linux-kernel - Re: [PATCH 5/5] arm64/vdso: Restrict splitting VVAR VMA

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Date:   Sun, 23 Feb 2020 15:30:13 -0800
From:   Andrei Vagin <avagin@...il.com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <dima@...sta.com>
Subject: Re: [PATCH 5/5] arm64/vdso: Restrict splitting VVAR VMA

On Thu, Feb 20, 2020 at 12:22:52PM +0000, Vincenzo Frascino wrote:
> Hi Andrei,
> 
> On 04/02/2020 17:59, Andrei Vagin wrote:
> > 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