma'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 than add jump trampolines to it
  from the place of vDSO in restoring application.

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 mremap to work also for 32-bit vDSO. Which does not work,
because of context.vdso pointer mentioned above. 

Changes:
v10: run selftest after fork() and treat child segfaults for a nice
     error reports.
v9: Added cover-letter with changelog and reasons for patches
v8: Add WARN_ON_ONCE on current->mm != new_vma->vm_mm;
    run test for x86_64 too;
    removed fixed VDSO_SIZE - check EINVAL mremap return for partial remapping
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;
    added test for mremapping vDSO
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

Dmitry Safonov (2):
  x86/vdso: add mremap hook to vm_special_mapping
  selftest/x86: add mremap vdso test

 arch/x86/entry/vdso/vma.c                      |  47 +++++++++--
 include/linux/mm_types.h                       |   3 +
 mm/mmap.c                                      |  10 +++
 tools/testing/selftests/x86/Makefile           |   3 +-
 tools/testing/selftests/x86/test_mremap_vdso.c | 111 +++++++++++++++++++++++++
 5 files changed, 168 insertions(+), 6 deletions(-)
 create mode 100644 tools/testing/selftests/x86/test_mremap_vdso.c

-- 
2.9.0

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#1437751 — Re: [PATCHv10 1/2] x86/vdso: add mremap hook to vm_special_mapping

On Tue, Jun 28, 2016 at 4:35 AM, Dmitry Safonov <dsafonov@virtuozzo.com> 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

Acked-by: Andy Lutomirski <luto@kernel.org>

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#1439387 — Re: [PATCHv10 2/2] selftest/x86: add mremap vdso test

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

> Or print that mremap for vDSO is unsupported:
> [root@localhost ~]# ./test_mremap_vdso_32
> 	AT_SYSINFO_EHDR is 0xf773c000
> [NOTE]	Moving vDSO: [0xf773c000, 0xf773d000] -> [0xf7737000, 0xf7738000]
> [FAIL]	mremap() of the vDSO does not work on this kernel!

Hm, I tried this on a 64-bit kernel and got:

triton:~/tip/tools/testing/selftests/x86> ./test_mremap_vdso_32 
        AT_SYSINFO_EHDR is 0xf7773000
[NOTE]  Moving vDSO: [0xf7773000, 0xf7774000] -> [0xf776e000, 0xf776f000]
Segmentation fault

Thanks,

	Ingo

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