On Thu, Feb 20, 2020 at 12:22:52PM +0000, Vincenzo Frascino wrote:

Hi Andrei,

On 04/02/2020 17:59, Andrei Vagin wrote:

quoted

Forbid splitting VVAR VMA resulting in a stricter ABI and reducing the
amount of corner-cases to consider while working further on VDSO time
namespace support.

As the offset from timens to VVAR page is computed compile-time, the pages
in VVAR should stay together and not being partically mremap()'ed.

I agree on the concept, but why do we need to redefine mremap?
special_mapping_mremap() (mm/mmap.c +3317) seems doing already the same thing if
we leave mremap == NULL as is.

Hmmm. I have read the code of special_mapping_mremap() and I don't see where
it restricts splitting the vvar mapping.

Here is the code what I see in the source:

static int special_mapping_mremap(struct vm_area_struct *new_vma)
{
        struct vm_special_mapping *sm = new_vma->vm_private_data;

        if (WARN_ON_ONCE(current->mm != new_vma->vm_mm))
                return -EFAULT;

        if (sm->mremap)
                return sm->mremap(sm, new_vma);

        return 0;
}

And I have checked that without this patch, I can remap only one page of
the vvar mapping.

Thanks,
Andrei


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