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Old 09-17-2010, 02:51 PM
Steve Conklin
 
Default SRU: Fix (hopefully) the last stack guard page effect

On Thu, 2010-09-16 at 06:14 -0700, Brad Figg wrote:
> On 09/14/2010 09:04 AM, Stefan Bader wrote:
> > SRU Justification:
> >
> > Impact: When introducing the stack guard page, a follow up patch tried to
> > minimize the effects user-space sees from this change. Unfortunately a
> > lot of the checks did not take into account that user-space may mlock a
> > portion of the stack, but not all of it. This causes the stack vma to
> > be split and a lot of assumptions to go down the drain. Most places have
> > been fixed by now but looking at /proc/<pid>/maps when a stack vma has
> > been split still hides a page on every area and not only on head of it
> > as it was intended.
> >
> > Fix: The following upstream change makes one inline available to other
> > code and uses that when checking for the guard page in the proc output.
> >
> > Testcase: A program that mlocks an area of the stack and then dumps its
> > maps will see holes before the patch and none after.
> >
> > Note: When providing incremental patches to Linus, the cc for stable got
> > lost, though I pinged Greg, but have not received an answer yet. In the
> > end it hopefully comes from stable. I have not yet created a bug report
> > for it as I am not really sure how urgent this is to fix.
> >
> > -Stefan
> >
> > ---
> >
> > From 39aa3cb3e8250db9188a6f1e3fb62ffa1a717678 Mon Sep 17 00:00:00 2001
> > From: Stefan Bader<stefan.bader@canonical.com>
> > Date: Tue, 31 Aug 2010 15:52:27 +0200
> > Subject: [PATCH] mm: (pre-stable) Move vma_stack_continue into mm.h
> >
> > So it can be used by all that need to check for that.
> >
> > Signed-off-by: Stefan Bader<stefan.bader@canonical.com>
> > Signed-off-by: Linus Torvalds<torvalds@linux-foundation.org>
> > (cherry-picked from commit 39aa3cb3e8250db9188a6f1e3fb62ffa1a717678 upstream)
> > Signed-off-by: Stefan Bader<stefan.bader@canonical.com>
> > ---
> > fs/proc/task_mmu.c | 3 ++-
> > include/linux/mm.h | 6 ++++++
> > mm/mlock.c | 6 ------
> > 3 files changed, 8 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 439fc1f..271afc4 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -224,7 +224,8 @@ static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
> > /* We don't show the stack guard page in /proc/maps */
> > start = vma->vm_start;
> > if (vma->vm_flags& VM_GROWSDOWN)
> > - start += PAGE_SIZE;
> > + if (!vma_stack_continue(vma->vm_prev, vma->vm_start))
> > + start += PAGE_SIZE;
> >
> > seq_printf(m, "%08lx-%08lx %c%c%c%c %08llx %02x:%02x %lu %n",
> > start,
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index e6b1210..74949fb 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -864,6 +864,12 @@ int set_page_dirty(struct page *page);
> > int set_page_dirty_lock(struct page *page);
> > int clear_page_dirty_for_io(struct page *page);
> >
> > +/* Is the vma a continuation of the stack vma above it? */
> > +static inline int vma_stack_continue(struct vm_area_struct *vma, unsigned long addr)
> > +{
> > + return vma&& (vma->vm_end == addr)&& (vma->vm_flags& VM_GROWSDOWN);
> > +}
> > +
> > extern unsigned long move_page_tables(struct vm_area_struct *vma,
> > unsigned long old_addr, struct vm_area_struct *new_vma,
> > unsigned long new_addr, unsigned long len);
> > diff --git a/mm/mlock.c b/mm/mlock.c
> > index cbae7c5..b70919c 100644
> > --- a/mm/mlock.c
> > +++ b/mm/mlock.c
> > @@ -135,12 +135,6 @@ void munlock_vma_page(struct page *page)
> > }
> > }
> >
> > -/* Is the vma a continuation of the stack vma above it? */
> > -static inline int vma_stack_continue(struct vm_area_struct *vma, unsigned long addr)
> > -{
> > - return vma&& (vma->vm_end == addr)&& (vma->vm_flags& VM_GROWSDOWN);
> > -}
> > -
> > static inline int stack_guard_page(struct vm_area_struct *vma, unsigned long addr)
> > {
> > return (vma->vm_flags& VM_GROWSDOWN)&&
>
>
> Acked-by: Brad Figg <brad.figg@canonical.com>
>
> --
> Brad Figg brad.figg@canonical.com http://www.canonical.com
>
Acked-by: Steve Conklin <sconklin@canonical.com>
--
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