[PATCH v2 2/6] mm/util: Allow to pass a specific task size when getting mmapping base

[sxwjean]

From: Xiongwei Song <sxwjean@xxxxxxxxx>

In x86, a 64bit task may invoke a 32 bit syscall, which is in compat
syscall. Then we have to provide 32bit mapping base.

Signed-off-by: Xiongwei Song <sxwjean@xxxxxxxxx>
---
 mm/util.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/util.c b/mm/util.c
index 40b1a8837c0b..38326ef21a3b 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -385,14 +385,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
 #define MIN_GAP		(SZ_128M)
 #define MAX_GAP		(STACK_TOP / 6 * 5)
 
-static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
+static unsigned long mmap_base(unsigned long rnd, unsigned long task_size,
+			       struct rlimit *rlim_stack)
 {
 	unsigned long gap = rlim_stack->rlim_cur;
 	unsigned long pad = stack_guard_gap;
 
 	/* Account for stack randomization if necessary */
 	if (current->flags & PF_RANDOMIZE)
-		pad += (STACK_RND_MASK << PAGE_SHIFT);
+		pad += (in_compat_syscall() ? 0x7ff : STACK_RND_MASK) << PAGE_SHIFT;
 
 	/* Values close to RLIM_INFINITY can overflow. */
 	if (gap + pad > gap)
@@ -403,7 +404,7 @@ static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
 	else if (gap > MAX_GAP)
 		gap = MAX_GAP;
 
-	return PAGE_ALIGN(STACK_TOP - gap - rnd);
+	return PAGE_ALIGN(task_size - gap - rnd);
 }
 
 void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
@@ -418,7 +419,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
 		mm->mmap_base = mm->mmap_legacy_base;
 		mm->get_unmapped_area = arch_get_unmapped_area;
 	} else {
-		mm->mmap_base = mmap_base(random_factor, rlim_stack);
+		mm->mmap_base = mmap_base(random_factor, STACK_TOP, rlim_stack);
 		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
 	}
 }
-- 
2.30.2