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

[Leon Romanovsky]

On Mon, Jul 13, 2015 at 3:29 PM, Kirill A. Shutemov
<kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> Leon Romanovsky wrote:
>> Hi Kirill,
>>
>> On Mon, Jul 13, 2015 at 1:54 PM, Kirill A. Shutemov
>> <kirill.shutemov@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
>> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> > Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> > ---
>> >  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 *".
I see, it behaves similiar to global "const char *" variable definition.
Thank you for clarification.

>
> --
>  Kirill



-- 
Leon Romanovsky | Independent Linux Consultant
        www.leon.nu | leon@xxxxxxx
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