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Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs

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Leon Romanovsky wrote:
> Hi Kirill,
> 
> On Mon, Jul 13, 2015 at 1:54 PM, Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > MPX setups private anonymous mapping, but uses vma->vm_ops too.
> > This can confuse core VM, as it relies on vm->vm_ops to distinguish
> > file VMAs from anonymous.
> >
> > As result we will get SIGBUS, because handle_pte_fault() thinks it's
> > file VMA without vm_ops->fault and it doesn't know how to handle the
> > situation properly.
> >
> > Let's fix that by not setting ->vm_ops.
> >
> > We don't really need ->vm_ops here: MPX VMA can be detected with VM_MPX
> > flag. And vma_merge() will not merge MPX VMA with non-MPX VMA, because
> > ->vm_flags won't match.
> >
> > The only thing left is name of VMA. I'm not sure if it's part of ABI, or
> > we can just drop it. The patch keep it by providing arch_vma_name() on x86.
> >
> > Build tested only.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Andy Lutomirski <luto@amacapital.net>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > ---
> >  arch/x86/mm/mmap.c |  7 +++++++
> >  arch/x86/mm/mpx.c  | 20 +-------------------
> >  2 files changed, 8 insertions(+), 19 deletions(-)
> >
> > diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> > index 9d518d693b4b..844b06d67df4 100644
> > --- a/arch/x86/mm/mmap.c
> > +++ b/arch/x86/mm/mmap.c
> > @@ -126,3 +126,10 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
> >                 mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> >         }
> >  }
> > +
> > +const char *arch_vma_name(struct vm_area_struct *vma)
> > +{
> > +       if (vma->vm_flags & VM_MPX)
> > +               return "[mpx]";
> > +       return NULL;
> > +}
> 
> I sure that I'm missing something important. This function stores
> "[mpx]" string on this function stack and returns the pointer to that
> address. In current flow, this address is visible and accessible,
> however in can be a different in general case.

The string is not on stack. String literals are in .rodata and caller is
not allowed to modify it since it's "const char *".

-- 
 Kirill
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[PATCH 0/5] Make vma_is_anonymous() reliable "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-13 13:00 +0200
  [PATCH 4/5] mm, madvise: use vma_is_anonymous() to check for anon VMA "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-13 13:00 +0200
  [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-13 13:00 +0200
    Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs Leon Romanovsky <leon@leon.nu> - 2015-07-13 14:10 +0200
      Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-13 14:40 +0200
        Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs Leon Romanovsky <leon@leon.nu> - 2015-07-13 14:50 +0200
    Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-16 13:10 +0200
      Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs Dave Hansen <dave.hansen@linux.intel.com> - 2015-07-16 18:00 +0200
        Re: [PATCH 2/5] x86, mpx: do not set ->vm_ops on mpx VMAs "Kirill A. Shutemov" <kirill@shutemov.name> - 2015-07-16 18:10 +0200
          [PATCH 0/1] mm, mpx: add "vm_flags_t vm_flags" arg to  do_mmap_pgoff() Oleg Nesterov <oleg@redhat.com> - 2015-07-17 00:30 +0200
            [PATCH 1/1] mm, mpx: add "vm_flags_t vm_flags" arg to  do_mmap_pgoff() Oleg Nesterov <oleg@redhat.com> - 2015-07-17 00:30 +0200
  [PATCH 3/5] mm: make sure all file VMAs have ->vm_ops set "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-13 13:00 +0200
  [PATCH 1/5] mm: mark most vm_operations_struct const "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-13 13:00 +0200
  [PATCH 5/5] mm, memcontrol: use vma_is_anonymous() to check for anon VMA "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2015-07-13 13:00 +0200
    Re: [PATCH 5/5] mm, memcontrol: use vma_is_anonymous() to check for  anon VMA Johannes Weiner <hannes@cmpxchg.org> - 2015-07-13 15:10 +0200

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