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    SubjectRe: [PATCH]Fix: 32bit binary has 64bit address of stack vma
    On Mon, 12 Jan 2009, Andrew Morton wrote:
    > On Mon, 12 Jan 2009 15:05:51 -0800
    > Mike Waychison <mikew@google.com> wrote:
    >
    > > > Subject: fs/exec.c: fix value of vma->vm_pgoff for the stack VMA of 32-bit processes
    > > > From: Ying Han <yinghan@google.com>
    > > >
    > > > With a 32 bit binary running on a 64 bit system, the /proc/pid/maps for
    > > > the [stack] VMA displays a 64-bit address:
    > > >
    > > > ff96c000-ff981000 rwxp 7ffffffea000 00:00 0 [stack]
    > > >
    > > > This is because vma->vm_pgoff for that VMA is incorrectly being stored in
    > > > units of offset-in-bytes. It should be stored in units of offset-in-pages.
    > > >
    > >
    > > The problem is that the offset was stored without taking into account
    > > the shift.
    >
    > Sigh. Is it efficient to have me sitting here reverse-engineering the
    > code, writing your changelog?
    >
    >
    >
    > From: Ying Han <yinghan@google.com>
    >
    > With a 32 bit binary running on a 64 bit system, the /proc/pid/maps for
    > the [stack] VMA displays a 64-bit address:
    >
    > ff96c000-ff981000 rwxp 7ffffffea000 00:00 0 [stack]
    >
    > This is because vma->vm_pgoff for that VMA is not being updated for the
    > shift of the stack VMA.
    >
    > Signed-off-by: Ying Han <yinghan@google.com>
    > Cc: Mike Waychison <mikew@google.com>
    > Cc: Hugh Dickins <hugh@veritas.com>
    > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    > ---

    Sorry for all the waste of your time, but
    I'm afraid this is entirely bogus, as Kamezawa-san has kindly
    discovered for us (in: Re: mmotm 2009-01-12-16-53 uploaded thread).

    The code was correct before, there was no shifting issue, and you
    cannot muck around with the vm_pgoff of a vma like this patch does.
    Thank Nick for the CONFIG_DEBUG_VM BUG_ON which warns that this
    patch has inadvertently made these stack pages unreclaimable
    (because their indexes no longer match up with vm_pgoff).

    All there is here is a cosmetic issue: we're used to seeing 8-hex-
    digit offsets in the /proc/pid/maps of a 32-bit process. Though it
    could perfectly well mmap() a large file from a very high offset and
    show more hex digits there, couldn't it?

    Because this 32-bit process has been started by a 64-bit kernel, it
    started off working with the 64-bit stack position before moving into
    place, and that happens to get reflected in the final vm_pgoff.

    You prefer only to see 8-hex-digit offsets for anonymous vmas of 32-bit
    processes? If so, then perhaps Ying Han could supply a cosmetic patch
    for /proc/pid/maps to mask off the upper bits in that case.

    Hugh

    >
    > fs/exec.c | 5 +++--
    > 1 file changed, 3 insertions(+), 2 deletions(-)
    >
    > diff -puN fs/exec.c~fs-execc-fix-value-of-vma-vm_pgoff-for-the-stack-vma-of-32-bit-processes fs/exec.c
    > --- a/fs/exec.c~fs-execc-fix-value-of-vma-vm_pgoff-for-the-stack-vma-of-32-bit-processes
    > +++ a/fs/exec.c
    > @@ -509,6 +509,7 @@ static int shift_arg_pages(struct vm_are
    > unsigned long length = old_end - old_start;
    > unsigned long new_start = old_start - shift;
    > unsigned long new_end = old_end - shift;
    > + unsigned long new_pgoff = new_start >> PAGE_SHIFT;
    > struct mmu_gather *tlb;
    >
    > BUG_ON(new_start > new_end);
    > @@ -523,7 +524,7 @@ static int shift_arg_pages(struct vm_are
    > /*
    > * cover the whole range: [new_start, old_end)
    > */
    > - vma_adjust(vma, new_start, old_end, vma->vm_pgoff, NULL);
    > + vma_adjust(vma, new_start, old_end, new_pgoff, NULL);
    >
    > /*
    > * move the page tables downwards, on failure we rely on
    > @@ -556,7 +557,7 @@ static int shift_arg_pages(struct vm_are
    > /*
    > * shrink the vma to just the new range.
    > */
    > - vma_adjust(vma, new_start, new_end, vma->vm_pgoff, NULL);
    > + vma_adjust(vma, new_start, new_end, new_pgoff, NULL);
    >
    > return 0;
    > }
    > _


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