[PATCH v2 3/6] mm/util: Support CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES

[sxwjean@me.com]

From: Xiongwei Song <sxwjean@gmail.com>

With CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES enabled, archs can support 64bit
process invoke 32bit syscall via compatible mmap(), which uses
mmap_compat_base and mmap_compat_legacy_base of mm_struct, which only
x86 uses so far.

Here we assume other archs will support CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
as well.

Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
---
 arch/x86/include/asm/processor.h |  4 ++++
 mm/util.c                        | 21 +++++++++++++++++++--
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 577f342dbfb2..fb7a4f21d412 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -787,6 +787,10 @@ extern void start_thread(struct pt_regs *regs, unsigned long new_ip,
 #define __TASK_UNMAPPED_BASE(task_size)	(PAGE_ALIGN(task_size / 3))
 #define TASK_UNMAPPED_BASE		__TASK_UNMAPPED_BASE(TASK_SIZE_LOW)
 
+#ifdef CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
+#define TASK_UNMAPPED_COMPAT_BASE __TASK_UNMAPPED_BASE(task_size_32bit())
+#endif
+
 #define KSTK_EIP(task)		(task_pt_regs(task)->ip)
 
 /* Get/set a process' ability to use the timestamp counter instruction */
diff --git a/mm/util.c b/mm/util.c
index 38326ef21a3b..ab3711c445e6 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -357,8 +357,9 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
-	if (is_compat_task())
+#if defined(CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS) \
+	|| defined(CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES)
+	if (is_compat_task() || in_compat_syscall())
 		rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
 	else
 #endif /* CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS */
@@ -422,6 +423,22 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
 		mm->mmap_base = mmap_base(random_factor, STACK_TOP, rlim_stack);
 		mm->get_unmapped_area = arch_get_unmapped_area_topdown;
 	}
+#ifdef CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
+	/*
+	 * The mmap syscall mapping base decision depends solely on the
+	 * syscall type (64-bit or compat). This applies for 64bit
+	 * applications and 32bit applications. The 64bit syscall uses
+	 * mmap_base, the compat syscall uses mmap_compat_base.
+	 */
+	if (mmap_is_legacy(rlim_stack)) {
+		mm->mmap_compat_legacy_base =
+			TASK_UNMAPPED_COMPAT_BASE + random_factor;
+		mm->mmap_compat_base = mm->mmap_compat_legacy_base;
+	} else {
+		mm->mmap_compat_base = mmap_base(random_factor,
+				       task_size_32bit(), rlim_stack);
+	}
+#endif
 }
 #elif defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)
 void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
-- 
2.30.2