[Lucid, Maverick] SRU: Fix (hopefully) the last stack guard page effect

[Stefan Bader]

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 at 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 at canonical.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry-picked from commit 39aa3cb3e8250db9188a6f1e3fb62ffa1a717678 upstream)
Signed-off-by: Stefan Bader <stefan.bader at 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) &&
-- 
1.7.0.4